Shale Characteristics And Organic Matter Accumulation Mechanism Of Wufeng-Longmaxi Formation In Northeastern Sichuan Basin

Late Ordovician to Early Silurian is an important period for the evolution of life and environmental changes of the Earth.High paleoproductivity conditions and anoxic sedimentary environment made a series of black organic-rich shale widely deposited in the whole world during this period.The corresponding strata in Yangtze area of China are Wufeng-Longmaxi Formation.Because of its high quality as the source rock,Wufeng-Longmaxi Formation is the key strata for shale gas exploration in South China.The strata of Wufeng-Longmaxi Formation in the middle and upper Yangtze region are depositied completely and well preserved,but there are some differences in palaeogeographic conditions,sedimentary environment and post-transformation degree in different areas,resulting in different reservoir physical properties,organic carbon content,biostratigraphic division and preservation conditions.Researchers have carried out a lot of shale gas exploration work in southeastern Sichuan Basin,and raised some important points on the sedimentary and diagenetic process of Wufeng-Longmaxi Formation,which provide important theoretical basis and reference materials for the discussion of related scientific issues in this paper.However,the related research in northeastern Sichuan is relatively weak,so this paper takes Wufeng-Longmaxi Formation of Houping Section in northeastern Sichuan as the main research focus.By means of low temperature nitrogen adsorption experiment,elemental geochemistry and biostratigraphy,the reservoir characteristics,sedimentary environment,organic matter enrichment model and graptolite biostratigraphic division of the ufeng-Longmaxi Formation is investigated.The pore size distribution,sedimentary environment,organic matter accumulation model and graptolite biostratigraphic division of the mudstone reservoir in different areas are compared.The following key points of view are obtained in this study:?1?By comparing the graptolite biostratigraphic division of five sections in Sichuan Basin,it is found that the thickness of organic-rich strata in northeastern Sichuan is more than 50m,and the distribution range is more widely on time scale.It can be developed in corresponding period from LM1 to LM9 graptolite zones.However,the thickness of organic-rich shale in southeastern Sichuan basin is significantly smaller than that in northeastern Sichuan basin,ranging from 15 to 38 m,mainly distributed in WF1-LM5 graptolite zones.Although the development of high-quality source rocks in southeastern Sichuan in Rhuddanian?LM2-LM5?is more advantageous,the organic-rich shale deposits have generally ended in southeastern Sichuan basin in Rhuddanian.In contrast,the northeastern Sichuan basin can be sustained to the Eronian and even the Telychian stage,showing that the sedimentary center of the organic-rich shale of the Longmaxi Formation is moving northward continuously.As a result,the thickness of high-quality source rocks in Wuxi-Chengkou area in northeastern Sichuan is about two times thicker than that in southeastern Sichuan.?2?The terrigenous elements Al,Ti,Zr and Th indicate that the detrital flux of Wufeng-Longmaxi Formation in Chengkou area and Qianjiang area is stable during the sedimentary period,and there is a slight increase trend in the upper of Longmaxi Formation.The detrital flux of Qianqian core section is more than Houping section,which is related to the location of Qianqian core section near the paleo-uplift.The redox sensitive elements?V,U,Mo?and related indicators show that the Wufeng Formation and the bottom of Longmaxi Formation are all deposited in anoxic environment,but the reduction degree of the sedimentary environment at the bottom of Longmaxi Formation is higher than that of Wufeng Formation.In addition,the reducing degree and duration of the anoxic environment at the Houping Section are higher than those of Qianqian core section.Based on the analysis of?Ce indication to sea level,it is possible that the slower regression conditions in the northeastern Sichuan basin in the late Early Silurian resulted in a longer duration of anoxic environment.The contents of P,Ba_exx and Si_ex indicate that the two groups of Wufeng-Longmaxi Formation mudstones have good paleoproductivity conditions during the sedimentary period,but the Houping Section has a slightly higher level of paleoproductivity.?3?Low-temperature nitrogen adsorption experiments show that the pore types of Wufeng-Longmaxi Formation of the Houping section are similar to Qianqian core section that located in eastern Sichuan,which are almostly open parallel plate-like pores.Both groups of mudstones have high BET surface area and pore volume,and micropore and mesopore with pore size ranging from<2 nm to 2-50 nm are mainly pore types,but the macropore volume of the Houping outcrop section is slightly higher than the Qianqian core section.This may be related to the geological modification and preservation conditions of Houping section.Generally,the BET surface area,pore volume and pore size distribution of the two groups of samples are basically similar,which indicates that the difference of pore development of the Wufeng-Longmaxi Formation in different areas of Sichuan Basin is very small.?4?The BET surface area and pore volume of the Wufeng-Longmaxi Formation in Houping Section and Well Qianqian core section are positively correlated with TOC,but not with quartz content.Clay minerals content in the bottom of Longmaxi Formation in Houping Section is negatively correlated with BET surface area and pore volume,but not with Qianqian core section,indicating that organic carbon content is an important factor affecting pore development of Wufeng-Longmaxi Formation,but mineral compositions have no obvious influence on pore development.?5?The correlation analysis between TOC and redox conditions and paleoproductivity shows that the enrichment of organic matter at Wufeng Formation and the bottom of Wufeng Formation is mainly controlled by anoxic environment,so the accumulation mode of organic matter belongs to"preservation mode".Mo/TOC and Mo-U covariant models indicate that the formation mechanism of anoxic environment at Wufeng Formation and the bottom of Wufeng Formation is different,the anoxic conditions of Wufeng Formation is affected by strong restricted conditions,but the anoxic conditions of Longmaxi Formation is controlled by the stratification of water bodies due to the large rise of sea level.