吉木萨尔凹陷二叠系芦草沟组页岩层系细粒沉积特征OA

The shale oil of the Lucaogou Formation in the Jimsar Sag,Xinjiang,is a typical representative of continental shale oil in China.The resources in favorable sweet spot areas reach 1.112 billion tons,featuring a large resource scale and high confirmation degree.It has been approved as China's first national shale oil construction demonstration area.With continuous advancement of development,the drilling rate of high-quality sweet spots is generally low,and there are significant differences in single-well production.The distribution of shale oil sweet spots exhibits strong heterogeneity,making it urgent to further deepen the research on the lithofacies and sedimentary characteristics of fine-grained rocks in the study area.By utilizing data and methods including core observation,drilling and logging,thin section analysis,and scanning electron microscopy,studies on the petrological characteristics and lithofacies of fine-grained rocks were conducted,and the fine-grained sedimentary characteristics were clarified.The results show that:1)the lithology of the study area is dominated by shales,with minor amounts of clastic rocks and carbonate rocks.Among them,shales have a large thickness and high proportion,while carbonate rocks and clastic rocks have low content,mostly developing as thin interbeds within thick shales.Shales are further divided into three major categories:mud shales,silty shales,and calcareous shales.2)The lithofacies of the Lucaogou Formation are classified into five types:sandy dolomite facies,felsic shale facies,gray-dolomitic shale facies,mixed shale facies,and the fine-silty sandstone facies,which is minimally developed.The"upper sweet spot section"(P2l22)is dominated by mixed shale facies as a whole;the eastern part of the sag is mainly composed of felsic shale facies,and lithofacies such as sandy dolomite facies and gray-dolomitic shale facies develop as interbeds within the thick dominant shale facies.The"lower sweet spot section"(P2l12)is also dominated by mixed shale facies;the edge of the sag is mainly felsic shale facies,and fine-silty sandstone facies is found in the near-provenance area in the north.3)There are obvious facies differences between the"upper and lower sweet spot sections"(P2l22 and P2l12):the"lower sweet spot section"(P2l12)is mainly a shore-shallow lake to semi-deep lake sedimentary system.Semi-deep lake to deep lake facies develop in the central part of the study area,characterized by high-frequency interbedding of mud shales,dolomitic mud shales,and micritic dolomitic shales,intercalated with minor mixed flats,dolomitic flats,and thin interbeds of hyperpycnal flow fine-silty sandstones(single-layer thickness＜2 m);the edge of the sag transitions to shore-shallow lake facies,featuring interbedding of mud shales and silty shales,with local development of beach-bar sands and hyperpycnal flow sandy strips.The"upper sweet spot section"(P2l22)is dominated by semi-deep lake to deep lake facies as a whole;its lithology consists of semi-deep lake mud shales intercalated with dolomitic shales,mixed dolomitic flats,mixed flat deposits,and thin layers of hyperpycnal flow siltstones.Shore-shallow lake facies argillaceous deposits develop at the eastern edge of the sag,with local low-energy beach-bar sand bodies.The research results play a promotional role in the subsequent efficient evaluation and development of the Permian Lucaogou Formation shale reservoirs.