Geochemical Characteristics And Tectonic Settings Of The Middle And Upper Permian Hydrocarbon Source Rocks In The Southeastern Margin Of Junggar Basin

The Junggar Basin is the second largest inland basin in China and is still rich in oil and gas resources,with oil reserves exceeding 10 billion tons.However,the tectonic and basin evolution of the southeastern margin of the Junggar Basin is complicated.Its oil and gas exploration degree and results are poor,but it has great exploration potential.In recent years,the research and exploration of the oil shale of the Permian Lucaogou Formation in the southern margin has been intensively carried out.The Permian Lucaogou Formation and Wutonggou Formation in the eastern Junggar Basin and its adjacent area have become the focus and hotspots of unconventional oil and gas exploration research in China.Two Permian hydrocarbon source rocks outcrop profiles in the southeastern margin,the Lucaogou Formation in the Jixin coal mine area and the Wutonggou Formation in the Xiaolongkou area were selected.We use multiple methods of sedimentology,petrology,organic geochemistry,biomarker compounds and elemental geochemistry to determine the organic matter abundance,type and maturity of the hydrocarbon source rocks of the Lucaogou Formation and the Wutonggou Formation in the study area,and analyze the factors which control the forming and preserving of the hydrocarbon resource rocks,the paleo-productivity,paleo-climate,paleo-salinity,paleo-water depth and paleo-redox environment.And the weathering status of the clastic particles source area,the parent rock lithology and the tectonic setting are also studied.We hope this study may contribute to the future oil and gas exploration and extraction work in the region.Main conclusions are as follows:1.The Lucaogou Formation and the Wutonggou Formation in the study area are a set of high-quality hydrocarbon source rocks.The total organic carbon content?average values 8.7%and 5.0%,respectively?and the saturated hydrocarbon component content?average values 37.8%and 42.0%,respectively?were high,and the organic matter abundance of the Lucaogou Formation was significantly higher than that of the Wutonggou Formation.The organic matter types of the Lucaogou Formation and the Wutonggou Formation in the study area were good(the average values of?¹³C/‰is-29.0‰and-25.6‰,respectively),and the kerogen type was mixed with humus and mucilage?type?^II?.The source rock samples of the Lucaogou formation show the mixed organic parent material input characteristics of more algae aquatic organisms and a certain amount of terrestrial plants.The source rock samples of the Wutonggou Formation show the characteristics of more terrestrial plants and a certain amount of algae aquatic organisms.The thermal evolution of the Lucaogou Formation and the Wutonggou Formation samples were in the medium-mature stage?R_c mean values 0.77 and 0.71,respectively?.2.The source rocks of the Lucaogou Formation and the Wutonggou Formation in the study area all have high paleo-productivity?P/Al average 156.16 and 109.33?.The Lucaogou Formation shows brackish water-salt water characteristics?GI average0.31?,andthe WutonggouFormationshowedfreshwater-brackishwater characteristics?GI average 0.08?.The sedimentary environment reflected by the Lucaogou and Wutonggou samples in the study area was generally weak reduction preservation condition?V/Cr average 3.38 and 2.89,respectively?,and the Lucaogou group is more reductive,especially in the middle part of the Lucaogou Formation?V/Cr average 4.86?.The water level change processes of the study area in the Middle and Late Permian experienced:semi-deep lake,to deep lake,to semi-deep lake?shore lakes?,and the corresponding sedimentary environments experienced:weak oxidation-weak reduction environment,to reduction environment,to weak oxidation-weak reduction environment.Relatively high levels of paleo-productivity,arid climate and good water stratification promotes the proliferation of aquatic organisms such as algae in the upper waters,providing a material basis for the formation of high-quality source rocks.The large water depth,good water stratification and relatively reduction preservation environment makes organic matter preserved perfectly,then contained in the rocks through burial and diagenesis.3.The bottom and top sections of the Lucaogou Formation in the study area were subjected to moderate weathering intensity indicating semi-arid to semi-humid climate?CIA ration 61⁷1?,and the middle part of the Lucaogou Formation were subjected to low weathering intensity indicating arid to semi-arid climate?CIA ratio 42⁵1?,the Wutonggou group samples were subjected to moderate weathering intensity indicating semi-arid to semi-humid climate?CIA ratio 68⁷1?.4.The REE composition of the clastic particles of the Lucaogou Formation and the Wutonggou Formation in the study area is consistent with PAAS?Post-Archean Australian shale?and UCC?Upper continental crust?;and the source rock lithology is medium-acid igneous rock?Eu/Eu*average 0.84?;according to the La-Th-Sc diagram,the tectonic settings of the parent rocks of the source area are mainly the continental island arc,while a small amount of it may be the ocean island arc.