Facies Characteristics and Sedimentary Evolution Model of the 7^thMember of Dameigou Formation in Yuqia Area, North Qaidam Basin

In recent years, shale oil incontinental basins of China has become a hotspot in oil and gas exploration and development.This paper studied the lithofacies characteristics and sedimentary evolution model of the 7^th member of Dameigou formation of Middle JurassicinYuqia area, north Qaidam Basin by means of observation of core and thin section, organic geochemistry and X-ray diffraction experiments.It provided theoretical basis for the development and distribution of high-quality source rocks.The results show that there are six types of lithofacies, which can be divided into three sedimentary stagesin the vertical direction.Stage A is the delta front and shallow lake sedimentary environment under warm and humid climate,which develops massive poor-organicclay and siliceous mudstone facies.Strong terrestrial input leads to the oxygen-rich water bodywith an average TOC of 0.79%.Therefore, shale oil and gas potential is poor at this stag.Stage B is a semi-deep to deep lake sedimentary environment under tectonic subsidence, which develops the massive rich organic clay mudstone facies.The deep-lakereducing environment provides good condition for the preservation of organic matter.A higher potential of oil and gas appears with an average TOC of 4.16%.Stage C is a semi-deep to deep lake sedimentary environment jointly controlled by arid climate and tectonic subsidence, which develops the laminar calcareous organic-rich matter,calcareous clay-rich and clayey mudstone facies.Good preservation conditions of organic matter by lake salinity stratification and high productivity of adaptive alga make stage C the highest shale oil and gas potential with an average TOC of 5.03%.