Depositional environmental controls on the genesis and characteristics of oil shale: Case study of the Middle Jurassic Shimengou Formation, northern Qaidam Basin, north-west China

Two different types of oil shales (OSs) from the Middle Jurassic Shimengou Formation on the northern margin of the Qaidam Basin, north-west China, provide a valuable opportunity to study the factors controlling the genesis of oil shales. This study presents the sedimentary characteristics of various types of oil shales from the Shimengou Formation and then discusses their possible genesis based on a comprehensive analysis on drillcores. The Shimengou Formation comprises a lower coal-bearing member (J(2)sh(1)) and an upper shale member (J(2)sh(2)). The J(2)sh(1) OS unit is composed of dark grey thin-layered shales interbedded with coal seams, with total organic carbon (TOC) content of 18.8-34.1 wt.% (averagely 21.5 wt.%), and belonging to type II2 organic matter. The J(2)sh(2) OS unit consists of dark grey to grey-brown thick-bedded shales and mudstone. The J(2)sh(2) OS has a moderate abundance in organic matter, TOC contents of 6.3-11.5 wt.% (averagely 9.59 wt.%), and types I and II1 organic matters. During the deposition of the J(2)sh(1) member, both braided river delta and freshwater-marsh facies dominate the succession, and OS developed in shallow lakes and swamps. Soon after, the depositional environment changed rapidly. Plentiful terrestrial and shallow water aquatic plants provided abundant organic matter for the sedimentation of OS and also promoted the formation of thinly-bedded, high-quality OS with type II2 organic matter. The frequent changes in depositional environments facilitated the formation of thin layers of oil shale. During the sedimentation of the J(2)sh(2) member, in similar tectonic settings, the stable warm-humid climate makes the lake water deeper than before, and the oil shale was deposited mainly in a relatively deep lake. This led to a reduced supply of terrestrial organic matter, which came mainly from lake algae, leading to the formation of OS with a large, uniform thickness and the presence of types I and II2 organic matter.