The marine-continental transitional shale of the Upper Permian Longtan Formation is widely distributed in Hunan and shows significant exploration potential.Frequent changes in lithofacies have however notably influenc...The marine-continental transitional shale of the Upper Permian Longtan Formation is widely distributed in Hunan and shows significant exploration potential.Frequent changes in lithofacies have however notably influenced the shale gas enrichment.The strata of the Longtan Formation in the Shaoyang Depression,central Hunan,were taken as the study object for this project.Three lithofacies assemblages were identified:shale interbedded with sandstone layer(SAL),sandstone interbedded with shale layer(ASL)and laminated shale layer(LSL).The SAL shale shows significant variability in hydrocarbon generation potential,which leads to shale gas characterized by'hydrocarbon generation in high total organic carbon(TOC)shale,retention in low TOC shale and accumulation in sandstone'.The ASL shale,influenced by the redox conditions of the depositional environment,shows a lower concentration of organic matter.This results in an enrichment model of'hydrocarbon generation and accumulation in shale,with sealing by sandstone'.The laminar structure of LSL shale causes both quartz and clay minerals to control the reservoir.Shale gas is characterized by'hydrocarbon generation in mud laminae,retention and accumulation in silty laminae,with multiple intra-source migration paths'.In the marine-continental transitional shale gas system,the enrichment intervals of different types of shale gas reservoirs exhibit significant variability.展开更多
Fine identification and division of lithofacies types of continental shale strata is an important basis for the evaluation of shale gas exploration and development potential.At present,however,there is no consensus on...Fine identification and division of lithofacies types of continental shale strata is an important basis for the evaluation of shale gas exploration and development potential.At present,however,there is no consensus on the identification standard and division scheme of shale lithofacies.Taking the continental shale strata of theMiddleeLower Jurassic in the SichuanBasin as an example,this paper established a lithofacies division method bymeans of core observation,whole-rockmineral X-ray diffraction analysis,thin section analysis,total organic carbon(TOC)measurement and heliumporosity measurement after analyzing whole-rock mineral composition and shale characteristics.Then lithofacies types of shale strata were identified and divided,and characteristics of lithofacies assemblages in different scales were investigated.Finally,their significance for shale gas exploration was discussed.The following research results were obtained.First,20 shale lithofacies types of 6 categories are totally identified in this continental shale strata using the newly established three-step lithofacies divisionmethod(whole-rock mineral composition partitione-TOC classification-correction and improvement of mineral texture and sedimentary structure).Among them,mediumehigh TOC clay shale lithofacies,laminaethin layer clay shale lithofacies and lowemedium TOC silty shale lithofacies are dominant,followed by lowemedium TOC shell limy clay shale lithofacies,and TOC bearing and low TOC silty clay shale lithofacies.Second,the average TOC and the average porosity of clay shale lithofacies and shell limestone clay shale lithofacies are higher than those of silty and silty clay shale lithofacies.It is indicated thatmineral composition and lithofacies types of shale have a certain impact on gas source and reservoir performance.Third,three types of assemblages are identified in the continental shale strata,including mudstoneelimestone assemblage,mudstoneesandstone assemblage and mudstoneelimestoneesandstone mixed assemblage,which reflect the sedimentary characteristics of distal region,proximal region and transitional region in the lacustrine environment,respectively;and that the characterization of different lithofacies assemblages is conducive to recognizing the differences between different shale sedimentary environments.Fourth,fine identification and statistic of the number and frequency of limy shell laminae and thin layers in the terrestrial organic-rich shale with high claymineral content can provide a basis for the fracturability evaluation of gas-rich zones and the optimization of optimum exploration and development intervals.展开更多
Whether clay-rich shale reservoirs with low-medium maturity can serve as primary exploration targets remains a focal point of debate in the academic community.Clarifying the exploration potential of clayrich shale res...Whether clay-rich shale reservoirs with low-medium maturity can serve as primary exploration targets remains a focal point of debate in the academic community.Clarifying the exploration potential of clayrich shale reservoirs is crucial for the future exploration and development of lacustrine shale.The Triassic Yanchang Formation in the Ordos Basin has been one of most productive lacustrine shale oil systems in China,with substantial oil production capacity already established.While the primary productive layers are currently fine-grained siltstone interbeds,however,it remains a highly debated issue whether the volumetrically more significant clay-rich reservoirs can become viable exploration targets in the near future.To address this issue,we examined the exploration potential of different lithofacies assemblages in Member 7(Mbr 7)of the Triassic Yanchang Formation,using a borehole in the Tongchuan area of the southern Ordos Basin as an example.We identified favorable exploration targets and assessed whether clay-rich reservoirs formed predominantly-freshwater conditions can become viable exploration targets.The results indicate the presence of six lithofacies in clay-rich reservoirs of Mbr 7 of the Yanchang Formation,with two main lithofacies assemblages:laminated organic-rich shale and massive mudstone.From the perspective of sandstone distribution,the sandstone interlayers within laminated organic-rich shale are primarily formed by gravity(hyperpycnal)flows,while sandstones deposited in delta front environments are typically associated with massive mudstone.Laminated organic-rich shale deposition occurred in an anoxic,deep-water environment characterized by high primary productivity,whereas massive mudstone formed in environments with high sedimentation rates and substantial terrigenous debris influx.Currently,the exploration potential of sandstone interlayers exceeds that of clay-rich reservoirs,with the greatest potential observed in the sandstone interlayers associated with laminated organic-rich shale formed by gravity(hyperpycnal)flows.Comparative analysis reveals that clay-rich reservoirs with low to medium maturity present great challenges for exploitation,making interlayer-type reservoirs the main focus of exploration at this stage.Nevertheless,clay-rich reservoirs in closed systems with high thermal maturity and organic matter content also hold considerable potential.展开更多
基金financial support from the National Natural Science Foundation of China(Grant Nos.U23B20155 and 42202140)the Science and Technology Innovation Program of Hunan Province(2023RC1021)+1 种基金the China Geological Survey(DD20221659)the Science and Technology Bureau,Changsha,China(kq2208261)。
文摘The marine-continental transitional shale of the Upper Permian Longtan Formation is widely distributed in Hunan and shows significant exploration potential.Frequent changes in lithofacies have however notably influenced the shale gas enrichment.The strata of the Longtan Formation in the Shaoyang Depression,central Hunan,were taken as the study object for this project.Three lithofacies assemblages were identified:shale interbedded with sandstone layer(SAL),sandstone interbedded with shale layer(ASL)and laminated shale layer(LSL).The SAL shale shows significant variability in hydrocarbon generation potential,which leads to shale gas characterized by'hydrocarbon generation in high total organic carbon(TOC)shale,retention in low TOC shale and accumulation in sandstone'.The ASL shale,influenced by the redox conditions of the depositional environment,shows a lower concentration of organic matter.This results in an enrichment model of'hydrocarbon generation and accumulation in shale,with sealing by sandstone'.The laminar structure of LSL shale causes both quartz and clay minerals to control the reservoir.Shale gas is characterized by'hydrocarbon generation in mud laminae,retention and accumulation in silty laminae,with multiple intra-source migration paths'.In the marine-continental transitional shale gas system,the enrichment intervals of different types of shale gas reservoirs exhibit significant variability.
基金supported by the National Major Science and Technology Project“Evaluation of shale gas exploration potential in continental strata”(No.:2017ZX05036004)Sinopec Technology Development Project“Main control factors of shale gas enrichment and favorable targets in Ziliujing Formation in Northeast Sichuan”(No.:P19017-2).
文摘Fine identification and division of lithofacies types of continental shale strata is an important basis for the evaluation of shale gas exploration and development potential.At present,however,there is no consensus on the identification standard and division scheme of shale lithofacies.Taking the continental shale strata of theMiddleeLower Jurassic in the SichuanBasin as an example,this paper established a lithofacies division method bymeans of core observation,whole-rockmineral X-ray diffraction analysis,thin section analysis,total organic carbon(TOC)measurement and heliumporosity measurement after analyzing whole-rock mineral composition and shale characteristics.Then lithofacies types of shale strata were identified and divided,and characteristics of lithofacies assemblages in different scales were investigated.Finally,their significance for shale gas exploration was discussed.The following research results were obtained.First,20 shale lithofacies types of 6 categories are totally identified in this continental shale strata using the newly established three-step lithofacies divisionmethod(whole-rock mineral composition partitione-TOC classification-correction and improvement of mineral texture and sedimentary structure).Among them,mediumehigh TOC clay shale lithofacies,laminaethin layer clay shale lithofacies and lowemedium TOC silty shale lithofacies are dominant,followed by lowemedium TOC shell limy clay shale lithofacies,and TOC bearing and low TOC silty clay shale lithofacies.Second,the average TOC and the average porosity of clay shale lithofacies and shell limestone clay shale lithofacies are higher than those of silty and silty clay shale lithofacies.It is indicated thatmineral composition and lithofacies types of shale have a certain impact on gas source and reservoir performance.Third,three types of assemblages are identified in the continental shale strata,including mudstoneelimestone assemblage,mudstoneesandstone assemblage and mudstoneelimestoneesandstone mixed assemblage,which reflect the sedimentary characteristics of distal region,proximal region and transitional region in the lacustrine environment,respectively;and that the characterization of different lithofacies assemblages is conducive to recognizing the differences between different shale sedimentary environments.Fourth,fine identification and statistic of the number and frequency of limy shell laminae and thin layers in the terrestrial organic-rich shale with high claymineral content can provide a basis for the fracturability evaluation of gas-rich zones and the optimization of optimum exploration and development intervals.
基金supported by the National Natural Science Foundation of China(Projects 42090022)。
文摘Whether clay-rich shale reservoirs with low-medium maturity can serve as primary exploration targets remains a focal point of debate in the academic community.Clarifying the exploration potential of clayrich shale reservoirs is crucial for the future exploration and development of lacustrine shale.The Triassic Yanchang Formation in the Ordos Basin has been one of most productive lacustrine shale oil systems in China,with substantial oil production capacity already established.While the primary productive layers are currently fine-grained siltstone interbeds,however,it remains a highly debated issue whether the volumetrically more significant clay-rich reservoirs can become viable exploration targets in the near future.To address this issue,we examined the exploration potential of different lithofacies assemblages in Member 7(Mbr 7)of the Triassic Yanchang Formation,using a borehole in the Tongchuan area of the southern Ordos Basin as an example.We identified favorable exploration targets and assessed whether clay-rich reservoirs formed predominantly-freshwater conditions can become viable exploration targets.The results indicate the presence of six lithofacies in clay-rich reservoirs of Mbr 7 of the Yanchang Formation,with two main lithofacies assemblages:laminated organic-rich shale and massive mudstone.From the perspective of sandstone distribution,the sandstone interlayers within laminated organic-rich shale are primarily formed by gravity(hyperpycnal)flows,while sandstones deposited in delta front environments are typically associated with massive mudstone.Laminated organic-rich shale deposition occurred in an anoxic,deep-water environment characterized by high primary productivity,whereas massive mudstone formed in environments with high sedimentation rates and substantial terrigenous debris influx.Currently,the exploration potential of sandstone interlayers exceeds that of clay-rich reservoirs,with the greatest potential observed in the sandstone interlayers associated with laminated organic-rich shale formed by gravity(hyperpycnal)flows.Comparative analysis reveals that clay-rich reservoirs with low to medium maturity present great challenges for exploitation,making interlayer-type reservoirs the main focus of exploration at this stage.Nevertheless,clay-rich reservoirs in closed systems with high thermal maturity and organic matter content also hold considerable potential.
