Two oil shale sequences in the Middle Jurassic Shimengou Formation in the Yuqia area, northern Qaidam Basin in Northwest China were evaluated based on geochemical and proximate analyses. The characteristics and accumulation conditions of oil shale in the Lower Coal-bearing Member (J2sh1) and Upper Shale Member (J2sh2) are different. The oil shale in J2sh1 with a high, terrigenous organic matter (OM)-derived total organic carbon (TOC) content is of medium-high oil yield, low ash and moisture contents, and low-medium calorific value and sulfur content. The oil shale in J2sh2 having a medium, alginite and bituminite-originated TOC content is characterized by a low-medium oil yield, high ash content, and low calorific value and moisture and sulfur contents. The oil shale in J2sh1 was deposited in a limnic environment under a varied climate from hot-humid to warm-humid, whereas that in J2sh2 was deposited in a semi-deep and deep lake environment under a stable warm-humid climate. Climatic conditions may have controlled the quality, distribution and thickness of oil shale by influencing the origin of organic matter and sedimentary environment. High paleoproductivity in J2sh2 (or high terrigenous detrital matter input in J2sh1) and strong water salinity stratification are responsible for the accumulation of high-quality oil shale.
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