Depositional Environment And Pore Characteristics Of The Odorvician-silurian Fine-grained Sedimentary Rocks,Northwestern Guizhou,South China

The Ordovician-Silurian Wufeng-Longmaxi fine-grained sedimentary rocks in Northwestern Guizhou record significant shift in paleoenvironment,and have been considered as a promising shale gas reservoir.Based on outcrop and drilling core data,we did description of outcrop and drilling cores,thin section observation,scanning electron microscope observation,analysis of rock minerals,and geochemical analysis.The purpose of this study is to analyze sedimentary characteristics of lithofacies,reconstruct sequence stratigraphy and depositional facies,rebuild paleoenvironment,analyze characteristics of pore system,and discuss distribution of potential reservoir within sequence stratigraphy.Seven lithofacies have been encountered from Wufeng Formation to Longmaxi Formation:bioturbated claystone,black siliceous shale,muddy fossiliferous rocks,black siliceous-argillaceous shale,black calcareous siltstone,gray siliceous-argillaceous siltstone,and dark gray argillaceous-siliceous siltstone.The main sedimentary processes are suspension and bed-load transport by bottom current.The bioturbated shale is fully bioturbated,indicating oxic condition during early Wufengian age.Appearance of fecal pellets within black siliceous shale and black siliceous-argillaceous shale refutes existence of consistent anoxic conditions.Two third-order depositional sequences have been recognized:sq1 and sq2.Including transgressive systems tract and highstand systems track,sq1 is composed of shallow muddy shelf and deep siliceous shelf,exhibiting relatively flat distribution trend.Unlike sq1,sq2 is composed of lowstand systems track,transgressive systems track,and highstand systems track.Shallow calcareous shelf,deep siliceous/muddy shelf and shallow silty shelf have been recognized within sq2.The depo-center of sq2 is in the northern part of study area.The characteristics of isotopic data demonstrate that organic carbon isotope(?¹³C_org)is controlled by origins of organic matter in different depositional environment,while variation of sulfur isotope from iron sulfides(?³⁴S_sulfide)is mainly related to sedimentation rates and redox conditions.Both of them are related to changes of relatively sea level.Impact of redox conditions,organic paleoproductivity,and clastic dilution on enrichment of organic matter has been discussed by elemental indices.The redox conditions are considered as a main factor for enrichment of organic matter,and clastic dilution is the secondary factor.However,paleoproductivity does not have impact on enrichment of organic matter.Interparticle pores,intraparticle pores,phyllosilicate framework pores,and organic pores have been recognized in different lithofacies.Interparticle pores between detrital grains and primary phyllosilicate framework pores would be mainly destroyed by mechanical compaction during shallow to intermediate burial,whereas interparitcle pores between authigenic silica and organic pores would be largely survived during burial processes.Black siliceous shale shale that enriched in organic matter and authigenic silica is considered as the most promising unit of well-developed pore systems.Finally,given impacts on organic matter enrichment and pore systems development,late transgressive systems tract and highstand systems track within sq1 as well as transgressive systems tract and early highstand systems within sq2 are proved as potential shale-gas reservoirs.