Paleogene Shale Organic Facies And Maturity Research In Yingxiongling Area,Southwestern Qaidam Basin

Yingxiongling sag is an important hydrocarbon generation sag in the west of Qaidam Basin.The E₃²section is the key area of this study.It is of great significance for the subsequent evaluation of shale oil resources and exploration and development of shale oil to clarify the spatial distribution of different organic facies source rocks in the E₃²section of Yingxiongling sag and characterize the maturity of source rocks.In this paper,the thickness,spatial distribution and spatial variation of source rock maturity of organic facies in E₃²section are studied,and the following understandings are obtained.The organic matter abundance of Paleogene E₃²source rocks in western Qaidam Basin is generally low,but the soluble hydrocarbon content is high and the source rock type is good.The Paleogene E₃²formation is relatively thick,with a thickness of more than 2000 m,and mainly develops brackish-saline lacustrine source rocks.The thickness of source rocks in western Qaidam Basin is generally large,ranging from 400to 1100 m,mainly developed in Yingxiongling Sag and Mangya Sag.Based on the covariation relationship of HI-TOC,organic petrological characteristics and geochemical characteristics of source rocks,the source rocks are divided into C facies,D/E facies and F facies source rocks.Among them,the HI of C-facies source rock is above 400 mg HC/g TOC,TOC is more than 1%,rich in alginate,and less terrestrial input.The thickness of C-facies source rock in western Qaidam Basin ranges from 40 to370 m,mainly distributed in Ganchaigou area of Yingxiongling structural belts,with a thickness of more than 200 m.The HI of D/E source rock is 200～400mg HC/g TOC,TOC value is between 0.5%～1%,terrestrial input is relatively more,only sporadic alginate.The thickness of D/E source rock in western Qaidam Basin is 370～920m,mainly distributed in Shizigou area and Ganchaigou area.The thickness of E₃²formation in Yingxiongling area is large,ranging from 1 to 2km,and the geothermal gradient is high,with an average of 3.5°C/100m,so the maturity span of source rock is large.Vertically,the maturity of the top boundary of E₃²formation is very different from that of the bottom boundary of E₃²formation.The source rock in Ganchaigou area enters the hydrocarbon generation threshold at about2800m,and 2800～4400m is the oil window.On the plane,the degree of thermal evolution of source rocks is also different.In general,from the northwest-southeast（tectonic downdip direction）,the maturity of source rocks gradually increases,and the east-west direction changes greatly.