Based on the experimental results of casting thin section,low temperature nitrogen adsorption,high pressure mercury injection,nuclear magnetic resonance T2 spectrum,contact angle and oil-water interfacial tension,the ...Based on the experimental results of casting thin section,low temperature nitrogen adsorption,high pressure mercury injection,nuclear magnetic resonance T2 spectrum,contact angle and oil-water interfacial tension,the relationship between pore throat structure and crude oil mobility characteristics of full particle sequence reservoirs in the Lower Permian Fengcheng Formation of Mahu Sag,Junggar Basin,are revealed.(1)With the decrease of reservoir particle size,the volume of pores connected by large throats and the volume of large pores show a decreasing trend,and the distribution and peak ranges of throat and pore radius shift to smaller size in an orderly manner.The upper limits of throat radius,porosity and permeability of unconventional reservoirs in Fengcheng Formation are approximately 0.7μm,8%and 0.1×10^(−3)μm^(2),respectively.(2)As the reservoir particle size decreases,the distribution and peak ranges of pores hosting retained oil and movable oil are shifted to a smaller size in an orderly manner.With the increase of driving pressure,the amount of retained and movable oil of the larger particle reservoir samples shows a more obvious trend of decreasing and increasing,respectively.(3)With the increase of throat radius,the driving pressure of reservoir with different particle levels presents three stages,namely rapid decrease,slow decrease and stabilization.The oil driving pressures of various reservoirs and the differences of them decrease with the increase of temperature and obviously decrease with the increase of throat radius.According to the above experimental analysis,it is concluded that the deep shale oil of Fengcheng Formation in Mahu Sag has great potential for production under geological conditions.展开更多
China's lacustrine shale oil reserves are abundant,making it a key area for future exploration and development.Most lacustrine shales feature a mix of mineral compositions and interlayer sedimentary structures.Hig...China's lacustrine shale oil reserves are abundant,making it a key area for future exploration and development.Most lacustrine shales feature a mix of mineral compositions and interlayer sedimentary structures.High-quality reservoirs exhibit significant heterogeneity,which influences the stress distribution during fracturing,leading to complex fracture network patterns.This complexity presents challenges for the comprehensive well logging evaluation of the geological-engineering"double sweet spots"in shale oil,severely restricting efficient development.This study focuses on the impact of shale sedimentary layering on the radial slowness of dipole shear waves.It employs rock physics experiments combined with advanced well logging techniques to explore the relationship between reservoir anisotropy caused by sedimentary layering and reservoir quality,thereby establishing a logging evaluation method for vertical identification of"sweet spots"in lacustrine shale oil.The shales in the Fengcheng Formation of the Mahu Sag into three types according to sedimentary structure scale:laminated,interlayer,and massive.Each type has different mineral compositions,affecting reservoir quality and fracturing potential.Laminated shales develop more fractures under stress along the beddings,showing moderate anisotropy,with reservoir capacity dependent on intercrystalline porosity within carbonate layers.Interlayer shales easily form complex fracture networks,exhibiting significant anisotropy,and their reservoir capacity depends on the porosity within sandy bands.Massive mudstones have the fewest fractures under stress,appearing isotropic with reservoir capacity dependent on matrix pore size.The intensity of reservoir anisotropy correlates positively with storage capacity and the propensity to form irregular and complex fracture networks during hydraulic fracturing.In sections without natural fractures,a larger difference between fast and slow shear waves corresponds to a radial profile shift towards warm tones,indicating stronger anisotropy and better reservoir quality,thus forming complex fracture networks during fracturing.Conversely,a smaller difference leads to a profile energy shift towards cooler tones,indicating stronger isotropy and poorer reservoir quality,hindering the formation of complex fracture networks during hydraulic fracturing.In sections with natural fractures,the difference between fast and slow shear waves exhibits erratic behavior,showing a cross-pattern in radial profiles,indicating strong anisotropy.The presence of natural fractures can synergize with induced fracture networks to form more complex systems,significantly enhancing reservoir productivity.展开更多
The Fengcheng Formation is a crucial source rock and the primary reservoir for oil accumulation in the Mahu Sag.Crude oils are distributed throughout the Fengcheng Formation,ranging from the edge to the interior of th...The Fengcheng Formation is a crucial source rock and the primary reservoir for oil accumulation in the Mahu Sag.Crude oils are distributed throughout the Fengcheng Formation,ranging from the edge to the interior of the sag in the southern Mahu Sag.These crude oils originate from in-situ source rocks in shallowly buried areas and the inner deep sag.During migration,the crude oil from the inner deep sag affects the source rocks close to carrier beds,leading to changes in the organic geochemical characteristics of the source rocks.These changes might alter source rock evaluations and oil-source correlation.Based on data such as total organic carbon(TOC),Rock-Eval pyrolysis of source rocks,and gas chromatography-mass spectrometry(GC-MS)of the saturated fraction,and considering the geological characteristics of the study area,we define the identification characteristics of source rock affected by migrated hydrocarbons and establish the various patterns of influence that migrated hydrocarbons have on the source rock of the Fengcheng Formation in the southern Mahu Sag.The source rocks of the Fengcheng Formation are mostly fair to good,containing mainly Type II organic matter and being thermally mature enough to generate oil.Source rocks affected by migrated hydrocarbons exhibit relatively high hydrocarbon contents(S1/TOC>110 mg HC/g TOC,Extract/TOC>30%,HC:hydrocarbon),relatively low Rock-Eval Tmax values,and relatively high tricyclic terpane contents with a descending and mountain-shaped distribution.Furthermore,biomarker composition parameters indicate a higher thermal maturity than in-situ source rocks.Through a comparison of the extract biomarker fingerprints of adjacent reservoirs and mudstones in different boreholes,three types of influence patterns of migrated hydrocarbons are identified:the edge-influence of thin sandstone-thick mudstone,the mixed-influence of sandstone-mudstone interbedded,and the full-influence of thick sandstone-thin mudstone.This finding reminds us that the influence of migrated hydrocarbons must be considered when evaluating source rocks and conducting oil-source correlation.展开更多
Based on core and thin section data,the source rock samples from the Fengcheng Formation in the Mahu Sag of the Junggar Basin were analyzed in terms of zircon SIMS U-Pb geochronology,organic carbon isotopic compositio...Based on core and thin section data,the source rock samples from the Fengcheng Formation in the Mahu Sag of the Junggar Basin were analyzed in terms of zircon SIMS U-Pb geochronology,organic carbon isotopic composition,major and trace element contents,as well as petrology.Two zircon U-Pb ages of(306.0±5.2)Ma and(303.5±3.7)Ma were obtained from the first member of the Fengcheng Formation.Combined with carbon isotopic stratigraphy,it is inferred that the depositional age of the Fengcheng Formation is about 297-306 Ma,spanning the Carboniferous-Permian boundary and corresponding to the interglacial period between C4 and P1 glacial events.Multiple increases in Hg/TOC ratios and altered volcanic ash were found in the shale rocks of the Fengcheng Formation,indicating that multiple phases of volcanic activity occurred during its deposition.An interval with a high B/Ga ratio was found in the middle of the second member of the Fengcheng Formation,associated with the occurrence of evaporite minerals and reedmergnerite,indicating that the high salinity of the water mass was related to hydrothermal activity.Comprehensive analysis suggests that the warm and humid climate during the deposition of Fengcheng Formation is conducive to the growth of organic matter such as algae and bacteria in the lake,and accelerates the continental weathering,driving the input of nutrients.Volcanic activities supply a large amount of nutrients and stimulate primary productivity.The warm climate and high salinity are conducive to water stratification,leading to water anoxia that benefits organic matter preservation.The above factors interact and jointly control the enrichment of organic matter in the Fengcheng Formation of Mahu Sag.展开更多
The organic-rich mudstones and dolostones of the Permian Fengcheng Formation(Fm.)are typically alkaline lacustrine source rocks,which are typified by impressively abundantβ-carotane.Abundant β-carotane has been well...The organic-rich mudstones and dolostones of the Permian Fengcheng Formation(Fm.)are typically alkaline lacustrine source rocks,which are typified by impressively abundantβ-carotane.Abundant β-carotane has been well acknowledged as an effective indicator of biological sources or depositional environments.However,the specific biological sources of β-carotane and the coupling control of biological sources and environmental factors on the enrichment of β-carotane in the Fengcheng Fm.remains obscure.Based on a comprehensive investigation of the bulk,molecular geochemistry,and organic petrology of sedimentary rocks and the biochemistry of phytoplankton in modern alkaline lakes,we proposed a new understanding of the biological precursors of β-carotane and elucidated the enrichment mechanism of β-carotane in the Fengcheng Fm.The results show that the biological precursors crucially control the enrichment of β-carotane in the Fengcheng Fm.The haloalkaliphilic cyanobacteria are the primary biological sources of β-carotane,which is suggested by a good positive correlation between the 2-methylhopane index,7-+8-methyl heptadecanes/C_(max),C_(29%),and β-carotane/C_(max)in sedimentary rocks and the predominance of cyanobacteria with abundantβ-carotene in modern alkaline lakes.The enrichment of β-carotane requires the reducing condition,and the paleoredox state that affects the enrichment of β-carotane appears to have a threshold.The paleoclimate conditions do not considerably impact the enrichment of β-carotane,but they have some influence on the water's paleosalinity by affecting evaporation and precipitation.While it does not directly affect the enrichment of β-carotane in the Fengcheng Fm.,paleosalinity does have an impact on the cyanobacterial precursor supply and the preservation conditions.展开更多
Based on the organic geochemical data and the molecular and stable carbon isotopic compositions of natural gas of the Lower Permian Fengcheng Formation in the western Central Depression of Junggar Basin,combined with ...Based on the organic geochemical data and the molecular and stable carbon isotopic compositions of natural gas of the Lower Permian Fengcheng Formation in the western Central Depression of Junggar Basin,combined with sedimentary environment analysis and hydrocarbon-generating simulation,the gas-generating potential of the Fengcheng source rock is evaluated,the distribution of large-scale effective source kitchen is described,the genetic types of natural gas are clarified,and four types of favorable exploration targets are selected.The results show that:(1)The Fengcheng Formation is a set of oil-prone source rocks,and the retained liquid hydrocarbon is conducive to late cracking into gas,with characteristics of high gas-generating potential and late accumulation;(2)The maximum thickness of Fengcheng source rock reaches 900 m.The source rock has entered the main gas-generating stage in Penyijingxi and Shawan sags,and the area with gas-generating intensity greater than 20×10^(8) m^(3)/km^(2) is approximately 6500 km^(2).(3)Around the western Central Depression,highly mature oil-type gas with light carbon isotope composition was identified to be derived from the Fengcheng source rocks mainly,while the rest was coal-derived gas from the Carboniferous source rock;(4)Four types of favorable exploration targets with exploration potential were developed in the western Central Depression which are structural traps neighboring to the source,stratigraphic traps neighboring to the source,shale-gas type within the source,and structural traps within the source.Great attention should be paid to these targets.展开更多
The dolomitic rocks of the Fengcheng Formation are considered to be formed under special geologic conditions, and are significant hydrocarbon reservoir rocks in the Wu-Xia area in the Junggar Basin. Analyses of petrol...The dolomitic rocks of the Fengcheng Formation are considered to be formed under special geologic conditions, and are significant hydrocarbon reservoir rocks in the Wu-Xia area in the Junggar Basin. Analyses of petrologic characteristics and stable isotope composition indicate that the dolomitizing host rock is volcanic and the dolomitizing fluids probably consists of brine from shore- shallow lakes with great evaporation and salinity in the Fengcheng Formation, which have formed under arid climatic conditions, as well as residual Mg-rich seawater from the underlying Jiamuhe Formation and Carboniferous. Dust tuff in the area has significant plagioclase content. Anorthite and labradorite hydrolysis by CO2 can be coupled with calcite precipitation. Late Mg-rich brine percolated and replaced calcite formed in the early time, which lead to precipitate dolostones with different occurrences, such as graniphyric, random bedded or lumpy. The diagenetic dolostones with different occurrences resulting from particular formation conditions occurred in different tectonic settings. The dolomitizing fluid has been driven by the thermal convection flow generated by volcanic eruptions. At the same time, the overthrusts of the Wu-Xia growth fault have speeded up the flow of deep Mg-rich water upwards, and induced the water to quickly penetrate and horizontally migrate in the strata. Fracture is the major and the most important reservoir space in dolomitic reservoir of the Fengcheng Formation. Fracture and fault plays a decisive role in controlling the formation of dolostone and the distribution of favorable reservoirs. The deliverability of oil and gas is determined by the development and match relations of dissolved pores and fractures to a certain degree.展开更多
Shales in the Carboniferous–Permian Fengcheng(FC) and Lucaogou(LCG) formations in Junggar Basin are important organic rich rocks containing significant oil resources.To evaluate the difference in sedimentary environm...Shales in the Carboniferous–Permian Fengcheng(FC) and Lucaogou(LCG) formations in Junggar Basin are important organic rich rocks containing significant oil resources.To evaluate the difference in sedimentary environment conditions and hydrocarbon-generating potential between the FC and LCG formations.Total organic carbon(TOC),Rock-Eval pyrolysis,solvent extraction,column fractionation,stable carbon isotope,gas chromatography-mass spectrometry(GC-MS) of saturated hydrocarbons and organic petrology from the source rocks of FC and LCG formations.were analyzed.The biomarker composition indicates that during the deposition of FC,LCG-1 to LCG-2,the sedimentary environment for the source rock formations changed with gradual decrease of salinity,from anoxic to dyoxic/suboxic in redox conditions,and from strong stratification to weakened stratification of water.The FC Formation source rock,with main telalginite(planktonic green algae),archaebacteria and minor terrestrial organic matter,deposited in the environment characterized by high salinity and strongly reducing condition.Its TOC content is relatively low with a high original hydrocarbongenerating potential of unit organic material.The LCG Formation source rock deposited in the environment with low salinity and large variations,the organic matter is mainly sourced from telalginite(planktonic green algae),lamalginite,bacteria and higher plants,resulting in strong heterogeneity of the source rock.The abundance of TOC is high,but the original hydrocarbon generation potential of unit organic matter is lower than that of FC Formation.The results provide a geochemical basis for further study of saline-brackish water sedimentary environment shales in the Junggar Basin.展开更多
Based on the combination of core observation,experimental analysis and testingand geological analysis,the main controlling factors of shale oil enrichment in the Lower Permian Fengcheng Formation in the Mahu Sag of th...Based on the combination of core observation,experimental analysis and testingand geological analysis,the main controlling factors of shale oil enrichment in the Lower Permian Fengcheng Formation in the Mahu Sag of the Junggar Basin are clarified,and a shale oil enrichment model is established.The results show that the enrichment of shale oil in the Fengcheng Formation in the Mahu Sag is controlled by the organic abundance,organic type,reservoir capacity and the amount of migration hydrocarbon in shale.The abundance of organic matter provides the material basis for shale oil enrichment,and the shales containing typesⅠandⅡorganic matters have good oil content.The reservoir capacity controls shale oil enrichment.Macropores are the main space for shale oil enrichment in the Fengcheng Formation,and pore size and fracture scale directly control the degree of shale oil enrichment.The migration of hydrocarbons in shale affects shale oil enrichment.The shale that has expelled hydrocarbons has poor oil content,while the shale that has received hydrocarbons migrated from other strata has good oil content.Lithofacies reflect the hydrocarbon generation and storage capacity comprehensively.The laminated felsic shale,laminated lime-dolomitic shale and thick-layered felsic shale have good oil content,and they are favorable lithofacies for shale oil enrichment.Under the control of these factors,relative migration of hydrocarbons occurred within the Fengcheng shale,which leads to the the difference in the enrichment process of shale oil.Accordingly,the enrichment mode of shale oil in Fengcheng Formation is established as"in-situ enrichment"and"migration enrichment".By superimposing favorable lithofacies and main controlling factors of enrichment,the sweet spot of shale oil in the Fengcheng Formation can be selected which has great significance for the exploration and development of shale oil.展开更多
By using the latest geological,seismic,drilling and logging data,this article studies the basic conditions for the formation of the total petroleum system and the orderly coexisting characteristics and accumulation mo...By using the latest geological,seismic,drilling and logging data,this article studies the basic conditions for the formation of the total petroleum system and the orderly coexisting characteristics and accumulation models of conventional&unconventional reservoirs in the Lower Permian Fengcheng Formation in the Junggar Basin.Controlled by thermal evolution,hydrocarbon generation and expulsion process of the high-quality source rocks in alkaline lake as well as the characteristics of multi-type reservoirs(conglomerate,sandstone,dolomite and shale),conventional structure-lithologic reservoirs and tight oil and shale oil reservoirs controlled by source-reservoir structure have been formed.On the plane,mature conventional reservoirs,medium-high mature tight oil,and medium-high mature shale oil reservoirs coexist orderly from the slope area around Mahu sag to the sag.Based on the orderly coexisting characteristics of conventional and unconventional reservoirs in the Fengcheng Formation,it is clear that oil and gas in the Fengcheng Formation accumulate continuously over a large area in three accumulation models:integrated source-reservoir,source-reservoir in close contact,and separated source-reservoir model.The three accumulation models differ in relationship between source-reservoir structure,reservoir lithology and spatial distribution,hydrocarbon migration,oil and gas type.It is pointed out that the conventional&unconventional oil and gas should be explored and developed as a whole to achieve an overall breakthrough of the total petroleum system.This study is expected to enrich the geological theory of oil and gas enrichment in continental basins and to provide an analogy for exploration and research in other hydrocarbon-rich sags.展开更多
Alkaline-lacustrine deposition and its evolution model in Permian Fengcheng Formation at the Mahu sag, Junggar Basin were investigated through core and thin-section observation, geochemical and elemental analysis, log...Alkaline-lacustrine deposition and its evolution model in Permian Fengcheng Formation at the Mahu sag, Junggar Basin were investigated through core and thin-section observation, geochemical and elemental analysis, logging response and lithofacies identification. Six lithofacies are developed in the Fengcheng Formation. The Feng 2 Member(P1 f2) is dominated by lithofacies with alkaline minerals, while the upper part of the Feng 1 Member(P1 f1) and the lower part of the Feng 3 Member(P1 f3) are primarily organic-rich mudstones that are interbedded with dolomite and dolomitic rock. Paleoenvironment evolution of Fengcheng Formation can be divided into 5 stages, which was controlled by volcanic activity and paleoclimate. The first stage(the early phase of P1 f1) was characterized by intensive volcanic activity and arid climate, developing pyroclastics and sedimentary volcaniclastic rocks. The secondary stage(the later phase of P1 f1) had weak volcanic activity and humid climate that contributed to the development of organic-rich mudstone, forming primary source rock in the Fengcheng Formation. The increasing arid climate at the third stage(the early phase of P1 f2) resulted in shrinking of lake basin and increasing of salinity, giving rise to dolomite and dolomitic rocks. The continuous aird climate, low lake level and high salinity at the fourth stage(the later phase of P1 f2) generated special alkaline minerals, e.g., trona, indicating the formation of alkaline-lacustrine. The humid climate made lake level rise and desalted lake water, therefore, the fifth stage(P1 f3) dominated by the deposition of terrigenous clastic rocks and dolomitic rocks.展开更多
The tight reservoirs of the Fengcheng Formation at the southern margin of the Mahu Sag have strong heterogeneity due to the diversity in their pore types, sizes, and structures. The microscopic characteristics of tigh...The tight reservoirs of the Fengcheng Formation at the southern margin of the Mahu Sag have strong heterogeneity due to the diversity in their pore types, sizes, and structures. The microscopic characteristics of tight reservoirs and the mechanisms that generate them are of significance in identifying the distribution of high-quality reservoirs and in improving the prediction accuracy of sweet spots in tight oil reservoirs. In this paper, high-pressure mercury intrusion (HPMI) and nuclear magnetic resonance (NMR) experiments were carried out on samples from the tight reservoirs in the study area. These experimental results were combined with cluster analysis, fractal theory, and microscopic observations to qualitatively and quantitatively evaluate pore types, sizes, and structures. A classification scheme was established that divides the reservoir into four types, based on the microstructure characteristics of samples, and the genetic mechanisms that aided the development of reservoir microstructure were analyzed. The results show that the lower limit for the tight reservoir in the Fengcheng Formation is Φ of 3.5% and K of 0.03 mD. The pore throat size and distribution span gradually decrease from Type I, through Type II and Type III reservoirs to non-reservoirs, and the pore type also evolves from dominantly intergranular pores to intercrystalline pores. The structural trend shows a decrease in the ball-stick pore-throat system and an increase in the branch-like pore-throat system. The dual effects of sedimentation and diagenesis shape the microscopic characteristics of pores and throats. The sorting, roundness, and particle size of the original sediments determine the original physical properties of the reservoir. The diagenetic environment of ‘two alkalinity stages and one acidity stage’ influenced the evolution of pore type and size. Although the cementation of authigenic minerals in the early alkaline environment adversely affected reservoir properties, it also alleviated the damage of the later compaction to some extent. Dissolution in the mid-term acidic environment greatly improved the physical properties of this tight reservoir, making dissolution pores an important reservoir space. The late alkaline environment occurred after large-scale oil and gas accumulation. During this period, the cementation of authigenic minerals had a limited effect on the reservoir space occupied by crude oil. It had a more significant impact on the sand bodies not filled with oil, making them function as barriers.展开更多
Origin of authigenic dolomites in the dolomitic reservoir of the Permian Fengcheng Formation in the Mahu Sag of Junggar Basin is unclear.Occurrence and genetic evolution of the authigenic dolomites in dolomitic rock r...Origin of authigenic dolomites in the dolomitic reservoir of the Permian Fengcheng Formation in the Mahu Sag of Junggar Basin is unclear.Occurrence and genetic evolution of the authigenic dolomites in dolomitic rock reservoir of the Fengcheng Formation in the Mahu Sag were analyzed by polarized and fluorescence thin sections,scanning electron microscope(SEM),electron microprobe(EMP),C,O and Sr isotopes analysis,and other techniques.(1)Dolomites were mainly precipitated in three stages:penecontemporaneous-shallow burial stage(early stage of the Middle Permian),middle burial stage(middle stage of the Middle Permian),and middle-deep burial stage,with the former two stages in dominance.(2)Dolomitization fluid was high-salinity brine originating from alkaline lake.In the penecontemporaneous-shallow burial stage,Mg^(2+)was mainly supplied by alkaline-lake fluid and devitrification of volcanic glass.In the middle burial stage,Mg^(2+)mainly came from the transformation of clay minerals,devitrification of volcanic glass and dissolution of aluminosilicates such as feldspar.(3)Regular changes of Mg,Mn,Fe,Sr,Si and other elements during the growth of dolomite were mainly related to the alkaline-lake fluid,and to different influences of devitrification and diagenetic alteration of volcanic materials during the burial.(4)In the penecontemporaneous stage,induced by alkaline-lake microorganisms,the micritic-microcrystalline dolomites were formed by primary precipitation,replacement of aragonite and high-Mg calcite,and other processes;in the shallow burial stage,the silt-sized dolomites were formed by continuous growth of micritic-microcrystalline dolomite and replacement of calcites,tuffs and other substances;in the middle burial stage,the dolomites,mainly silt-and fine-sized,were formed by replacement of volcanic materials.The research results are referential for investigating the formation mechanism and distribution patterns of tight dolomitic reservoirs in the Mahu Sag and other similar oil and gas bearing areas.展开更多
Dolomites and eruptive rocks are well-developed in the Permian Fengcheng Formation in Junggar Basin in China, in which oil and gas are accumulated extensively. Until now, high-yield industrial oil and gas flows have b...Dolomites and eruptive rocks are well-developed in the Permian Fengcheng Formation in Junggar Basin in China, in which oil and gas are accumulated extensively. Until now, high-yield industrial oil and gas flows have been obtained in the dolomitic tuff of the second unit of the Fengcheng Formation, which demonstrates the huge exploration potential of the thick layer of massive dolomitic tuff. The lithology of the second unit of the Fengcheng Formation in this area has gradually transformed from the dolomite, dolomitic tuff to siltstone from east to west. Moreover, the well testing shows that the reservoir is oil-saturated, and the production rate mainly depends on the reservoir’s physical properties and fracture development. In this study, different types of data including core data, well log and seismic data are used cooperatively to characterize the sedimentary, structure and fracture features of the Fengcheng Formation, and then characterize the promising target zone in the study area. The result indicates that hydrocarbons are most accumulated along the deep fault in the Wu-Xia fault zone, which will be the favorable zone for the next progressive exploration.展开更多
The late Paleozoic Fengcheng Formation shale(LPF shale)in the Junggar Basin,NW China,is the oldest alkaline source rock discovered in the world,providing a unique perspective with which to explore organic matter(OM)en...The late Paleozoic Fengcheng Formation shale(LPF shale)in the Junggar Basin,NW China,is the oldest alkaline source rock discovered in the world,providing a unique perspective with which to explore organic matter(OM)enrichment in alkaline lake environments.Combined with the organic carbon isotope profile and paleoenvironmental proxies,this study reveals that the LPF shale was deposited in an arid climate with high salinity and a strong reducing environment,accompanied by frequent volcanic activity.High TOC values are concentrated in two intervals with frequent fluctuations in OM types.A negative excursion due to changes in sedimentary OM source is found in theδ^(13)C_(org)profile.The excursion corresponds to the OM enrichment interval and is accompanied by abnormally high values of Sr/Ba and Sr/Cu.This implies that the extreme arid climate has led to high salinity,resulting in strong reducibility and changes in paleontological assemblages,which in turn controlled the differential enrichment of OM.The Fengcheng Fm.high-quality source rocks are the result of the combined action of climatic events,volcanism,high-salinity water environment and superior hydrocarbon-generating organisms.The results provide new insights into the formation conditions of terrestrial alkaline high-quality source rocks and the factors controlling alkaline OM enrichment.展开更多
基金Supported by Leading Talent Program of Autonomous Region(2022TSYCLJ0070)PetroChina Prospective and Basic Technological Project(2021DJ0108)Natural Science Foundation for Outstanding Young People in Shandong Province(ZR2022YQ30).
文摘Based on the experimental results of casting thin section,low temperature nitrogen adsorption,high pressure mercury injection,nuclear magnetic resonance T2 spectrum,contact angle and oil-water interfacial tension,the relationship between pore throat structure and crude oil mobility characteristics of full particle sequence reservoirs in the Lower Permian Fengcheng Formation of Mahu Sag,Junggar Basin,are revealed.(1)With the decrease of reservoir particle size,the volume of pores connected by large throats and the volume of large pores show a decreasing trend,and the distribution and peak ranges of throat and pore radius shift to smaller size in an orderly manner.The upper limits of throat radius,porosity and permeability of unconventional reservoirs in Fengcheng Formation are approximately 0.7μm,8%and 0.1×10^(−3)μm^(2),respectively.(2)As the reservoir particle size decreases,the distribution and peak ranges of pores hosting retained oil and movable oil are shifted to a smaller size in an orderly manner.With the increase of driving pressure,the amount of retained and movable oil of the larger particle reservoir samples shows a more obvious trend of decreasing and increasing,respectively.(3)With the increase of throat radius,the driving pressure of reservoir with different particle levels presents three stages,namely rapid decrease,slow decrease and stabilization.The oil driving pressures of various reservoirs and the differences of them decrease with the increase of temperature and obviously decrease with the increase of throat radius.According to the above experimental analysis,it is concluded that the deep shale oil of Fengcheng Formation in Mahu Sag has great potential for production under geological conditions.
基金supported by supportedby the China Postdoctoral Science Foundation under Grant Nos.2024M753612 and GZC20233101Supported by the Strategic Cooperation Technology Projects of CNPC and CUPB(ZLZX2020-05)the Science Foundation of China University of Petroleum,Beijing(No.2462023XKBH012).
文摘China's lacustrine shale oil reserves are abundant,making it a key area for future exploration and development.Most lacustrine shales feature a mix of mineral compositions and interlayer sedimentary structures.High-quality reservoirs exhibit significant heterogeneity,which influences the stress distribution during fracturing,leading to complex fracture network patterns.This complexity presents challenges for the comprehensive well logging evaluation of the geological-engineering"double sweet spots"in shale oil,severely restricting efficient development.This study focuses on the impact of shale sedimentary layering on the radial slowness of dipole shear waves.It employs rock physics experiments combined with advanced well logging techniques to explore the relationship between reservoir anisotropy caused by sedimentary layering and reservoir quality,thereby establishing a logging evaluation method for vertical identification of"sweet spots"in lacustrine shale oil.The shales in the Fengcheng Formation of the Mahu Sag into three types according to sedimentary structure scale:laminated,interlayer,and massive.Each type has different mineral compositions,affecting reservoir quality and fracturing potential.Laminated shales develop more fractures under stress along the beddings,showing moderate anisotropy,with reservoir capacity dependent on intercrystalline porosity within carbonate layers.Interlayer shales easily form complex fracture networks,exhibiting significant anisotropy,and their reservoir capacity depends on the porosity within sandy bands.Massive mudstones have the fewest fractures under stress,appearing isotropic with reservoir capacity dependent on matrix pore size.The intensity of reservoir anisotropy correlates positively with storage capacity and the propensity to form irregular and complex fracture networks during hydraulic fracturing.In sections without natural fractures,a larger difference between fast and slow shear waves corresponds to a radial profile shift towards warm tones,indicating stronger anisotropy and better reservoir quality,thus forming complex fracture networks during fracturing.Conversely,a smaller difference leads to a profile energy shift towards cooler tones,indicating stronger isotropy and poorer reservoir quality,hindering the formation of complex fracture networks during hydraulic fracturing.In sections with natural fractures,the difference between fast and slow shear waves exhibits erratic behavior,showing a cross-pattern in radial profiles,indicating strong anisotropy.The presence of natural fractures can synergize with induced fracture networks to form more complex systems,significantly enhancing reservoir productivity.
文摘The Fengcheng Formation is a crucial source rock and the primary reservoir for oil accumulation in the Mahu Sag.Crude oils are distributed throughout the Fengcheng Formation,ranging from the edge to the interior of the sag in the southern Mahu Sag.These crude oils originate from in-situ source rocks in shallowly buried areas and the inner deep sag.During migration,the crude oil from the inner deep sag affects the source rocks close to carrier beds,leading to changes in the organic geochemical characteristics of the source rocks.These changes might alter source rock evaluations and oil-source correlation.Based on data such as total organic carbon(TOC),Rock-Eval pyrolysis of source rocks,and gas chromatography-mass spectrometry(GC-MS)of the saturated fraction,and considering the geological characteristics of the study area,we define the identification characteristics of source rock affected by migrated hydrocarbons and establish the various patterns of influence that migrated hydrocarbons have on the source rock of the Fengcheng Formation in the southern Mahu Sag.The source rocks of the Fengcheng Formation are mostly fair to good,containing mainly Type II organic matter and being thermally mature enough to generate oil.Source rocks affected by migrated hydrocarbons exhibit relatively high hydrocarbon contents(S1/TOC>110 mg HC/g TOC,Extract/TOC>30%,HC:hydrocarbon),relatively low Rock-Eval Tmax values,and relatively high tricyclic terpane contents with a descending and mountain-shaped distribution.Furthermore,biomarker composition parameters indicate a higher thermal maturity than in-situ source rocks.Through a comparison of the extract biomarker fingerprints of adjacent reservoirs and mudstones in different boreholes,three types of influence patterns of migrated hydrocarbons are identified:the edge-influence of thin sandstone-thick mudstone,the mixed-influence of sandstone-mudstone interbedded,and the full-influence of thick sandstone-thin mudstone.This finding reminds us that the influence of migrated hydrocarbons must be considered when evaluating source rocks and conducting oil-source correlation.
基金Supported by the National Natural Science Foundation of China(41802177,42272188,42303056)PetroChina Prospective and Basic Technological Project(2022DJ0507)+1 种基金Research Fund of PetroChina Basic Scientific Research and Strategic Reserve Technology(2020D-5008-04)National Natural Science of Sichuan Province(23NSFSC546)。
文摘Based on core and thin section data,the source rock samples from the Fengcheng Formation in the Mahu Sag of the Junggar Basin were analyzed in terms of zircon SIMS U-Pb geochronology,organic carbon isotopic composition,major and trace element contents,as well as petrology.Two zircon U-Pb ages of(306.0±5.2)Ma and(303.5±3.7)Ma were obtained from the first member of the Fengcheng Formation.Combined with carbon isotopic stratigraphy,it is inferred that the depositional age of the Fengcheng Formation is about 297-306 Ma,spanning the Carboniferous-Permian boundary and corresponding to the interglacial period between C4 and P1 glacial events.Multiple increases in Hg/TOC ratios and altered volcanic ash were found in the shale rocks of the Fengcheng Formation,indicating that multiple phases of volcanic activity occurred during its deposition.An interval with a high B/Ga ratio was found in the middle of the second member of the Fengcheng Formation,associated with the occurrence of evaporite minerals and reedmergnerite,indicating that the high salinity of the water mass was related to hydrothermal activity.Comprehensive analysis suggests that the warm and humid climate during the deposition of Fengcheng Formation is conducive to the growth of organic matter such as algae and bacteria in the lake,and accelerates the continental weathering,driving the input of nutrients.Volcanic activities supply a large amount of nutrients and stimulate primary productivity.The warm climate and high salinity are conducive to water stratification,leading to water anoxia that benefits organic matter preservation.The above factors interact and jointly control the enrichment of organic matter in the Fengcheng Formation of Mahu Sag.
基金financial support from the National Key Research and Development Program of China(2019YFC0605502)the National Natural Science Foundation of China(42302156)+1 种基金the Major Projects of Petro China Science and Technology Fund(2021DJ0206)the Natural Science Foundation of China University of Petroleum(22CX06046A)。
文摘The organic-rich mudstones and dolostones of the Permian Fengcheng Formation(Fm.)are typically alkaline lacustrine source rocks,which are typified by impressively abundantβ-carotane.Abundant β-carotane has been well acknowledged as an effective indicator of biological sources or depositional environments.However,the specific biological sources of β-carotane and the coupling control of biological sources and environmental factors on the enrichment of β-carotane in the Fengcheng Fm.remains obscure.Based on a comprehensive investigation of the bulk,molecular geochemistry,and organic petrology of sedimentary rocks and the biochemistry of phytoplankton in modern alkaline lakes,we proposed a new understanding of the biological precursors of β-carotane and elucidated the enrichment mechanism of β-carotane in the Fengcheng Fm.The results show that the biological precursors crucially control the enrichment of β-carotane in the Fengcheng Fm.The haloalkaliphilic cyanobacteria are the primary biological sources of β-carotane,which is suggested by a good positive correlation between the 2-methylhopane index,7-+8-methyl heptadecanes/C_(max),C_(29%),and β-carotane/C_(max)in sedimentary rocks and the predominance of cyanobacteria with abundantβ-carotene in modern alkaline lakes.The enrichment of β-carotane requires the reducing condition,and the paleoredox state that affects the enrichment of β-carotane appears to have a threshold.The paleoclimate conditions do not considerably impact the enrichment of β-carotane,but they have some influence on the water's paleosalinity by affecting evaporation and precipitation.While it does not directly affect the enrichment of β-carotane in the Fengcheng Fm.,paleosalinity does have an impact on the cyanobacterial precursor supply and the preservation conditions.
基金Supported by the National Natural Science Foundation of China(41802177,42272188)PetroChina Basic Technology Research and Development Project(2021DJ0206,2022DJ0507)Research Fund of PetroChina Basic Scientific Research and Strategic Reserve Technology(2020D-5008-04).
文摘Based on the organic geochemical data and the molecular and stable carbon isotopic compositions of natural gas of the Lower Permian Fengcheng Formation in the western Central Depression of Junggar Basin,combined with sedimentary environment analysis and hydrocarbon-generating simulation,the gas-generating potential of the Fengcheng source rock is evaluated,the distribution of large-scale effective source kitchen is described,the genetic types of natural gas are clarified,and four types of favorable exploration targets are selected.The results show that:(1)The Fengcheng Formation is a set of oil-prone source rocks,and the retained liquid hydrocarbon is conducive to late cracking into gas,with characteristics of high gas-generating potential and late accumulation;(2)The maximum thickness of Fengcheng source rock reaches 900 m.The source rock has entered the main gas-generating stage in Penyijingxi and Shawan sags,and the area with gas-generating intensity greater than 20×10^(8) m^(3)/km^(2) is approximately 6500 km^(2).(3)Around the western Central Depression,highly mature oil-type gas with light carbon isotope composition was identified to be derived from the Fengcheng source rocks mainly,while the rest was coal-derived gas from the Carboniferous source rock;(4)Four types of favorable exploration targets with exploration potential were developed in the western Central Depression which are structural traps neighboring to the source,stratigraphic traps neighboring to the source,shale-gas type within the source,and structural traps within the source.Great attention should be paid to these targets.
基金supported by the National Program for Fundamental Research and Development(973 Program 2011CB201104)
文摘The dolomitic rocks of the Fengcheng Formation are considered to be formed under special geologic conditions, and are significant hydrocarbon reservoir rocks in the Wu-Xia area in the Junggar Basin. Analyses of petrologic characteristics and stable isotope composition indicate that the dolomitizing host rock is volcanic and the dolomitizing fluids probably consists of brine from shore- shallow lakes with great evaporation and salinity in the Fengcheng Formation, which have formed under arid climatic conditions, as well as residual Mg-rich seawater from the underlying Jiamuhe Formation and Carboniferous. Dust tuff in the area has significant plagioclase content. Anorthite and labradorite hydrolysis by CO2 can be coupled with calcite precipitation. Late Mg-rich brine percolated and replaced calcite formed in the early time, which lead to precipitate dolostones with different occurrences, such as graniphyric, random bedded or lumpy. The diagenetic dolostones with different occurrences resulting from particular formation conditions occurred in different tectonic settings. The dolomitizing fluid has been driven by the thermal convection flow generated by volcanic eruptions. At the same time, the overthrusts of the Wu-Xia growth fault have speeded up the flow of deep Mg-rich water upwards, and induced the water to quickly penetrate and horizontally migrate in the strata. Fracture is the major and the most important reservoir space in dolomitic reservoir of the Fengcheng Formation. Fracture and fault plays a decisive role in controlling the formation of dolostone and the distribution of favorable reservoirs. The deliverability of oil and gas is determined by the development and match relations of dissolved pores and fractures to a certain degree.
文摘Shales in the Carboniferous–Permian Fengcheng(FC) and Lucaogou(LCG) formations in Junggar Basin are important organic rich rocks containing significant oil resources.To evaluate the difference in sedimentary environment conditions and hydrocarbon-generating potential between the FC and LCG formations.Total organic carbon(TOC),Rock-Eval pyrolysis,solvent extraction,column fractionation,stable carbon isotope,gas chromatography-mass spectrometry(GC-MS) of saturated hydrocarbons and organic petrology from the source rocks of FC and LCG formations.were analyzed.The biomarker composition indicates that during the deposition of FC,LCG-1 to LCG-2,the sedimentary environment for the source rock formations changed with gradual decrease of salinity,from anoxic to dyoxic/suboxic in redox conditions,and from strong stratification to weakened stratification of water.The FC Formation source rock,with main telalginite(planktonic green algae),archaebacteria and minor terrestrial organic matter,deposited in the environment characterized by high salinity and strongly reducing condition.Its TOC content is relatively low with a high original hydrocarbongenerating potential of unit organic material.The LCG Formation source rock deposited in the environment with low salinity and large variations,the organic matter is mainly sourced from telalginite(planktonic green algae),lamalginite,bacteria and higher plants,resulting in strong heterogeneity of the source rock.The abundance of TOC is high,but the original hydrocarbon generation potential of unit organic matter is lower than that of FC Formation.The results provide a geochemical basis for further study of saline-brackish water sedimentary environment shales in the Junggar Basin.
文摘Based on the combination of core observation,experimental analysis and testingand geological analysis,the main controlling factors of shale oil enrichment in the Lower Permian Fengcheng Formation in the Mahu Sag of the Junggar Basin are clarified,and a shale oil enrichment model is established.The results show that the enrichment of shale oil in the Fengcheng Formation in the Mahu Sag is controlled by the organic abundance,organic type,reservoir capacity and the amount of migration hydrocarbon in shale.The abundance of organic matter provides the material basis for shale oil enrichment,and the shales containing typesⅠandⅡorganic matters have good oil content.The reservoir capacity controls shale oil enrichment.Macropores are the main space for shale oil enrichment in the Fengcheng Formation,and pore size and fracture scale directly control the degree of shale oil enrichment.The migration of hydrocarbons in shale affects shale oil enrichment.The shale that has expelled hydrocarbons has poor oil content,while the shale that has received hydrocarbons migrated from other strata has good oil content.Lithofacies reflect the hydrocarbon generation and storage capacity comprehensively.The laminated felsic shale,laminated lime-dolomitic shale and thick-layered felsic shale have good oil content,and they are favorable lithofacies for shale oil enrichment.Under the control of these factors,relative migration of hydrocarbons occurred within the Fengcheng shale,which leads to the the difference in the enrichment process of shale oil.Accordingly,the enrichment mode of shale oil in Fengcheng Formation is established as"in-situ enrichment"and"migration enrichment".By superimposing favorable lithofacies and main controlling factors of enrichment,the sweet spot of shale oil in the Fengcheng Formation can be selected which has great significance for the exploration and development of shale oil.
基金Supported by the China National Science and Technology Major Project(2017ZX05001-004,2016ZX05046-006)Petrochina Science and Technology Major Project(2019E-2601,2019E-2602)。
文摘By using the latest geological,seismic,drilling and logging data,this article studies the basic conditions for the formation of the total petroleum system and the orderly coexisting characteristics and accumulation models of conventional&unconventional reservoirs in the Lower Permian Fengcheng Formation in the Junggar Basin.Controlled by thermal evolution,hydrocarbon generation and expulsion process of the high-quality source rocks in alkaline lake as well as the characteristics of multi-type reservoirs(conglomerate,sandstone,dolomite and shale),conventional structure-lithologic reservoirs and tight oil and shale oil reservoirs controlled by source-reservoir structure have been formed.On the plane,mature conventional reservoirs,medium-high mature tight oil,and medium-high mature shale oil reservoirs coexist orderly from the slope area around Mahu sag to the sag.Based on the orderly coexisting characteristics of conventional and unconventional reservoirs in the Fengcheng Formation,it is clear that oil and gas in the Fengcheng Formation accumulate continuously over a large area in three accumulation models:integrated source-reservoir,source-reservoir in close contact,and separated source-reservoir model.The three accumulation models differ in relationship between source-reservoir structure,reservoir lithology and spatial distribution,hydrocarbon migration,oil and gas type.It is pointed out that the conventional&unconventional oil and gas should be explored and developed as a whole to achieve an overall breakthrough of the total petroleum system.This study is expected to enrich the geological theory of oil and gas enrichment in continental basins and to provide an analogy for exploration and research in other hydrocarbon-rich sags.
基金Supported by the China National Science and Technology Major Project(2017ZX05001)the PetroChina Science and Technology Major Project(2016B-0302)
文摘Alkaline-lacustrine deposition and its evolution model in Permian Fengcheng Formation at the Mahu sag, Junggar Basin were investigated through core and thin-section observation, geochemical and elemental analysis, logging response and lithofacies identification. Six lithofacies are developed in the Fengcheng Formation. The Feng 2 Member(P1 f2) is dominated by lithofacies with alkaline minerals, while the upper part of the Feng 1 Member(P1 f1) and the lower part of the Feng 3 Member(P1 f3) are primarily organic-rich mudstones that are interbedded with dolomite and dolomitic rock. Paleoenvironment evolution of Fengcheng Formation can be divided into 5 stages, which was controlled by volcanic activity and paleoclimate. The first stage(the early phase of P1 f1) was characterized by intensive volcanic activity and arid climate, developing pyroclastics and sedimentary volcaniclastic rocks. The secondary stage(the later phase of P1 f1) had weak volcanic activity and humid climate that contributed to the development of organic-rich mudstone, forming primary source rock in the Fengcheng Formation. The increasing arid climate at the third stage(the early phase of P1 f2) resulted in shrinking of lake basin and increasing of salinity, giving rise to dolomite and dolomitic rocks. The continuous aird climate, low lake level and high salinity at the fourth stage(the later phase of P1 f2) generated special alkaline minerals, e.g., trona, indicating the formation of alkaline-lacustrine. The humid climate made lake level rise and desalted lake water, therefore, the fifth stage(P1 f3) dominated by the deposition of terrigenous clastic rocks and dolomitic rocks.
基金supported by a Major Projects grant of the China National Petroleum Corporation(Project No.2021DJ1003).
文摘The tight reservoirs of the Fengcheng Formation at the southern margin of the Mahu Sag have strong heterogeneity due to the diversity in their pore types, sizes, and structures. The microscopic characteristics of tight reservoirs and the mechanisms that generate them are of significance in identifying the distribution of high-quality reservoirs and in improving the prediction accuracy of sweet spots in tight oil reservoirs. In this paper, high-pressure mercury intrusion (HPMI) and nuclear magnetic resonance (NMR) experiments were carried out on samples from the tight reservoirs in the study area. These experimental results were combined with cluster analysis, fractal theory, and microscopic observations to qualitatively and quantitatively evaluate pore types, sizes, and structures. A classification scheme was established that divides the reservoir into four types, based on the microstructure characteristics of samples, and the genetic mechanisms that aided the development of reservoir microstructure were analyzed. The results show that the lower limit for the tight reservoir in the Fengcheng Formation is Φ of 3.5% and K of 0.03 mD. The pore throat size and distribution span gradually decrease from Type I, through Type II and Type III reservoirs to non-reservoirs, and the pore type also evolves from dominantly intergranular pores to intercrystalline pores. The structural trend shows a decrease in the ball-stick pore-throat system and an increase in the branch-like pore-throat system. The dual effects of sedimentation and diagenesis shape the microscopic characteristics of pores and throats. The sorting, roundness, and particle size of the original sediments determine the original physical properties of the reservoir. The diagenetic environment of ‘two alkalinity stages and one acidity stage’ influenced the evolution of pore type and size. Although the cementation of authigenic minerals in the early alkaline environment adversely affected reservoir properties, it also alleviated the damage of the later compaction to some extent. Dissolution in the mid-term acidic environment greatly improved the physical properties of this tight reservoir, making dissolution pores an important reservoir space. The late alkaline environment occurred after large-scale oil and gas accumulation. During this period, the cementation of authigenic minerals had a limited effect on the reservoir space occupied by crude oil. It had a more significant impact on the sand bodies not filled with oil, making them function as barriers.
基金Supported the Major National Oil and Gas Projects of China(2016ZX05046-006).
文摘Origin of authigenic dolomites in the dolomitic reservoir of the Permian Fengcheng Formation in the Mahu Sag of Junggar Basin is unclear.Occurrence and genetic evolution of the authigenic dolomites in dolomitic rock reservoir of the Fengcheng Formation in the Mahu Sag were analyzed by polarized and fluorescence thin sections,scanning electron microscope(SEM),electron microprobe(EMP),C,O and Sr isotopes analysis,and other techniques.(1)Dolomites were mainly precipitated in three stages:penecontemporaneous-shallow burial stage(early stage of the Middle Permian),middle burial stage(middle stage of the Middle Permian),and middle-deep burial stage,with the former two stages in dominance.(2)Dolomitization fluid was high-salinity brine originating from alkaline lake.In the penecontemporaneous-shallow burial stage,Mg^(2+)was mainly supplied by alkaline-lake fluid and devitrification of volcanic glass.In the middle burial stage,Mg^(2+)mainly came from the transformation of clay minerals,devitrification of volcanic glass and dissolution of aluminosilicates such as feldspar.(3)Regular changes of Mg,Mn,Fe,Sr,Si and other elements during the growth of dolomite were mainly related to the alkaline-lake fluid,and to different influences of devitrification and diagenetic alteration of volcanic materials during the burial.(4)In the penecontemporaneous stage,induced by alkaline-lake microorganisms,the micritic-microcrystalline dolomites were formed by primary precipitation,replacement of aragonite and high-Mg calcite,and other processes;in the shallow burial stage,the silt-sized dolomites were formed by continuous growth of micritic-microcrystalline dolomite and replacement of calcites,tuffs and other substances;in the middle burial stage,the dolomites,mainly silt-and fine-sized,were formed by replacement of volcanic materials.The research results are referential for investigating the formation mechanism and distribution patterns of tight dolomitic reservoirs in the Mahu Sag and other similar oil and gas bearing areas.
文摘Dolomites and eruptive rocks are well-developed in the Permian Fengcheng Formation in Junggar Basin in China, in which oil and gas are accumulated extensively. Until now, high-yield industrial oil and gas flows have been obtained in the dolomitic tuff of the second unit of the Fengcheng Formation, which demonstrates the huge exploration potential of the thick layer of massive dolomitic tuff. The lithology of the second unit of the Fengcheng Formation in this area has gradually transformed from the dolomite, dolomitic tuff to siltstone from east to west. Moreover, the well testing shows that the reservoir is oil-saturated, and the production rate mainly depends on the reservoir’s physical properties and fracture development. In this study, different types of data including core data, well log and seismic data are used cooperatively to characterize the sedimentary, structure and fracture features of the Fengcheng Formation, and then characterize the promising target zone in the study area. The result indicates that hydrocarbons are most accumulated along the deep fault in the Wu-Xia fault zone, which will be the favorable zone for the next progressive exploration.
基金supported by the National Natural Science Foundation of China(Grant Nos.41872128,42202133)the Strategic Cooperation Technology Projects of the CNPC and CUPB(Grant No.ZLZX2020-01-05)。
文摘The late Paleozoic Fengcheng Formation shale(LPF shale)in the Junggar Basin,NW China,is the oldest alkaline source rock discovered in the world,providing a unique perspective with which to explore organic matter(OM)enrichment in alkaline lake environments.Combined with the organic carbon isotope profile and paleoenvironmental proxies,this study reveals that the LPF shale was deposited in an arid climate with high salinity and a strong reducing environment,accompanied by frequent volcanic activity.High TOC values are concentrated in two intervals with frequent fluctuations in OM types.A negative excursion due to changes in sedimentary OM source is found in theδ^(13)C_(org)profile.The excursion corresponds to the OM enrichment interval and is accompanied by abnormally high values of Sr/Ba and Sr/Cu.This implies that the extreme arid climate has led to high salinity,resulting in strong reducibility and changes in paleontological assemblages,which in turn controlled the differential enrichment of OM.The Fengcheng Fm.high-quality source rocks are the result of the combined action of climatic events,volcanism,high-salinity water environment and superior hydrocarbon-generating organisms.The results provide new insights into the formation conditions of terrestrial alkaline high-quality source rocks and the factors controlling alkaline OM enrichment.
