Natural fractures are of crucial importance for oil and gas reservoirs,especially for those with ultralow permeability and porosity.The deep-marine shale gas reservoirs of the Wufeng and Longmaxi Formations are typica...Natural fractures are of crucial importance for oil and gas reservoirs,especially for those with ultralow permeability and porosity.The deep-marine shale gas reservoirs of the Wufeng and Longmaxi Formations are typical targets for the study of natural fracture characteristics.Detailed descriptions of full-diameter shale drill core,together with 3D Computed Tomography scans and Formation MicroScanner Image data acquisition,were carried out to characterize microfracture morphology in order to obtain the key parameters of natural fractures in such system.The fracture type,orientation,and their macroscopic and microscopic distribution features are evaluated.The results show that the natural fracture density appears to remarkably decrease in the Wufeng and Longmaxi Formations with increasing the burial depth.Similar trends have been observed for fracture length and aperture.Moreover,the natural fracture density diminishes as the formation thickness increases.There are three main types of natural fractures,which we interpret as(I)mineral-filled fractures(by pyrite and calcite),i.e.,veins,(II)those induced by tectonic stress,and(III)those formed by other processes(including diagenetic shrinkage and fluid overpressure).Natural fracture orientations estimated from the studied natural fractures in the Luzhou block are not consistent with the present-day stress field.The difference in tortuosity between horizontally and vertically oriented fractures reveals their morphological complexity.In addition,natural fracture density,host rock formation thickness,average total organic carbon and effective porosity are found to be important factors for evaluating shale gas reservoirs.The study also reveals that the high density of natural fractures is decisive to evaluate the shale gas potential.The results may have significant implications for evaluating favorable exploration areas of shale gas reservoirs and can be applied to optimize hydraulic fracturing for permeability enhancement.展开更多
The lower Cambrian Niutitang Formation is of crucial importance for shale gas target reservoirs in western Hubei,China;however,little work has been done in this field,and its shale gas accumulation and enrichment mech...The lower Cambrian Niutitang Formation is of crucial importance for shale gas target reservoirs in western Hubei,China;however,little work has been done in this field,and its shale gas accumulation and enrichment mechanism are still unclear.Based on survey wells,outcrop data,and large numbers of tests,the geological conditions of shale gas accumulation were studied;moreover,the factors that influence the gas content were thoroughly discussed.The results show that the Niutitang Formation(Є1n)can be divided into three sections:the first section(Є_(1)n^(1)),the second section(Є_(1)n^(2)),and the third section(Є1n3).TheЄ_(1)n^(2) is the main shale gas reservoir.The deep shelf facies is the main sedimentary facies and can be divided into three main lithofacies:argillaceous siltstone,carbonaceous shale and carbonaceous siliceous rock.The total organic carbon(TOC)content shows gentle growth trends until bottom of theЄ_(1)n^(2) and then decreases rapidly within theЄ_(1)n^(1),and the TOC content mainly ranges from 2%to 4%horizontally.The calcite and dolomite dissolution pores,clay intergranular pores and organic pores are the main pore types and the micropore types are clearly related to the mineral compositions and the TOC content.Vertically,the gas content is mainly affected by the TOC content.Horizontally,wells with high gas contents are distributed only southeast of the Huangling anticline,and the combination of structural styles,fault and fracture development,and the distribution of the regional unconformity boundary between the upper Sinian Dengying Formation(Z2d)and theЄ_(1)n^(2) are the three most important factors affecting the gas content.The favorable areas must meet the following conditions:a deep shelf environment,the presence of theЄ_(1)n^(1),wide and gentle folds,far from large normal faults that are more than 5 km,moderate thermal evolution,and greater than 500 m burial depth;this includes the block with the YD2–ZD2 wells,and the block with the Y1 and YD4 wells,which are distributed in the southern portion of the Huangling anticline and northern portion of the Xiannvshan fault.展开更多
基金The project is funded by the National Natural Science Foundation of China(Grant No.42202155)China Postdoctoral Science Foundation(No.2021MD703807),Major Special Project of the Ministry of Science and Technology of PetroChina(Nos.2022DJ8004 and 2021DJ1901)+4 种基金Heilongjiang Postdoctoral Foundation(No.LBH-Z20121)Natural Science Foundation of Hubei Province Project(No.2020CFB501)The Scientific Research Project of Department of Natural Resources of Hubei Province(No.ZRZY2020KJ10)The authors gratefully acknowledge financial support from the China Scholarship Council(No.202008230018)EGR acknowledges funding by the Spanish Ministry of Science and Innovation(MCIN)/State Research Agency of Spain(AEI)/European Regional Development Fund(ERDF)/10.13039/501100011033 for the“Ramón y Cajal”fellowship RYC2018-026335-I and research projects PGC2018-093903-B-C22 and PID2020-118999GB-I00.
文摘Natural fractures are of crucial importance for oil and gas reservoirs,especially for those with ultralow permeability and porosity.The deep-marine shale gas reservoirs of the Wufeng and Longmaxi Formations are typical targets for the study of natural fracture characteristics.Detailed descriptions of full-diameter shale drill core,together with 3D Computed Tomography scans and Formation MicroScanner Image data acquisition,were carried out to characterize microfracture morphology in order to obtain the key parameters of natural fractures in such system.The fracture type,orientation,and their macroscopic and microscopic distribution features are evaluated.The results show that the natural fracture density appears to remarkably decrease in the Wufeng and Longmaxi Formations with increasing the burial depth.Similar trends have been observed for fracture length and aperture.Moreover,the natural fracture density diminishes as the formation thickness increases.There are three main types of natural fractures,which we interpret as(I)mineral-filled fractures(by pyrite and calcite),i.e.,veins,(II)those induced by tectonic stress,and(III)those formed by other processes(including diagenetic shrinkage and fluid overpressure).Natural fracture orientations estimated from the studied natural fractures in the Luzhou block are not consistent with the present-day stress field.The difference in tortuosity between horizontally and vertically oriented fractures reveals their morphological complexity.In addition,natural fracture density,host rock formation thickness,average total organic carbon and effective porosity are found to be important factors for evaluating shale gas reservoirs.The study also reveals that the high density of natural fractures is decisive to evaluate the shale gas potential.The results may have significant implications for evaluating favorable exploration areas of shale gas reservoirs and can be applied to optimize hydraulic fracturing for permeability enhancement.
基金supported by the Scientific Research project of Department of Natural Resources of Hubei Province(No.ZRZY2020KJ10)Hubei Geological Bureau(No.KJ2019-3)+2 种基金the Shale Gas Geological Survey Projects of Department of Natural Resources of Hubei Province(Nos.HBCZ-17060223-170397 and DTCG-190409)We also thank the funds provided by Youth Foundation of the Northeast Petroleum University(No.2019QNL-21)Opening Fund of the Coal Reservoir Laboratory of National Engineering Research Center of CBM Development&Utilization,China University of Geosciences(Beijing)(No.2019MCQ02001)。
文摘The lower Cambrian Niutitang Formation is of crucial importance for shale gas target reservoirs in western Hubei,China;however,little work has been done in this field,and its shale gas accumulation and enrichment mechanism are still unclear.Based on survey wells,outcrop data,and large numbers of tests,the geological conditions of shale gas accumulation were studied;moreover,the factors that influence the gas content were thoroughly discussed.The results show that the Niutitang Formation(Є1n)can be divided into three sections:the first section(Є_(1)n^(1)),the second section(Є_(1)n^(2)),and the third section(Є1n3).TheЄ_(1)n^(2) is the main shale gas reservoir.The deep shelf facies is the main sedimentary facies and can be divided into three main lithofacies:argillaceous siltstone,carbonaceous shale and carbonaceous siliceous rock.The total organic carbon(TOC)content shows gentle growth trends until bottom of theЄ_(1)n^(2) and then decreases rapidly within theЄ_(1)n^(1),and the TOC content mainly ranges from 2%to 4%horizontally.The calcite and dolomite dissolution pores,clay intergranular pores and organic pores are the main pore types and the micropore types are clearly related to the mineral compositions and the TOC content.Vertically,the gas content is mainly affected by the TOC content.Horizontally,wells with high gas contents are distributed only southeast of the Huangling anticline,and the combination of structural styles,fault and fracture development,and the distribution of the regional unconformity boundary between the upper Sinian Dengying Formation(Z2d)and theЄ_(1)n^(2) are the three most important factors affecting the gas content.The favorable areas must meet the following conditions:a deep shelf environment,the presence of theЄ_(1)n^(1),wide and gentle folds,far from large normal faults that are more than 5 km,moderate thermal evolution,and greater than 500 m burial depth;this includes the block with the YD2–ZD2 wells,and the block with the Y1 and YD4 wells,which are distributed in the southern portion of the Huangling anticline and northern portion of the Xiannvshan fault.