Sedimentary Characterization And Its Effect On The Development Of Shale Reservoir In The Longmaxi Formation In Yanyuan Area,Southwest Sichuan Province

The lower Silurian Longmaxi shales are widely distributed in the west of Kangdian Oldlamd.However,limited research has been done to linking the sedimentary characteristics to the development of shale reservoir.In this paper,the Longmaxi shales have been studied in Yanyuan and its adjacent areas.Lots of research works have been done to analyze the sedimentary and reservoir characteristics.These research works include literature review,field survey,and experiment analysis?mineralogy,geochemistry,scanning electron microscope observation,and lower N₂ adsorption?.The main conclusions are made as follows:The Longmaxi shales are dominated by quartz?50.2-100%;average 78.8%?.Results show that quartz is derived from four origins,including biogenic,hydrothermal,diagenetic transformation,and terrrigenous input.Clay minerals are composed of illite and Illite-smectite?I/S?mixed minerals,accounting for 0-33.2%?average:10.2%?.Clay minerals are mainly derived from terrigenous clastic and diagenetic transformation.The average contents of feldspar and carbonate minerals are less than8%,while the average content of pyrite is less than 1%.Based on the mineral composition,the intrabasinal quartz,terrigenous silt,clay minerals,and carbonate minerals are regarded as the parameters.Five types of lithofacies are identified,including siliceous shale?inrabasinal quartz>50%?,silty shale?terrrigenous silt>50%?,calcareous shale?carbonate minerals>50%?,argillaceous shale?clay minerals>50%?,and mixed shale?the contents of the four parameters all lower than 50%?.The intrabasinal quartz has three distinct sources,including biogenic,hydrothermal,and diagentic transformation.The siliceous shale are further subdivide into biogenic siliceous shale?biogenic quartz>50%?,hydrothermal siliceous shale?hydrothermal quartz>50%?,authigenic siliceous shale?authigentic quartz>50%?,and mixed siliceous shale?all of them<50%?.Therefore,the Longmaxi shale are divided into totally eight lithofacies and five of them?biogenic siliceous shale,silty shale,mixed siliceous shale,calcareous shale,and hydrothermal siliceous shale?are identified in the study area.The contents of TOC,elements Cu,and Ni all show a decrease trend from biogenic siliceous shale,silty shale,mixed siliceous shale,hydrothermal siliceous shale to calcareous shale.Cu and Ni are widely used for reconstructing paleoproductivity.The strong positive relationships between Cu,Ni and TOC contents?R²>0.6?reveal that high paleoproductivity is the main controlling factor for organic carbon accumulation.Elements Al₂O₃ and Ti are widely exploited to reflect terrigenous input.The weak positive relationships between Al₂O₃,Ti and TOC contents?R²<0.4?indicate that high terrigenous input is favorable for the organic carbon accumulation.V/Cr,Ni/Co can be used to recognize redox conditions.The extremely weak positive correlations between V/Cr,Ni/Co and TOC contents?R²<0.2?denotes that redox conditions have little influence on organic carbon accumulation.The organic hydrocarbon generation,compaction,cementation,dissolution,and fracture-formed diagenesis are identified in Longmaxi shales.Organic matter pores,intergranular pores,intragranular pores,and fractures are identified in Longaxi shales.The specific surface area and total pore volume of shales are mainly controlled by organic matter pore and clay minerals-related pores.The amount,size and connectivity of organic matter pores reduce gradually,from biogenic siliceous shale,silty shale,mixed siliceous shale,calcareous shale to hydrothermal siliceous shale.Among the five lithofacies,intragranular dissolution pores are developed much better in silty and calcareous shales.Compared with biogenic siliceous shale and silty shale,intergranular pores in the other three lithofacies are poorly developed.Affected by regional tectonic stress and diagenesis,the structure and diagenetic fractures are developed much better in biogenic siliceous shale,silty shale,and hydrothermal siliceous shale.From biogenic siliceous shale,silty shale,mixed siliceous shale,calcareous shale to hydrothermal siliceous shale,the corresponding hysteresis loop of N₂ isotherm are changing from type H₂ to type H₄ based on the classification standard from International Union of Theoretical and Applied Chemistry?IUPAC?.The pore size distribution curve reveals that the biogenic siliceous shale and silty shale are dominated by micropores,while the other three lithofacies are dominated by mesopores.Therefore,the biogenic and silty shales show a much better pore strcture with larger specific surface areas,and pore volumes when compared to the other three lithofacies.The pore strcture of Longmaxi shales in the study area are mainly controlled by total organic carbon?TOC?content.The shales with high TOC content were mainly deposited in good sedimentary environment.In the five lithofacies,biogenic siliceous shale has the best pore strcture and deposited in the deep water shelf environment.Due to the high paleoproductivity of the surface water,The TOC content of biogenic siliceous shale is much high.On the contrary,the hydrothermal siliceous shale has a much lower TOC content even though it also deposited in the deep water shelf.The lower paleoproductivity of surface water led to its lower TOC content.In the biogenic siliceous shale,the organic matters were abundant and they often occurred along with prtite,clay minerals,and quartz,which was favorable for the generation of organic matter pores during the middle-late diagetic stage.By contrast,in the calcareous shale and hydrothermal siliceous shales,the organic matters were depleted and they oftern occurred alone,which was bad for the generation of organic matter pores during the middle-late diagenetic stage.