Centering on the critical bottlenecks in the development of shale oil in the Jiyang Depression of Shengli Oilfield,key scientific and engineering issues are proposed in aspects such as the storage space and occurrence...Centering on the critical bottlenecks in the development of shale oil in the Jiyang Depression of Shengli Oilfield,key scientific and engineering issues are proposed in aspects such as the storage space and occurrence state of shale oil,the formation mechanisms of multi-scale flow spaces,the mobilization mechanisms of crude oil in pores and fractures,and the enhanced oil recovery(EOR)mechanisms during the late stage of elastic development.The research progress and mechanistic insights in recent years are reviewed with respect to experimental techniques,characteristics of pore-fracture structure and fluid occurrence,fracture evolution mechanisms,shale oil flow mechanisms and EOR techniques.Through improving the experimental methods,optimizing the testing conditions,and developing new technologies,we deeply understand the occurrence state,storage space and flow pattern of shale oil,and reveal the distribution pattern of“oil-bearing in all pore sizes and oil-rich in large pores”and the differences in fluid phase states under the confinement effect of nano-scale pores in the shales of the Jiyang Depression;depict the characteristics of“restricted vertical expansion and complex fracture networks”of induced fractures and the dynamic evolution of fracture networks during the fracturing-soaking-production process;establish a“easy flow-slow flow-stagnant flow”three-zone model and the elastic drive+imbibition drive synergistic energy replenishment mechanism;and carry out high-pressure injection to further enhance the mass transfer and diffusion capacity of CO_(2)within the shale pore-fracture system,and compete for the desorption of alkanes to improve the mobilization degree of shale oil.The research achievements provide crucial support for the formation of the theory of continental shale oil development and the construction of the technical system.The future research efforts will focus on mine-scale multi-field coupling physical simulation equipment,microscopic to macroscopic multi-scale experimental methods,pore/fracture fine characterization and post-fracturing core fracture description technologies,multi-media fluid-structure coupling numerical simulation algorithms,and low-cost EOR and low-quality shale oil in-situ upgrading technologies,in order to promote the large-scale and profitable development of shale oil in the Jiyang Depression.展开更多
This paper systematically analyzes the reservoir-forming characteristics and cretaceous shale oil types in four major hydrocarbon-generating sags(Qingxi,Ying'er,Huahai,and Shida)of the Jiuquan Basin,based on the d...This paper systematically analyzes the reservoir-forming characteristics and cretaceous shale oil types in four major hydrocarbon-generating sags(Qingxi,Ying'er,Huahai,and Shida)of the Jiuquan Basin,based on the data of experiments for microscopic and geochemical analysis of reservoirs.The hydrothermal alteration-induced reservoir-forming model and its reservoir-controlling effect in the Qingxi Sag are discussed,and the exploration potential of shale oil in these four sags are evaluated.(1)The Qingxi Sag is widely developed with mud shale,dolomitic shale,and laminated argillaceous dolomite in the Cretaceous,which can be defined as mixed shale as a whole.The source rocks in this area are of good quality and high maturity,formed in a saline water sedimentary environment,and rich in dolomite,with a strong hydrocarbon generation capacity and excellent oil generation conditions.The reservoir space has been significantly modified by hydrothermal process,with well-developed dissolution pores and microfractures,recording favorable reservoir conditions for shale oil enrichment.Overall,this sag has large reservoir thickness and large resource volume,making it the most realistic shale oil exploration target in the Jiuquan Basin.However,it faces challenges such as great burial depth(deeper than 4500 m)and strong tectonic stress.(2)The Ying'er,Huahai,and Shida sags all feature sand-mud interbeds consisting of fan delta front thin sandbodies and lacustrine mud shale in the Cretaceous,having good source rock quality and favorable conditions for interbedded-type shale oil accumulation.The source rocks are insufficient in thermal evolution degree and unevenly distributed,and favorable shale oil resources are mainly endowed near the center of the sags.Reservoirs are primarily composed of siltstone to fine sandstone,suggesting relatively good reservoir conditions,generally with small burial depth(3000-4000 m)and the possibility of local sweet spots.It is noted that the Ying'er Sag has already produced low-mature to mature oil,qualifying it as a near-term realistic shale oil exploration area.展开更多
This paper proposes an approach to determing the optimal cluster spacing for volume fracturing in shale oil reservoirs based on three scales,i.e.microscopic capillary displacement,large-scale core imbibition,and macro...This paper proposes an approach to determing the optimal cluster spacing for volume fracturing in shale oil reservoirs based on three scales,i.e.microscopic capillary displacement,large-scale core imbibition,and macroscopic reservoir nuclear magnetic resonance(NMR)logging.Through flow experiments using capillary with different diameters and lengths,and large-scale core counter-current and dynamic imbibition tests,and combing with the NMR logging data of single wells,a graded optimization criterion for cluster spacing is established.The proposed approach was tested in the shale oil reservoir in the seventh member of the Triassic Yanchang Formation(Change 7 Member),the Ordos Basin.The following findings are obtained.First,in the Chang 7 reservoir,oil in pores smaller than 8μm requires a threshold pressure,and for 2-8μm pores,the movable drainage distance ranges from 0.7 m to 4.6 m under a pressure difference of 27 mPa.Second,the large-scale core imbibition tests show a counter-current imbibition distance of only 10 cm,but a dynamic imbibition distance up to 30 cm.Third,in-situ NMR logging results verified that the post-fracturing matrix drainage radius around fractures is 0-4 m,which is consistent with those of capillary flow experiments and large-scale core imbibition tests.The main pore-size range(2-8μm)of the Chang 7 reservoir corresponds to a permeability interval of(0.1-0.4)×10^(-3)μm^(2).Accordingly,a graded optimization criterion for cluster spacing is proposed as follows:for reservoirs with permeability less than 0.20×10^(-3)μm^(2),the cluster spacing should be reduced to smaller than 4.2 m;for reservoirs with permeability of(0.2-0.4)×10^(-3)μm^(2),the cluster spacing should be designed as 4.2-9.2 m.Field application on a pilot platform,where the cluster spacing was reduced to 4.0-6.0 m,yielded an increased initial oil production by approximately 36.6%over a 100-m horizontal reservoir section as compared with untested similar platforms.展开更多
In the Jimusaer Sag of the Junggar Basin,crude oils from the upper and lower sweet-spot intervals of the Permian Lucaogou Formation display a pronounced“light-heavy reversal”in oil properties that indicates a fundam...In the Jimusaer Sag of the Junggar Basin,crude oils from the upper and lower sweet-spot intervals of the Permian Lucaogou Formation display a pronounced“light-heavy reversal”in oil properties that indicates a fundamental mismatch between oil composition and host rock maturity.To resolve this anomaly,this study integrates geological,geochemical,and petrophysical datasets and systematically evaluates the combined roles of thermal evolution,organofacies,wettability,abnormal overpressure,and migration-related fractionation on shale oil composition.On this basis,a“staged charging-cumulative charging”model is proposed to explain compositional heterogeneity in lacustrine shale oils.The results demonstrate that crude-oil compositions are jointly controlled by the extent of biomarker depletion,the temporal evolution of hydrocarbon charging,and the openness of the source-reservoir system,rather than by thermal maturity or organofacies alone.The upper sweet-spot interval is interpreted to have functioned as a semi-open system during early stages,in which hydrocarbon generation and expulsion were broadly synchronous,leading to preferential loss of early-generated,biomarker-rich heavy components,whereas progressive shale diagenesis at later stages promoted the retention of highly mature,light hydrocarbons.In contrast,the lower sweet-spot interval represents a relatively closed system,where hydrocarbons generated during multiple stages continuously accumulated and were preserved as mixed charges;overprinting by multi-phase fluids progressively weakened sterane isomerization signals,rendering them unreliable indicators of individual charging events or final thermal maturity.This charging behavior provides a reasonable explanation for anomalously low or distorted biomarker parameters observed in intervals of low or similar maturity.Overall,the proposed charging model reconciles the observed reversal in crude-oil properties and,by shifting the interpretive focus from static maturity assessment to charging dynamics,offers a new theoretical basis for understanding lacustrine shale oil accumulation processes,and guiding sweet-spot selection and exploration-development strategies.展开更多
The Liushagang Formation in the Weixinan Depression,Beibu Gulf Basin,southern China,is one of the key stratigraphic units for offshore shale oil exploration in the country.The shale oil reservoirs in the formation are...The Liushagang Formation in the Weixinan Depression,Beibu Gulf Basin,southern China,is one of the key stratigraphic units for offshore shale oil exploration in the country.The shale oil reservoirs in the formation are characterized by low porosity,low permeability and strong heterogeneity,which constrain the precise evaluation of reservoir properties,the accurate prediction of sweet spots,and efficient development.This study integrates core observation,mineralogical analysis,and multi-scale pore characterization to systematically clarify the variations in reservoir properties and their controlling mechanisms.The results show that the physical properties of matrix-type,lamina-type,and interlayer-type reservoirs exhibit distinct stepwise variations:Among these,interlayer-type reservoirs show the greatest development potential(quartz content 65%,average porosity 15%,permeability>10 mD,and mobile fluid saturation 60%),whereas matrix-type reservoirs are the least favorable(dominated by 40 nm nanoscale pores,and clay content 45%).Mineral composition,sedimentary-diagenetic processes,and fault systems collectively control reservoir property heterogeneity.Quartz-rich rigid frameworks resist compaction,resulting in a porosity increase by approximately 2% for every 10%rise in q uartz content.The transformation of clay minerals induces stratified porosity zoning within the layered reservoirs,while fault systems enhance heterogeneity through the development of fracture networks and acid-induced dissolution.This study provides theoretical support for the evaluation and development of shale oil sweet spots in the Weixinan Depression and holds practical significance for the commercial development of shale oil in China's offshore areas.展开更多
To clarify fluid flow mechanisms and establish effective development conditions in continental shale oil reservoirs,a high-temperature,high-pressure steady-state flow system integrated with nuclear magnetic resonance(...To clarify fluid flow mechanisms and establish effective development conditions in continental shale oil reservoirs,a high-temperature,high-pressure steady-state flow system integrated with nuclear magnetic resonance(NMR)technology has been developed.The apparatus combines sample evacuation,rapid pressurization and saturation,and controlled displacement,enabling systematic investigation of single-phase shale oil flow under representative reservoir conditions.Related experiments allow proper quantification of the activation thresholds and relative contributions of different pore types to flow.A movable fluid index(MFI),defined using dual T_(2) cutoff values,is introduced accordingly and linked to key flow parameters.The results reveal distinct multi-scale characteristics of single-phase shale oil transport,namely micro-scale graded displacement and macro-scale segmented nonlinear behavior.As the injection-production pressure difference increases,flow pathways are activated progressively,beginning with fractures,followed by large and then smaller macropores,leading to a pronounced enhancement in apparent permeability.Although mesopores and micropores contribute little to direct flow,their indirect influence becomes increasingly important,and apparent permeability gradually approaches a stable limit at higher pressure difference.It is also shown that the MFI exhibits a strong negative correlation with the starting pressure gradient and a positive correlation with apparent permeability,providing a rapid and reliable indicator of shale oil flow capacity.Samples containing through-going fractures display consistently higher MFI values and superior flowability compared with those dominated by laminated fractures,highlighting the pivotal role of well-connected fracture networks generated by large-scale hydraulic fracturing in improving shale oil production.展开更多
The pore structure of shale oil reservoir significantly affects the occurrence and mobility of hydrocarbons.The potential of a new type of alkaline lake shale oil has been demonstrated,but there are few reports on the...The pore structure of shale oil reservoir significantly affects the occurrence and mobility of hydrocarbons.The potential of a new type of alkaline lake shale oil has been demonstrated,but there are few reports on the pore system of alkaline lake shale,which restricts the efficient exploration and development of shale oil.This study investigates the Fengcheng Formation shale in the Mahu sag of the Junggar Basin,employing methods such as low-temperature nitrogecn adsorption(LTNA),mercury intrusion capillary pressure(MICP),and nuclear magnetic resonance(NMR)to quantitatively characterize the multi-scale pore structure and fractal characteristics of shale,while evaluating the applicability of these methods.Based on a comprehensive analysis of material composition,different pore types,and fractal dimensions,the controlling factors for the development of different pore types and their seepage capacity are discussed.The results indicate that inorganic mineral pores are the main development in alkaline lake shale,with the pore morphology being characterized by slit-like and ink-bottle shapes.The multi-scale pore size distribution(PSD)shows that Ⅱ-micropores(10-100 nm)and mesopores(100–1000 nm)are the main contributors to the pore system.The development of Ⅱ-micropores is associated with feldspar and calcareous minerals,the development of Ⅰ-micropores(<10 nm)and mesopores is related to quartz content,while large pores are mainly found in interlayer fissures of clay minerals.The development of Ⅰ-micropores increases the roughness of pore surface and enhances the adsorption capacity of the pores,while the development of Ⅱ-micropores associated with calcareous minerals hinders pore seepage capacity.Mesopores and macropores(>1000 nm)exhibit good flowability.The high content of siliceous minerals plays a positive role in the pore system of alkaline lake shale.The shale with higher fractal dimension Dmin exhibits greater adsorption capacity,which hinders the accumulation of free-state shale oil.Different types of pore space play different roles in the occurrence of shale oil,with free-state shale oil primarily occurring in micro-fractures and inorganic mineral pores,and the pore size is exceeding 10 nm.展开更多
In-situ heating conversion is the most practical recovery method for lacustrine low-to-medium maturity shale oil.However,the energy output-input ratio must exceed the economic threshold to achieve commercial developme...In-situ heating conversion is the most practical recovery method for lacustrine low-to-medium maturity shale oil.However,the energy output-input ratio must exceed the economic threshold to achieve commercial development.This paper systematically investigates the mechanism of super-rich accumulation of organic matter in continental shale,sweet spot evaluation,optimal heating windows,and appropriate well types and patterns from the perspectives of enhancing energy output and reducing energy input.(1)The super-rich accumulation of organic matter in lacustrine shale is primarily controlled by the intensity,frequency,and preservation of external material inputs,and is related to moderate volcanic and hydrothermal activities,marine transgressions,with total organic carbon content greater than or equal to 6%.(2)The quality of organic-rich intervals is related to the type of source material and hydrocarbon generation potential.The in-situ conversion-derived hydrocarbon quality index(HQI)is established,and the zones exhibiting HQI>450 are defined as sweet spots.(3)Considering the characteristics of the organic matter conversion material field and seepage field,the temperature interval 300-370℃is recommended as the optimal heating window for the Chang 7_(3)sub-member of the Triassic Yanchang Formation in the Ordos Basin.Based on the advantages of thermal conductivity,permeability,and hydrocarbon expulsion efficiency along the bedding direction during in-situ heating,the“horizontal well heating+vertical well development”scheme is proposed,which has demonstrated significant enhancement in both recovery factor and energy output-input ratio,making it the optimal in-situ conversion process.The research findings provide a theoretical and technical foundation for the economical and efficient development of low-to-medium maturity shale oil.展开更多
The global energy demand is increasing rapidly,and it is imperative to develop shale hydrocarbon re-sources vigorously.The prerequisite for enhancing the exploitation efficiency of shale reservoirs is the systematic e...The global energy demand is increasing rapidly,and it is imperative to develop shale hydrocarbon re-sources vigorously.The prerequisite for enhancing the exploitation efficiency of shale reservoirs is the systematic elucidation of the occurrence characteristics,flow behavior,and enhanced oil recovery(EOR)mechanisms of shale oil within commonly developed nanopores.Molecular dynamics(MD)technique can simulate the occurrence,flow,and extraction processes of shale oil at the nanoscale,and then quantitatively characterize various fluid properties,flow characteristics,and action mechanisms under different reservoir conditions by calculating and analyzing a series of MD parameters.However,the existing review on the application of MD simulation in shale oil reservoirs is not systematic enough and lacks a summary of technical challenges and solutions.Therefore,recent MD studies on shale oil res-ervoirs were summarized and analyzed.Firstly,the applicability of force fields and ensembles of MD in shale reservoirs with different reservoir conditions and fluid properties was discussed.Subsequently,the calculation methods and application examples of MD parameters characterizing various properties of fluids at the microscale were summarized.Then,the application of MD simulation in the study of shale oil occurrence characteristics,flow behavior,and EOR mechanisms was reviewed,along with the elucidation of corresponding micro-mechanisms.Moreover,influencing factors of pore structure,wall properties,reservoir conditions,fluid components,injection/production parameters,formation water,and inorganic salt ions were analyzed,and some new conclusions were obtained.Finally,the main challenges associated with the application of MD simulations to shale oil reservoirs were discussed,and reasonable prospects for future MD research directions were proposed.The purpose of this review is to provide theoretical basis and methodological support for applying MD simulation to study shale oil reservoirs.展开更多
Benthic bivalves,the most widely distributed mollusks since the Mesozoic era,often inhabited environments where their fossilized remains are found adjacent to or intermingled with organic-rich shale.Recent Jurassic sh...Benthic bivalves,the most widely distributed mollusks since the Mesozoic era,often inhabited environments where their fossilized remains are found adjacent to or intermingled with organic-rich shale.Recent Jurassic shale oil exploration in the Sichuan Basin has revealed that bioclastic layers,composed of abundant fossil bivalves and closely associated with shales and,exhibit significant hydrocarbon potentials.However,the microscopic structures of these bivalve fossils and their role in hydrocarbon storage and migration remain poorly understood.In this study,we characterized the microporosity of bivalve shells within the Middle-Lower Jurassic bioclastic shale in the northeastern Sichuan Basin using a combination of 2D imaging(thin section,SEM),3D reconstruction(FIB-SEM),and permeability simulation.The micropores within the shell fossils range from 100 to 1000 nm in radius and are uniformly distributed in a grid-like pattern within the shell interior,where they host liquid hydrocarbons.The bioclastic carbonate layers exhibit an overall porosity of approximately 0.8%.Comparative analysis with extant bivalve shells suggests that these micropores represent residual pores from the nacreous brick wall structure.Due to the regular orientation of the shells and their microporous nacres,permeability coefficients along the long bivalve fossil axes are three to five times higher than those along the short axes.These residual micropores within the bioclastic fossil shells have a positive influence on both the storage and migration of shale oil and gas,making bioclastic fossil-bearing shalespromising sweet spots for shale oil and gas exploration in similar sedimentary environments.展开更多
The lamina(combination)types,reservoir characteristics and shale oil occurrence states of organic-rich shale in the Triassic Yanchang Formation Chang 73 sub-member in the Ordos Basin were systematically investigated t...The lamina(combination)types,reservoir characteristics and shale oil occurrence states of organic-rich shale in the Triassic Yanchang Formation Chang 73 sub-member in the Ordos Basin were systematically investigated to reveal the main controlling factors of shale oil occurrence under different lamina combinations.The differential enrichment mechanisms and patterns of shale oil were discussed using the shale oil micro-migration characterization and evaluation methods from the perspectives of relay hydrocarbon supply,stepwise migration,and multi-stage differentiation.The results are obtained in five aspects.First,Chang 73 shale mainly develops five types of lamina combination,i.e.non-laminated shale,sandy laminated shale,tuffaceous laminated shale,mixed laminated shale,and organic-rich laminated shale.Second,shales with different lamina combinations are obviously different in the reservoir space.Specifically,shales with sandy laminae and tuffaceous laminae have a large number of intergranular pores,dissolution pores and hydrocarbon generation-induced fractures.The multi-scale pore and fracture system constitutes the main place for liquid hydrocarbon occurrence.Third,the occurrence and distribution of shale oil in shale with different lamina combinations are jointly controlled by organic matter abundance,reservoir property,thermal evolution degree,mineral composition and laminae scale.The micro-nano-scale pore-fracture networks within shales containing rigid laminae,particularly sandy and tuffaceous laminations,primarily contain free-state light hydrocarbon components.In contrast,adsorption-phase heavy hydrocarbon components predominantly occupy surfaces of organic matter assemblages,clay mineral matrices,and framework mineral particulates.Fourth,there is obvious shale oil micro-migration between shales with different lamina combinations in Chang 73.Generally,such micro-migration is stepwise in a sequence of organic-rich laminated shale→tuffaceous laminated shale→mixed laminated shale→sandy lamiated shale→non-laminated shale.Fifth,the relay hydrocarbon supply of organic matter under the control of the spatial superposition of shales with various laminae,the stepwise migration via multi-scale pore and fracture network,and the multi-differentiation in shales with different lamina combinations under the control of organic-inorganic interactions fundamentally decide the differences of shale oil components between shales with different lamina combinations.展开更多
Fine-grained sediments are widely distributed and constitute the most abundant component in sedi-mentary systems,thus the research on their genesis and distribution is of great significance.In recent years,fine-graine...Fine-grained sediments are widely distributed and constitute the most abundant component in sedi-mentary systems,thus the research on their genesis and distribution is of great significance.In recent years,fine-grained sediment gravity-flows(FGSGF)have been recognized as an important transportation and depositional mechanism for accumulating thick successions of fine-grained sediments.Through a comprehensive review and synthesis of global research on FGSGF deposition,the characteristics,depositional mechanisms,and distribution patterns of fine-grained sediment gravity-flow deposits(FGSGFD)are discussed,and future research prospects are clarified.In addition to the traditionally recognized low-density turbidity current and muddy debris flow,wave-enhanced gravity flow,low-density muddy hyperpycnal flow,and hypopycnal plumes can all form widely distributed FGSGFD.At the same time,the evolution of FGSGF during transportation can result in transitional and hybrid gravity-flow deposits.The combination of multiple triggering mechanisms promotes the widespread develop-ment of FGSGFD,without temporal and spatial limitations.Different types and concentrations of clay minerals,organic matters,and organo-clay complexes are the keys to controlling the flow transformation of FGSGF from low-concentration turbidity currents to high-concentration muddy debris flows.Further study is needed on the interaction mechanism of FGSGF caused by different initiations,the evolution of FGSGF with the effect of organic-inorganic synergy,and the controlling factors of the distribution pat-terns of FGSGFD.The study of FGSGFD can shed some new light on the formation of widely developed thin-bedded siltstones within shales.At the same time,these insights may broaden the exploration scope of shale oil and gas,which have important geological significances for unconventional shale oil and gas.展开更多
Based on the analysis of typical lacustrine shale oil zones in China and their geological characteristics,this study elucidates the fundamental differences between the enrichment patterns of shale oil sweet spots and ...Based on the analysis of typical lacustrine shale oil zones in China and their geological characteristics,this study elucidates the fundamental differences between the enrichment patterns of shale oil sweet spots and conventional oil and gas.The key parameters and evaluation methods for assessing the large-scale production potential of lacustrine shale oil are proposed.The results show that shale oil is a petroleum resource that exists in organic-rich shale formations,in other words,it is preserved in its source bed,following a different process of generation-accumulation-enrichment from conventional oil and gas.Thus,the concept of“reservoir”seems to be inapplicable to shale oil.In China,lacustrine shale oil is distributed widely,but the geological characteristics and sweet spots enrichment patterns of shale oil vary significantly in lacustrine basins where the water environment and the tectonic evolution and diagenetic transformation frameworks are distinct.The core of the evaluation of lacustrine shale oil is“sweet spot volume”.The key factors for evaluating the large-scale production of continental shale oil are the oil storage capacity,oil-bearing capacity and oil producing capacity.The key parameters for evaluating these capacities are total porosity,oil content,and free oil content,respectively.It is recommended to determine the total porosity of shale by combining helium porosity measurement with nuclear magnetic resonance(NMR)method,the oil content of key layers by using organic solvent extraction,NMR method and high pressure mercury intrusion methods,and the free oil content by using NMR fluid distribution secondary spectral stripping decomposition and logging.The research results contribute supplemental insights on continental shale oil deliverability in China,and provide a scientific basis for the rapid exploration and large-scale production of lacustrine shale oil.展开更多
Based on recent advancements in shale oil exploration within the Ordos Basin,this study presents a comprehensive investigation of the paleoenvironment,lithofacies assemblages and distribution,depositional mechanisms,a...Based on recent advancements in shale oil exploration within the Ordos Basin,this study presents a comprehensive investigation of the paleoenvironment,lithofacies assemblages and distribution,depositional mechanisms,and reservoir characteristics of shale oil of fine-grained sediment deposition in continental freshwater lacustrine basins,with a focus on the Chang 7_(3) sub-member of Triassic Yanchang Formation.The research integrates a variety of exploration data,including field outcrops,drilling,logging,core samples,geochemical analyses,and flume simulation.The study indicates that:(1)The paleoenvironment of the Chang 7_(3) deposition is characterized by a warm and humid climate,frequent monsoon events,and a large water depth of freshwater lacustrine basin.The paleogeomorphology exhibits an asymmetrical pattern,with steep slopes in the southwest and gentle slopes in the northeast,which can be subdivided into microgeomorphological units,including depressions and ridges in lakebed,as well as ancient channels.(2)The Chang 7_(3) sub-member is characterized by a diverse array of fine-grained sediments,including very fine sandstone,siltstone,mudstone and tuff.These sediments are primarily distributed in thin interbedded and laminated arrangements vertically.The overall grain size of the sandstone predominantly falls below 62.5μm,with individual layer thicknesses of 0.05–0.64 m.The deposits contain intact plant fragments and display various sedimentary structure,such as wavy bedding,inverse-to-normal grading sequence,and climbing ripple bedding,which indicating a depositional origin associated with density flows.(3)Flume simulation experiments have successfully replicated the transport processes and sedimentary characteristics associated with density flows.The initial phase is characterized by a density-velocity differential,resulting in a thicker,coarser sediment layer at the flow front,while the upper layers are thinner and finer in grain size.During the mid-phase,sliding water effects cause the fluid front to rise and facilitate rapid forward transport.This process generates multiple“new fronts”,enabling the long-distance transport of fine-grained sandstones,such as siltstone and argillaceous siltstone,into the center of the lake basin.(4)A sedimentary model primarily controlled by hyperpynal flows was established for the southwestern part of the basin,highlighting that the frequent occurrence of flood events and the steep slope topography in this area are primary controlling factors for the development of hyperpynal flows.(5)Sandstone and mudstone in the Chang 7_(3) sub-member exhibit micro-and nano-scale pore-throat systems,shale oil is present in various lithologies,while the content of movable oil varies considerably,with sandstone exhibiting the highest content of movable oil.(6)The fine-grained sediment complexes formed by multiple episodes of sandstones and mudstones associated with density flow in the Chang 7_(3) formation exhibit characteristics of“overall oil-bearing with differential storage capacity”.The combination of mudstone with low total organic carbon content(TOC)and siltstone is identified as the most favorable exploration target at present.展开更多
In addition to the organic matter type,abundance,thermal maturity,and shale reservoir space,the preservation conditions of source rocks play a key factor in affecting the quantity and quality of retained hydrocarbons ...In addition to the organic matter type,abundance,thermal maturity,and shale reservoir space,the preservation conditions of source rocks play a key factor in affecting the quantity and quality of retained hydrocarbons in source rocks of lacustrine shale,yet this aspect has received little attention.This paper,based on the case analysis,explores how preservation conditions influence the enrichment of mobile hydrocarbons in shale oil.Research showns that good preservation conditions play three key roles.(1)Ensure the retention of sufficient light hydrocarbons(C_(1)–C_(13)),medium hydrocarbons(C_(14)–C_(25))and small molecular aromatics(including 1–2 benzene rings)in the formation,which enhances the fluidity and flow of shale oil;(2)Maintain a high energy field(abnormally high pressure),thus facilitating the maximum outflow of shale oil;(3)Ensure that the retained hydrocarbons have the miscible flow condition of multi-component hydrocarbons(light hydrocarbons,medium hydrocarbons,heavy hydrocarbons,and heteroatomic compounds),so that the heavy hydrocarbons(C_(25+))and heavy components(non-hydrocarbons and asphaltenes)have improved fluidity and maximum flow capacity.In conclusion,in addition to the advantages of organic matter type,abundance,thermal maturity,and reservoir space,good preservation conditions of shale layers are essential for the formation of economically viable shale oil reservoirs,which should be incorporated into the evaluation criteria of shale oil-rich areas/segments and considered a necessary factor when selecting favorable exploration targets.展开更多
Medium-high maturity continental shale oil is one of the hydrocarbon resources with the most potential for successful development in China.Nevertheless,the unique geological conditions of a multi-lithologic superposit...Medium-high maturity continental shale oil is one of the hydrocarbon resources with the most potential for successful development in China.Nevertheless,the unique geological conditions of a multi-lithologic superposition shield the vertical propagation of hydraulic fractures and limit the longitudinal reconstruction in reservoirs,posing a great challenge for large-scale volumetric fracturing.Radial wellbore crosslayer fracturing,which transforms the interaction between the hydraulic fractures and lithologic interface into longitudinal multilayer competitive initiation,could provide a potential solution for this engineering challenge.To determine the longitudinal propagation behaviors of fractures guided by radial wellbores,true triaxial fracturing experiments were performed on multilayer shale-sandstone samples,with a focus on the injection pressure response,fracture morphology,and cross-layer pattern.The effects of the radial borehole length L,vertical stress difference K_(v),injection rate Q,and viscosity m of the fracturing fluid were analyzed.The results indicate that radial wellbores can greatly facilitate fracture initiation and cross-layer propagation.Unlike conventional hydraulic fracturing,there are two distinct fracture propagation patterns in radial wellbore fracturing:cross-layering and skip-layering.The fracture height guided by a radial wellbore is positively correlated with K_(v),Q,and m.Increasing these parameters causes a shift in the fracture initiation from a single root to an asynchronous root/toe end and can improve the cross-layer propagation capacity.Critical parameter thresholds exist for fracture propagation through and across interlayers under the guidance of radial boreholes.A parameter combination of critical cross-layering/skip-layering or alternating displacement/viscosity is recommended to simultaneously improve the fracture height and degree of lateral activation.The degree of correlation of different parameters with the vertical fracture height can be written as L>Q/m>K_(v).Increasing the radial wellbore length can effectively facilitate fracture cross-/skip-layer propagation and reduce the critical threshold of injection parameters,which is conducive to maximizing the stimulated reservoir volume.展开更多
Key technologies that make productivity increase are revealed through analyzing the best practices and production data in major shale basins of North America.Trends of the key technologies and optimization designs for...Key technologies that make productivity increase are revealed through analyzing the best practices and production data in major shale basins of North America.Trends of the key technologies and optimization designs for shale oil and gas development are summarized and analyzed based on drilling and completion operations and well data.These technologies mainly include:(1)Optimizing well design and hydraulic fracturing design,including reducing cluster spacing,increasing proppant and fracturing fluid volumes,optimizing horizontal well lateral length and fracture stage length.The most effective method is to reduce cluster spacing to an optimized length.The second most effective method is to optimally increase proppant volumes.(2)Placing horizontal wells in the sweet spots and drilling the wells parallel or close to the minimum horizontal stress direction.(3)Using cube development with optimized well spacing to maximize resource recovery and reduce well interferences.Plus,in-situ stress impacts on hydraulic fracture propagation and hydrocarbon production are addressed.Determination of formation breakdown pressure is studied by considering the impacts of in-situ stresses,drilling and perforation directions.Whether or not the hydraulic fracturing can generate orthogonal fracture networks is also discussed.The key technologies and optimization design parameters proposed in this paper can be applied to guide new well placement,drilling and completion designs,and hydraulic fracture operations to increase productivity.展开更多
The brittleness index(BI)is crucial for predicting engineering sweet spots and designing fracturing operations in shale oil reservoir exploration and development.Seismic amplitude variation with offset(AVO)inversion i...The brittleness index(BI)is crucial for predicting engineering sweet spots and designing fracturing operations in shale oil reservoir exploration and development.Seismic amplitude variation with offset(AVO)inversion is commonly used to obtain the BI.Traditionally,velocity,density,and other parameters are firstly inverted,and the BI is then calculated,which often leads to accumulated errors.Moreover,due to the limited of well-log data in field work areas,AVO inversion typically faces the challenge of limited information,resulting in not high accuracy of BI derived by existing AVO inversion methods.To address these issues,we first derive an AVO forward approximation equation that directly characterizes the BI in P-wave reflection coefficients.Based on this,an intelligent AVO inversion method,which combines the advantages of traditional and intelligent approaches,for directly obtaining the BI is proposed.A TransUnet model is constructed to establish the strong nonlinear mapping relationship between seismic data and the BI.By incorporating a combined objective function that is constrained by both low-frequency parameters and training samples,the challenge of limited samples is effectively addressed,and the direct inversion of the BI is stably achieved.Tests on model data and applications on field data demonstrate the feasibility,advancement,and practicality of the proposed method.展开更多
Characterizing the petrophysical properties holds significant importance in shale oil reservoirs.Twodimensional(2-D)nuclear magnetic resonance(NMR),a nondestructive and noninvasive technique,has numerous applications ...Characterizing the petrophysical properties holds significant importance in shale oil reservoirs.Twodimensional(2-D)nuclear magnetic resonance(NMR),a nondestructive and noninvasive technique,has numerous applications in petrophysical characterization.However,the complex occurrence states of the fluids and the highly non-uniform distributions of minerals and organic matter pose challenges in the NMR-based petrophysical characterization.A novel T_(1)-T_(2)relaxation theory is introduced for the first time in this study.The transverse and longitudinal relaxivities of pore fluids are determined based on numerical investigation and experimental analysis.Additionally,an improved random walk algorithm is proposed to,on the basis of digital shale core,simulate the effects of the hydrogen index(HI)for the organic matter,echo spacing(T_(E)),pyrite content,clay mineral type,and clay content on T_(1)-T_(2)spectra at different NMR frequencies.Furthermore,the frequency conversion cross-plots for various petrophysical parameters influenced by the above factors are established.This study provides new insights into NMRbased petrophysical characterization and the frequency conversion of petrophysical parameters measured by laboratory NMR instruments and NMR logging in shale oil reservoirs.It is of great significance for the efficient exploration and environmentally friendly production of shale oil.展开更多
Mobility is a crucial metric for assessing sweet spots of continental shale oil.However,due to the complexity of shale oil reservoirs characteristics and the lack of systematic analyses of factors influencing mobility...Mobility is a crucial metric for assessing sweet spots of continental shale oil.However,due to the complexity of shale oil reservoirs characteristics and the lack of systematic analyses of factors influencing mobility,the difference in shale oil mobility under multiple lithofacies control remains unclear,causing significant challenges for mobility evaluation and sweet spot prediction.This study examines continental shales of the Fengcheng Formation in the Mahu Sag,employing scanning electron microscopy(SEM),nitrogen adsorption(NA),nuclear magnetic resonance(NMR),spontaneous imbibition(SI),and contact angle measurements(CAM)to investigate the pore structure,connectivity,and wettability properties of different lithofacies shale.Quantitative analyses of shale movable oil content and saturation were conducted using multistep temperature pyrolysis(MTP)and NMR centrifugation techniques.Furthermore,the influence of reservoir characteristics,geochemical characteristics,and lamination development on shale oil mobility were discussed.Results indicate that larger pore diameter,higher imbibition slopes,and lower fractal dimensions of movable fluid pores(D2)correspond to higher movable oil saturation.Organic matter exerts a dual effect on shale movable oil content.When the TOC is below a threshold,the movable oil content gradually increases with TOC.Laminations exhibit favorable reservoir properties and light oil enrichment,enhancing shale oil mobility.Massive siltstone(MS)develops interconnected intergranular pores with the best pore structure and connectivity,the lowest D2 values,and the highest shale oil mobility.Laminated felsic shale(LFS)and laminated calcareous shale(LCS)exhibit moderate mobility,where the development of microfractures enhances fluid flow by connecting isolated pores into pore-fracture networks.In contrast,massive felsic shale(MFS)and bedded felsic shale(BFS)primarily develop intragranular dissolution pores with more complex structures and poorer connectivity,resulting in weaker mobility.A more accurate approach for assessing shale oil mobility has been presented,taking into account both total oil content and movable oil saturation.More importantly,this study establishes a comprehensive conceptual model illustrating the potential relationships among shale lithofacies,reservoir characteristics,and movable oil flow space in the study area.This research not only provides a systematic approach for assessing shale oil mobility but also deepens the understanding of flow mechanisms of continental shale oil,offering theoretical guidance for optimizing sweet spots in the Fengcheng Formation shale oil reservoirs of the Mahu Sag.展开更多
基金Supported by the China National Science and Technology Major Project(2024ZD1405103)SINOPEC Science and Technology Major Project(P25184)。
文摘Centering on the critical bottlenecks in the development of shale oil in the Jiyang Depression of Shengli Oilfield,key scientific and engineering issues are proposed in aspects such as the storage space and occurrence state of shale oil,the formation mechanisms of multi-scale flow spaces,the mobilization mechanisms of crude oil in pores and fractures,and the enhanced oil recovery(EOR)mechanisms during the late stage of elastic development.The research progress and mechanistic insights in recent years are reviewed with respect to experimental techniques,characteristics of pore-fracture structure and fluid occurrence,fracture evolution mechanisms,shale oil flow mechanisms and EOR techniques.Through improving the experimental methods,optimizing the testing conditions,and developing new technologies,we deeply understand the occurrence state,storage space and flow pattern of shale oil,and reveal the distribution pattern of“oil-bearing in all pore sizes and oil-rich in large pores”and the differences in fluid phase states under the confinement effect of nano-scale pores in the shales of the Jiyang Depression;depict the characteristics of“restricted vertical expansion and complex fracture networks”of induced fractures and the dynamic evolution of fracture networks during the fracturing-soaking-production process;establish a“easy flow-slow flow-stagnant flow”three-zone model and the elastic drive+imbibition drive synergistic energy replenishment mechanism;and carry out high-pressure injection to further enhance the mass transfer and diffusion capacity of CO_(2)within the shale pore-fracture system,and compete for the desorption of alkanes to improve the mobilization degree of shale oil.The research achievements provide crucial support for the formation of the theory of continental shale oil development and the construction of the technical system.The future research efforts will focus on mine-scale multi-field coupling physical simulation equipment,microscopic to macroscopic multi-scale experimental methods,pore/fracture fine characterization and post-fracturing core fracture description technologies,multi-media fluid-structure coupling numerical simulation algorithms,and low-cost EOR and low-quality shale oil in-situ upgrading technologies,in order to promote the large-scale and profitable development of shale oil in the Jiyang Depression.
基金Supported by the CNPC Science and Technology Project(2023YQX10110)。
文摘This paper systematically analyzes the reservoir-forming characteristics and cretaceous shale oil types in four major hydrocarbon-generating sags(Qingxi,Ying'er,Huahai,and Shida)of the Jiuquan Basin,based on the data of experiments for microscopic and geochemical analysis of reservoirs.The hydrothermal alteration-induced reservoir-forming model and its reservoir-controlling effect in the Qingxi Sag are discussed,and the exploration potential of shale oil in these four sags are evaluated.(1)The Qingxi Sag is widely developed with mud shale,dolomitic shale,and laminated argillaceous dolomite in the Cretaceous,which can be defined as mixed shale as a whole.The source rocks in this area are of good quality and high maturity,formed in a saline water sedimentary environment,and rich in dolomite,with a strong hydrocarbon generation capacity and excellent oil generation conditions.The reservoir space has been significantly modified by hydrothermal process,with well-developed dissolution pores and microfractures,recording favorable reservoir conditions for shale oil enrichment.Overall,this sag has large reservoir thickness and large resource volume,making it the most realistic shale oil exploration target in the Jiuquan Basin.However,it faces challenges such as great burial depth(deeper than 4500 m)and strong tectonic stress.(2)The Ying'er,Huahai,and Shida sags all feature sand-mud interbeds consisting of fan delta front thin sandbodies and lacustrine mud shale in the Cretaceous,having good source rock quality and favorable conditions for interbedded-type shale oil accumulation.The source rocks are insufficient in thermal evolution degree and unevenly distributed,and favorable shale oil resources are mainly endowed near the center of the sags.Reservoirs are primarily composed of siltstone to fine sandstone,suggesting relatively good reservoir conditions,generally with small burial depth(3000-4000 m)and the possibility of local sweet spots.It is noted that the Ying'er Sag has already produced low-mature to mature oil,qualifying it as a near-term realistic shale oil exploration area.
基金Supported by the China National Oil and Gas Major Project(2025ZD1404800)PetroChina Science and Technology Major Project(2023ZZ15YJ03)CNPC Changqing Oilfield Company Major Special Project(2023DZZ04)。
文摘This paper proposes an approach to determing the optimal cluster spacing for volume fracturing in shale oil reservoirs based on three scales,i.e.microscopic capillary displacement,large-scale core imbibition,and macroscopic reservoir nuclear magnetic resonance(NMR)logging.Through flow experiments using capillary with different diameters and lengths,and large-scale core counter-current and dynamic imbibition tests,and combing with the NMR logging data of single wells,a graded optimization criterion for cluster spacing is established.The proposed approach was tested in the shale oil reservoir in the seventh member of the Triassic Yanchang Formation(Change 7 Member),the Ordos Basin.The following findings are obtained.First,in the Chang 7 reservoir,oil in pores smaller than 8μm requires a threshold pressure,and for 2-8μm pores,the movable drainage distance ranges from 0.7 m to 4.6 m under a pressure difference of 27 mPa.Second,the large-scale core imbibition tests show a counter-current imbibition distance of only 10 cm,but a dynamic imbibition distance up to 30 cm.Third,in-situ NMR logging results verified that the post-fracturing matrix drainage radius around fractures is 0-4 m,which is consistent with those of capillary flow experiments and large-scale core imbibition tests.The main pore-size range(2-8μm)of the Chang 7 reservoir corresponds to a permeability interval of(0.1-0.4)×10^(-3)μm^(2).Accordingly,a graded optimization criterion for cluster spacing is proposed as follows:for reservoirs with permeability less than 0.20×10^(-3)μm^(2),the cluster spacing should be reduced to smaller than 4.2 m;for reservoirs with permeability of(0.2-0.4)×10^(-3)μm^(2),the cluster spacing should be designed as 4.2-9.2 m.Field application on a pilot platform,where the cluster spacing was reduced to 4.0-6.0 m,yielded an increased initial oil production by approximately 36.6%over a 100-m horizontal reservoir section as compared with untested similar platforms.
基金Supported by the National Natural Science Foundation of China(42173030,42302161,42473034)State Science and Technology Major Project for New Oil and Gas Exploration and Development,Ministry of Science and Technology(2025ZD1400803)。
文摘In the Jimusaer Sag of the Junggar Basin,crude oils from the upper and lower sweet-spot intervals of the Permian Lucaogou Formation display a pronounced“light-heavy reversal”in oil properties that indicates a fundamental mismatch between oil composition and host rock maturity.To resolve this anomaly,this study integrates geological,geochemical,and petrophysical datasets and systematically evaluates the combined roles of thermal evolution,organofacies,wettability,abnormal overpressure,and migration-related fractionation on shale oil composition.On this basis,a“staged charging-cumulative charging”model is proposed to explain compositional heterogeneity in lacustrine shale oils.The results demonstrate that crude-oil compositions are jointly controlled by the extent of biomarker depletion,the temporal evolution of hydrocarbon charging,and the openness of the source-reservoir system,rather than by thermal maturity or organofacies alone.The upper sweet-spot interval is interpreted to have functioned as a semi-open system during early stages,in which hydrocarbon generation and expulsion were broadly synchronous,leading to preferential loss of early-generated,biomarker-rich heavy components,whereas progressive shale diagenesis at later stages promoted the retention of highly mature,light hydrocarbons.In contrast,the lower sweet-spot interval represents a relatively closed system,where hydrocarbons generated during multiple stages continuously accumulated and were preserved as mixed charges;overprinting by multi-phase fluids progressively weakened sterane isomerization signals,rendering them unreliable indicators of individual charging events or final thermal maturity.This charging behavior provides a reasonable explanation for anomalously low or distorted biomarker parameters observed in intervals of low or similar maturity.Overall,the proposed charging model reconciles the observed reversal in crude-oil properties and,by shifting the interpretive focus from static maturity assessment to charging dynamics,offers a new theoretical basis for understanding lacustrine shale oil accumulation processes,and guiding sweet-spot selection and exploration-development strategies.
基金jointly supported by the National Natural Science Foundation of China(42474156)the Technical Service Project of China Oilfield Services Limited(YJB23YF001)。
文摘The Liushagang Formation in the Weixinan Depression,Beibu Gulf Basin,southern China,is one of the key stratigraphic units for offshore shale oil exploration in the country.The shale oil reservoirs in the formation are characterized by low porosity,low permeability and strong heterogeneity,which constrain the precise evaluation of reservoir properties,the accurate prediction of sweet spots,and efficient development.This study integrates core observation,mineralogical analysis,and multi-scale pore characterization to systematically clarify the variations in reservoir properties and their controlling mechanisms.The results show that the physical properties of matrix-type,lamina-type,and interlayer-type reservoirs exhibit distinct stepwise variations:Among these,interlayer-type reservoirs show the greatest development potential(quartz content 65%,average porosity 15%,permeability>10 mD,and mobile fluid saturation 60%),whereas matrix-type reservoirs are the least favorable(dominated by 40 nm nanoscale pores,and clay content 45%).Mineral composition,sedimentary-diagenetic processes,and fault systems collectively control reservoir property heterogeneity.Quartz-rich rigid frameworks resist compaction,resulting in a porosity increase by approximately 2% for every 10%rise in q uartz content.The transformation of clay minerals induces stratified porosity zoning within the layered reservoirs,while fault systems enhance heterogeneity through the development of fracture networks and acid-induced dissolution.This study provides theoretical support for the evaluation and development of shale oil sweet spots in the Weixinan Depression and holds practical significance for the commercial development of shale oil in China's offshore areas.
基金supported by the National Science and Technology Major Project of China(Grant No.2024ZD 1004302)the Key Scientific and Technological Research project of SINOPEC(Grant No.P25186).
文摘To clarify fluid flow mechanisms and establish effective development conditions in continental shale oil reservoirs,a high-temperature,high-pressure steady-state flow system integrated with nuclear magnetic resonance(NMR)technology has been developed.The apparatus combines sample evacuation,rapid pressurization and saturation,and controlled displacement,enabling systematic investigation of single-phase shale oil flow under representative reservoir conditions.Related experiments allow proper quantification of the activation thresholds and relative contributions of different pore types to flow.A movable fluid index(MFI),defined using dual T_(2) cutoff values,is introduced accordingly and linked to key flow parameters.The results reveal distinct multi-scale characteristics of single-phase shale oil transport,namely micro-scale graded displacement and macro-scale segmented nonlinear behavior.As the injection-production pressure difference increases,flow pathways are activated progressively,beginning with fractures,followed by large and then smaller macropores,leading to a pronounced enhancement in apparent permeability.Although mesopores and micropores contribute little to direct flow,their indirect influence becomes increasingly important,and apparent permeability gradually approaches a stable limit at higher pressure difference.It is also shown that the MFI exhibits a strong negative correlation with the starting pressure gradient and a positive correlation with apparent permeability,providing a rapid and reliable indicator of shale oil flow capacity.Samples containing through-going fractures display consistently higher MFI values and superior flowability compared with those dominated by laminated fractures,highlighting the pivotal role of well-connected fracture networks generated by large-scale hydraulic fracturing in improving shale oil production.
基金financially supported by the National Natural Science Foundation of China(Nos.42272137,42202160)AAPG Foundation Grants-in-Aid and the Strategic Cooperation Technology Projecti of CNPC and CUPB(No.ZLZX2020-01-05)。
文摘The pore structure of shale oil reservoir significantly affects the occurrence and mobility of hydrocarbons.The potential of a new type of alkaline lake shale oil has been demonstrated,but there are few reports on the pore system of alkaline lake shale,which restricts the efficient exploration and development of shale oil.This study investigates the Fengcheng Formation shale in the Mahu sag of the Junggar Basin,employing methods such as low-temperature nitrogecn adsorption(LTNA),mercury intrusion capillary pressure(MICP),and nuclear magnetic resonance(NMR)to quantitatively characterize the multi-scale pore structure and fractal characteristics of shale,while evaluating the applicability of these methods.Based on a comprehensive analysis of material composition,different pore types,and fractal dimensions,the controlling factors for the development of different pore types and their seepage capacity are discussed.The results indicate that inorganic mineral pores are the main development in alkaline lake shale,with the pore morphology being characterized by slit-like and ink-bottle shapes.The multi-scale pore size distribution(PSD)shows that Ⅱ-micropores(10-100 nm)and mesopores(100–1000 nm)are the main contributors to the pore system.The development of Ⅱ-micropores is associated with feldspar and calcareous minerals,the development of Ⅰ-micropores(<10 nm)and mesopores is related to quartz content,while large pores are mainly found in interlayer fissures of clay minerals.The development of Ⅰ-micropores increases the roughness of pore surface and enhances the adsorption capacity of the pores,while the development of Ⅱ-micropores associated with calcareous minerals hinders pore seepage capacity.Mesopores and macropores(>1000 nm)exhibit good flowability.The high content of siliceous minerals plays a positive role in the pore system of alkaline lake shale.The shale with higher fractal dimension Dmin exhibits greater adsorption capacity,which hinders the accumulation of free-state shale oil.Different types of pore space play different roles in the occurrence of shale oil,with free-state shale oil primarily occurring in micro-fractures and inorganic mineral pores,and the pore size is exceeding 10 nm.
基金Supported by the National Natural Science Foundation of China Enterprise Innovation and Development Joint Fund Project(U22B6004)National Natural Science Foundation of China and Youth Science Fund Project(4250021468)CNPC Changqing Oilfield Company Key Core Technology Research Project(KJZX2023-01)。
文摘In-situ heating conversion is the most practical recovery method for lacustrine low-to-medium maturity shale oil.However,the energy output-input ratio must exceed the economic threshold to achieve commercial development.This paper systematically investigates the mechanism of super-rich accumulation of organic matter in continental shale,sweet spot evaluation,optimal heating windows,and appropriate well types and patterns from the perspectives of enhancing energy output and reducing energy input.(1)The super-rich accumulation of organic matter in lacustrine shale is primarily controlled by the intensity,frequency,and preservation of external material inputs,and is related to moderate volcanic and hydrothermal activities,marine transgressions,with total organic carbon content greater than or equal to 6%.(2)The quality of organic-rich intervals is related to the type of source material and hydrocarbon generation potential.The in-situ conversion-derived hydrocarbon quality index(HQI)is established,and the zones exhibiting HQI>450 are defined as sweet spots.(3)Considering the characteristics of the organic matter conversion material field and seepage field,the temperature interval 300-370℃is recommended as the optimal heating window for the Chang 7_(3)sub-member of the Triassic Yanchang Formation in the Ordos Basin.Based on the advantages of thermal conductivity,permeability,and hydrocarbon expulsion efficiency along the bedding direction during in-situ heating,the“horizontal well heating+vertical well development”scheme is proposed,which has demonstrated significant enhancement in both recovery factor and energy output-input ratio,making it the optimal in-situ conversion process.The research findings provide a theoretical and technical foundation for the economical and efficient development of low-to-medium maturity shale oil.
基金supported by the National Natural Science Foundation of China(52304021,52104022,52204031)the Natural Science Foundation of Sichuan Province(2022NSFSC0205,2024NSFSC0201,2023NSFSC0947)the National Science and Technology Major Projects of China(2017ZX05049006-010).
文摘The global energy demand is increasing rapidly,and it is imperative to develop shale hydrocarbon re-sources vigorously.The prerequisite for enhancing the exploitation efficiency of shale reservoirs is the systematic elucidation of the occurrence characteristics,flow behavior,and enhanced oil recovery(EOR)mechanisms of shale oil within commonly developed nanopores.Molecular dynamics(MD)technique can simulate the occurrence,flow,and extraction processes of shale oil at the nanoscale,and then quantitatively characterize various fluid properties,flow characteristics,and action mechanisms under different reservoir conditions by calculating and analyzing a series of MD parameters.However,the existing review on the application of MD simulation in shale oil reservoirs is not systematic enough and lacks a summary of technical challenges and solutions.Therefore,recent MD studies on shale oil res-ervoirs were summarized and analyzed.Firstly,the applicability of force fields and ensembles of MD in shale reservoirs with different reservoir conditions and fluid properties was discussed.Subsequently,the calculation methods and application examples of MD parameters characterizing various properties of fluids at the microscale were summarized.Then,the application of MD simulation in the study of shale oil occurrence characteristics,flow behavior,and EOR mechanisms was reviewed,along with the elucidation of corresponding micro-mechanisms.Moreover,influencing factors of pore structure,wall properties,reservoir conditions,fluid components,injection/production parameters,formation water,and inorganic salt ions were analyzed,and some new conclusions were obtained.Finally,the main challenges associated with the application of MD simulations to shale oil reservoirs were discussed,and reasonable prospects for future MD research directions were proposed.The purpose of this review is to provide theoretical basis and methodological support for applying MD simulation to study shale oil reservoirs.
基金supported by the National Natural Science Foundation of China(No.42173030)the Open Project from the Key Laboratory of Shale Gas Exploration,Ministry of Natural Resources(KLSGE-202406).
文摘Benthic bivalves,the most widely distributed mollusks since the Mesozoic era,often inhabited environments where their fossilized remains are found adjacent to or intermingled with organic-rich shale.Recent Jurassic shale oil exploration in the Sichuan Basin has revealed that bioclastic layers,composed of abundant fossil bivalves and closely associated with shales and,exhibit significant hydrocarbon potentials.However,the microscopic structures of these bivalve fossils and their role in hydrocarbon storage and migration remain poorly understood.In this study,we characterized the microporosity of bivalve shells within the Middle-Lower Jurassic bioclastic shale in the northeastern Sichuan Basin using a combination of 2D imaging(thin section,SEM),3D reconstruction(FIB-SEM),and permeability simulation.The micropores within the shell fossils range from 100 to 1000 nm in radius and are uniformly distributed in a grid-like pattern within the shell interior,where they host liquid hydrocarbons.The bioclastic carbonate layers exhibit an overall porosity of approximately 0.8%.Comparative analysis with extant bivalve shells suggests that these micropores represent residual pores from the nacreous brick wall structure.Due to the regular orientation of the shells and their microporous nacres,permeability coefficients along the long bivalve fossil axes are three to five times higher than those along the short axes.These residual micropores within the bioclastic fossil shells have a positive influence on both the storage and migration of shale oil and gas,making bioclastic fossil-bearing shalespromising sweet spots for shale oil and gas exploration in similar sedimentary environments.
基金Supported by the National Natural Science Foundation of China(42302184)Innovation Group Project of Basic Research in Gansu Province,China(22JR5RA045)。
文摘The lamina(combination)types,reservoir characteristics and shale oil occurrence states of organic-rich shale in the Triassic Yanchang Formation Chang 73 sub-member in the Ordos Basin were systematically investigated to reveal the main controlling factors of shale oil occurrence under different lamina combinations.The differential enrichment mechanisms and patterns of shale oil were discussed using the shale oil micro-migration characterization and evaluation methods from the perspectives of relay hydrocarbon supply,stepwise migration,and multi-stage differentiation.The results are obtained in five aspects.First,Chang 73 shale mainly develops five types of lamina combination,i.e.non-laminated shale,sandy laminated shale,tuffaceous laminated shale,mixed laminated shale,and organic-rich laminated shale.Second,shales with different lamina combinations are obviously different in the reservoir space.Specifically,shales with sandy laminae and tuffaceous laminae have a large number of intergranular pores,dissolution pores and hydrocarbon generation-induced fractures.The multi-scale pore and fracture system constitutes the main place for liquid hydrocarbon occurrence.Third,the occurrence and distribution of shale oil in shale with different lamina combinations are jointly controlled by organic matter abundance,reservoir property,thermal evolution degree,mineral composition and laminae scale.The micro-nano-scale pore-fracture networks within shales containing rigid laminae,particularly sandy and tuffaceous laminations,primarily contain free-state light hydrocarbon components.In contrast,adsorption-phase heavy hydrocarbon components predominantly occupy surfaces of organic matter assemblages,clay mineral matrices,and framework mineral particulates.Fourth,there is obvious shale oil micro-migration between shales with different lamina combinations in Chang 73.Generally,such micro-migration is stepwise in a sequence of organic-rich laminated shale→tuffaceous laminated shale→mixed laminated shale→sandy lamiated shale→non-laminated shale.Fifth,the relay hydrocarbon supply of organic matter under the control of the spatial superposition of shales with various laminae,the stepwise migration via multi-scale pore and fracture network,and the multi-differentiation in shales with different lamina combinations under the control of organic-inorganic interactions fundamentally decide the differences of shale oil components between shales with different lamina combinations.
基金supported by National Natural Science Foundation of China(Grant Nos.42072126,42372139)the Natural Science Foundation of Sichuan Province(Grant Nos.2022NSFSC0990).
文摘Fine-grained sediments are widely distributed and constitute the most abundant component in sedi-mentary systems,thus the research on their genesis and distribution is of great significance.In recent years,fine-grained sediment gravity-flows(FGSGF)have been recognized as an important transportation and depositional mechanism for accumulating thick successions of fine-grained sediments.Through a comprehensive review and synthesis of global research on FGSGF deposition,the characteristics,depositional mechanisms,and distribution patterns of fine-grained sediment gravity-flow deposits(FGSGFD)are discussed,and future research prospects are clarified.In addition to the traditionally recognized low-density turbidity current and muddy debris flow,wave-enhanced gravity flow,low-density muddy hyperpycnal flow,and hypopycnal plumes can all form widely distributed FGSGFD.At the same time,the evolution of FGSGF during transportation can result in transitional and hybrid gravity-flow deposits.The combination of multiple triggering mechanisms promotes the widespread develop-ment of FGSGFD,without temporal and spatial limitations.Different types and concentrations of clay minerals,organic matters,and organo-clay complexes are the keys to controlling the flow transformation of FGSGF from low-concentration turbidity currents to high-concentration muddy debris flows.Further study is needed on the interaction mechanism of FGSGF caused by different initiations,the evolution of FGSGF with the effect of organic-inorganic synergy,and the controlling factors of the distribution pat-terns of FGSGFD.The study of FGSGFD can shed some new light on the formation of widely developed thin-bedded siltstones within shales.At the same time,these insights may broaden the exploration scope of shale oil and gas,which have important geological significances for unconventional shale oil and gas.
基金Supported by the National Key R&D Program of China(2024YFE0114000)Science and Technology Project of China National Petroleum Corporation(2024DJ8702).
文摘Based on the analysis of typical lacustrine shale oil zones in China and their geological characteristics,this study elucidates the fundamental differences between the enrichment patterns of shale oil sweet spots and conventional oil and gas.The key parameters and evaluation methods for assessing the large-scale production potential of lacustrine shale oil are proposed.The results show that shale oil is a petroleum resource that exists in organic-rich shale formations,in other words,it is preserved in its source bed,following a different process of generation-accumulation-enrichment from conventional oil and gas.Thus,the concept of“reservoir”seems to be inapplicable to shale oil.In China,lacustrine shale oil is distributed widely,but the geological characteristics and sweet spots enrichment patterns of shale oil vary significantly in lacustrine basins where the water environment and the tectonic evolution and diagenetic transformation frameworks are distinct.The core of the evaluation of lacustrine shale oil is“sweet spot volume”.The key factors for evaluating the large-scale production of continental shale oil are the oil storage capacity,oil-bearing capacity and oil producing capacity.The key parameters for evaluating these capacities are total porosity,oil content,and free oil content,respectively.It is recommended to determine the total porosity of shale by combining helium porosity measurement with nuclear magnetic resonance(NMR)method,the oil content of key layers by using organic solvent extraction,NMR method and high pressure mercury intrusion methods,and the free oil content by using NMR fluid distribution secondary spectral stripping decomposition and logging.The research results contribute supplemental insights on continental shale oil deliverability in China,and provide a scientific basis for the rapid exploration and large-scale production of lacustrine shale oil.
基金Supported by the CNPC Major Science and Technology Project(2021DJ1806).
文摘Based on recent advancements in shale oil exploration within the Ordos Basin,this study presents a comprehensive investigation of the paleoenvironment,lithofacies assemblages and distribution,depositional mechanisms,and reservoir characteristics of shale oil of fine-grained sediment deposition in continental freshwater lacustrine basins,with a focus on the Chang 7_(3) sub-member of Triassic Yanchang Formation.The research integrates a variety of exploration data,including field outcrops,drilling,logging,core samples,geochemical analyses,and flume simulation.The study indicates that:(1)The paleoenvironment of the Chang 7_(3) deposition is characterized by a warm and humid climate,frequent monsoon events,and a large water depth of freshwater lacustrine basin.The paleogeomorphology exhibits an asymmetrical pattern,with steep slopes in the southwest and gentle slopes in the northeast,which can be subdivided into microgeomorphological units,including depressions and ridges in lakebed,as well as ancient channels.(2)The Chang 7_(3) sub-member is characterized by a diverse array of fine-grained sediments,including very fine sandstone,siltstone,mudstone and tuff.These sediments are primarily distributed in thin interbedded and laminated arrangements vertically.The overall grain size of the sandstone predominantly falls below 62.5μm,with individual layer thicknesses of 0.05–0.64 m.The deposits contain intact plant fragments and display various sedimentary structure,such as wavy bedding,inverse-to-normal grading sequence,and climbing ripple bedding,which indicating a depositional origin associated with density flows.(3)Flume simulation experiments have successfully replicated the transport processes and sedimentary characteristics associated with density flows.The initial phase is characterized by a density-velocity differential,resulting in a thicker,coarser sediment layer at the flow front,while the upper layers are thinner and finer in grain size.During the mid-phase,sliding water effects cause the fluid front to rise and facilitate rapid forward transport.This process generates multiple“new fronts”,enabling the long-distance transport of fine-grained sandstones,such as siltstone and argillaceous siltstone,into the center of the lake basin.(4)A sedimentary model primarily controlled by hyperpynal flows was established for the southwestern part of the basin,highlighting that the frequent occurrence of flood events and the steep slope topography in this area are primary controlling factors for the development of hyperpynal flows.(5)Sandstone and mudstone in the Chang 7_(3) sub-member exhibit micro-and nano-scale pore-throat systems,shale oil is present in various lithologies,while the content of movable oil varies considerably,with sandstone exhibiting the highest content of movable oil.(6)The fine-grained sediment complexes formed by multiple episodes of sandstones and mudstones associated with density flow in the Chang 7_(3) formation exhibit characteristics of“overall oil-bearing with differential storage capacity”.The combination of mudstone with low total organic carbon content(TOC)and siltstone is identified as the most favorable exploration target at present.
基金Supported by the National Natural Science Foundation of China(U22B6004)Project of PetroChina Research Institute of Petroleum Exploration and Development(2022yjcq03)Core Technology Key Project of China Petroleum Changqing Oilfield Company(KJZX2023-01).
文摘In addition to the organic matter type,abundance,thermal maturity,and shale reservoir space,the preservation conditions of source rocks play a key factor in affecting the quantity and quality of retained hydrocarbons in source rocks of lacustrine shale,yet this aspect has received little attention.This paper,based on the case analysis,explores how preservation conditions influence the enrichment of mobile hydrocarbons in shale oil.Research showns that good preservation conditions play three key roles.(1)Ensure the retention of sufficient light hydrocarbons(C_(1)–C_(13)),medium hydrocarbons(C_(14)–C_(25))and small molecular aromatics(including 1–2 benzene rings)in the formation,which enhances the fluidity and flow of shale oil;(2)Maintain a high energy field(abnormally high pressure),thus facilitating the maximum outflow of shale oil;(3)Ensure that the retained hydrocarbons have the miscible flow condition of multi-component hydrocarbons(light hydrocarbons,medium hydrocarbons,heavy hydrocarbons,and heteroatomic compounds),so that the heavy hydrocarbons(C_(25+))and heavy components(non-hydrocarbons and asphaltenes)have improved fluidity and maximum flow capacity.In conclusion,in addition to the advantages of organic matter type,abundance,thermal maturity,and reservoir space,good preservation conditions of shale layers are essential for the formation of economically viable shale oil reservoirs,which should be incorporated into the evaluation criteria of shale oil-rich areas/segments and considered a necessary factor when selecting favorable exploration targets.
基金supported by the National Natural Science Foun-dation of China(52421002,U24B6001,52204019,and 52192624)the Open Foundation of the Shanxi Key Laboratory of Carbon Dioxide Sequestration and Enhanced Oil Recovery。
文摘Medium-high maturity continental shale oil is one of the hydrocarbon resources with the most potential for successful development in China.Nevertheless,the unique geological conditions of a multi-lithologic superposition shield the vertical propagation of hydraulic fractures and limit the longitudinal reconstruction in reservoirs,posing a great challenge for large-scale volumetric fracturing.Radial wellbore crosslayer fracturing,which transforms the interaction between the hydraulic fractures and lithologic interface into longitudinal multilayer competitive initiation,could provide a potential solution for this engineering challenge.To determine the longitudinal propagation behaviors of fractures guided by radial wellbores,true triaxial fracturing experiments were performed on multilayer shale-sandstone samples,with a focus on the injection pressure response,fracture morphology,and cross-layer pattern.The effects of the radial borehole length L,vertical stress difference K_(v),injection rate Q,and viscosity m of the fracturing fluid were analyzed.The results indicate that radial wellbores can greatly facilitate fracture initiation and cross-layer propagation.Unlike conventional hydraulic fracturing,there are two distinct fracture propagation patterns in radial wellbore fracturing:cross-layering and skip-layering.The fracture height guided by a radial wellbore is positively correlated with K_(v),Q,and m.Increasing these parameters causes a shift in the fracture initiation from a single root to an asynchronous root/toe end and can improve the cross-layer propagation capacity.Critical parameter thresholds exist for fracture propagation through and across interlayers under the guidance of radial boreholes.A parameter combination of critical cross-layering/skip-layering or alternating displacement/viscosity is recommended to simultaneously improve the fracture height and degree of lateral activation.The degree of correlation of different parameters with the vertical fracture height can be written as L>Q/m>K_(v).Increasing the radial wellbore length can effectively facilitate fracture cross-/skip-layer propagation and reduce the critical threshold of injection parameters,which is conducive to maximizing the stimulated reservoir volume.
文摘Key technologies that make productivity increase are revealed through analyzing the best practices and production data in major shale basins of North America.Trends of the key technologies and optimization designs for shale oil and gas development are summarized and analyzed based on drilling and completion operations and well data.These technologies mainly include:(1)Optimizing well design and hydraulic fracturing design,including reducing cluster spacing,increasing proppant and fracturing fluid volumes,optimizing horizontal well lateral length and fracture stage length.The most effective method is to reduce cluster spacing to an optimized length.The second most effective method is to optimally increase proppant volumes.(2)Placing horizontal wells in the sweet spots and drilling the wells parallel or close to the minimum horizontal stress direction.(3)Using cube development with optimized well spacing to maximize resource recovery and reduce well interferences.Plus,in-situ stress impacts on hydraulic fracture propagation and hydrocarbon production are addressed.Determination of formation breakdown pressure is studied by considering the impacts of in-situ stresses,drilling and perforation directions.Whether or not the hydraulic fracturing can generate orthogonal fracture networks is also discussed.The key technologies and optimization design parameters proposed in this paper can be applied to guide new well placement,drilling and completion designs,and hydraulic fracture operations to increase productivity.
基金supposed by the National Nature Science Foundation of China(Grant No.42304131)the Natural Science Foundation of Heilongjiang Province(Grant No.LH2023D012)+1 种基金the Heilongjiang Postdoctoral Fund(Grant No.LBH-Z22092)the Basic Research Fund for Universities in Xinjiang Uygur Autonomous Region(Grant No.XJEDU2023P166)。
文摘The brittleness index(BI)is crucial for predicting engineering sweet spots and designing fracturing operations in shale oil reservoir exploration and development.Seismic amplitude variation with offset(AVO)inversion is commonly used to obtain the BI.Traditionally,velocity,density,and other parameters are firstly inverted,and the BI is then calculated,which often leads to accumulated errors.Moreover,due to the limited of well-log data in field work areas,AVO inversion typically faces the challenge of limited information,resulting in not high accuracy of BI derived by existing AVO inversion methods.To address these issues,we first derive an AVO forward approximation equation that directly characterizes the BI in P-wave reflection coefficients.Based on this,an intelligent AVO inversion method,which combines the advantages of traditional and intelligent approaches,for directly obtaining the BI is proposed.A TransUnet model is constructed to establish the strong nonlinear mapping relationship between seismic data and the BI.By incorporating a combined objective function that is constrained by both low-frequency parameters and training samples,the challenge of limited samples is effectively addressed,and the direct inversion of the BI is stably achieved.Tests on model data and applications on field data demonstrate the feasibility,advancement,and practicality of the proposed method.
基金funded by the National Natural Science Foundation of China(42174131).
文摘Characterizing the petrophysical properties holds significant importance in shale oil reservoirs.Twodimensional(2-D)nuclear magnetic resonance(NMR),a nondestructive and noninvasive technique,has numerous applications in petrophysical characterization.However,the complex occurrence states of the fluids and the highly non-uniform distributions of minerals and organic matter pose challenges in the NMR-based petrophysical characterization.A novel T_(1)-T_(2)relaxation theory is introduced for the first time in this study.The transverse and longitudinal relaxivities of pore fluids are determined based on numerical investigation and experimental analysis.Additionally,an improved random walk algorithm is proposed to,on the basis of digital shale core,simulate the effects of the hydrogen index(HI)for the organic matter,echo spacing(T_(E)),pyrite content,clay mineral type,and clay content on T_(1)-T_(2)spectra at different NMR frequencies.Furthermore,the frequency conversion cross-plots for various petrophysical parameters influenced by the above factors are established.This study provides new insights into NMRbased petrophysical characterization and the frequency conversion of petrophysical parameters measured by laboratory NMR instruments and NMR logging in shale oil reservoirs.It is of great significance for the efficient exploration and environmentally friendly production of shale oil.
基金supported by National Natural Science Foundation of China(42272137 and U24A20592)the Strategic Cooperation Technology Projecti of CNPC and CUPB(ZLZX2020-01-05)AAPG Foundation Grants-in-Aid.
文摘Mobility is a crucial metric for assessing sweet spots of continental shale oil.However,due to the complexity of shale oil reservoirs characteristics and the lack of systematic analyses of factors influencing mobility,the difference in shale oil mobility under multiple lithofacies control remains unclear,causing significant challenges for mobility evaluation and sweet spot prediction.This study examines continental shales of the Fengcheng Formation in the Mahu Sag,employing scanning electron microscopy(SEM),nitrogen adsorption(NA),nuclear magnetic resonance(NMR),spontaneous imbibition(SI),and contact angle measurements(CAM)to investigate the pore structure,connectivity,and wettability properties of different lithofacies shale.Quantitative analyses of shale movable oil content and saturation were conducted using multistep temperature pyrolysis(MTP)and NMR centrifugation techniques.Furthermore,the influence of reservoir characteristics,geochemical characteristics,and lamination development on shale oil mobility were discussed.Results indicate that larger pore diameter,higher imbibition slopes,and lower fractal dimensions of movable fluid pores(D2)correspond to higher movable oil saturation.Organic matter exerts a dual effect on shale movable oil content.When the TOC is below a threshold,the movable oil content gradually increases with TOC.Laminations exhibit favorable reservoir properties and light oil enrichment,enhancing shale oil mobility.Massive siltstone(MS)develops interconnected intergranular pores with the best pore structure and connectivity,the lowest D2 values,and the highest shale oil mobility.Laminated felsic shale(LFS)and laminated calcareous shale(LCS)exhibit moderate mobility,where the development of microfractures enhances fluid flow by connecting isolated pores into pore-fracture networks.In contrast,massive felsic shale(MFS)and bedded felsic shale(BFS)primarily develop intragranular dissolution pores with more complex structures and poorer connectivity,resulting in weaker mobility.A more accurate approach for assessing shale oil mobility has been presented,taking into account both total oil content and movable oil saturation.More importantly,this study establishes a comprehensive conceptual model illustrating the potential relationships among shale lithofacies,reservoir characteristics,and movable oil flow space in the study area.This research not only provides a systematic approach for assessing shale oil mobility but also deepens the understanding of flow mechanisms of continental shale oil,offering theoretical guidance for optimizing sweet spots in the Fengcheng Formation shale oil reservoirs of the Mahu Sag.
