The Geochemical Evaluation Of Ordovician Source Rocks In Sichuan Basin

Located in the eastern part of Sichuan Province in Western China,Sichuan basin is a foreland basin superimposing on the upper Yangtze carbonate platform in the Late Triassic.Ordovician is completely developed and belongs to a set of marine sediments dominated by carbonate rocks.The Overlying Silurian and lower Cambrian can supply hydrocarbon for it,but the research on its own hydrocarbon generation potential is relatively weak.In particular,the effectiveness of the middle and lower Ordovician source rocks is still unclear.Combining with the organic and inorganic geochemical characteristics,this paper aims at the high evolution degree of marine mudstone in the Sichuan basin.Distribution range and thickness of mudstones are finely described.The relationship between the development of source rocks and sedimentary environment is systematically discussed,and the potential source rock and hydrocarbon generating potential of Ordovician were comprehensively identified in the study area.The distribution of Ordovician mudstone is thick in the West and thin in the East of the study area.From the bottom to the top,the lithology changes from the bottom of thick bioclastic limestones and dolomites with dark mudstones and varied thickness,and the central section of sedimentary rocks mixed by shale rocks,clastic rocks and carbonate rocks to dark mudstones.While the sedimentary environment changes from the restricted platform or shoal to the open platform-shallow shelf.Wufeng group of upper Ordovician have the highest TOC value(1.51%~5.16%).The second one is Tongzi group,of which the TOC value is up to 1.17.The TOC value of Meitan group is up to 0.42% and that of the rest groups are less than 0.4%.Most mudstones have high chloroform bitumen "A" value,and the organic matter type of Ordovician shale formations is mostly II2-III,which gives priority to gas.The distribution of inorganic elements reflects high shale contents?medium productive forces and weak water power in Meitan and Tongzi formation of the southeast of Sichuan basin,which means they have certain hydrocarbon generating potential.Combined with cluster analysis and principal component analysis,Wufeng group of upper Ordovician has the highest score,which was matched with high organic acid and anoxic sedimentary environment.And this layer has conditions for the development of high quality hydrocarbon source rocks.The Meitan group and Tongzi group of lower Ordovician have medium score,of which the organic acid content is relatively low;the water was also turbulent and its level changed,leading to the result of poor source rocks.Corresponding to the lowest organic acid content and sedimentary environment of shallow water with oxidation and high energy,other layers in and above Middle Ordovician have the lowest comprehensive score,As the quantity of organic matter is less and burial condition is bad for the development of source rock,these layers are comprehensively evaluated as non-source rocks.