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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 (J2sh1) and an upper shale member (J2sh2). The J2sh1 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 J2sh2 OS unit consists of dark grey to grey‐brown thick‐bedded shales and mudstone. The J2sh2 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 J2sh1 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 J2sh2 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. [ABSTRACT FROM AUTHOR]

Based on the detailed observations of the whole core, bulk of organic geochemical analysis, and combined with regional geological characteristics, the evolution of depositional environments and enrichment pattern of organic matter of mudstone of the Meihe Formation in the Meihe Basin was studied, and this will be enriched the theory of oil shale and coal metallogenic mechanism of the Meihe Basin. Three types of sedimentary facies were identified in the Meihe Formation including alluvial fan, fan-delta and lake facies. Mid-fan subfacies of alluvial fan is developed in the bottom of Sandy-Conglomerate Member, and fan delta deposited in the Lower Coal-bearing Member. In the middle Mudstone Member, the mainly depositional environment is semi deep-deep lake. Lacustrine bog and shallow lake are developed in the Upper Coal-bearing Member and the Green-rock Member separately. It should be noted that, oil shale developed in the semi deep to deep lake, and coal layers deposited in the swamp. The organic matter content in mud-stone shows a decreasing trend after increasing upward, meanwhile the type of kerogen changes from type III to II2 and II1, then becomes to type III, and this caused the organic facies changes from E to A, and becomes E finally. Comprehensive studies show that, the base level, depositional environment evolution, organic matter supply and preservation conditions caused by tectonic and paleoclimate evolution result in different types and abundance of organic matter in mudstone. Due to the limit terrestrial debris supply, good preservation condition and explosion of higher plants, it is favorable for coal seam to occur in the stable shallow water area in the mid-stage of initial subsidence and the beginning of the basin shrinkage stage. While with the high lake productivity and good preservation conditions, oil shale deposited in the stable deep water area during the largest basin subsidence stage. [ABSTRACT FROM AUTHOR]

Clay mineral and inorganic geochemical characteristics of Eocene Huadian formation in Huadian basin have been found through X-ray diffraction, scanning electron microscopy and inorganic geochemical element tests. Clay minerals of Huadian formation of the study area contain montmorillonite, kaolinite and illite. The content of montmorillonite is the highest, and illite/smectite mixed layer is rare. Mn, Sr, P, Cd, Sr/Cu, B/Ga, Sr/Ba and Ca/(Ca+Fe) change with an exactly similar trend vertically, and Cu, Cs, Rb/Sr, V/(V+Ni) and 8Ce change with another similar trend. Clay minerals and inorganic geochemical evidence reveal the characteristics of paleoclimate and paleolimnology:the upper section containing coal and the lower section containing pyrite of Huadian formation belong to terrestrial sedimentary environment under the condition of warm and humid climate and low salinity ; the middle section containing oil shale belongs to frequently alternating freshwater and saltwater environment, alternating humid and drought climate. The largest expansion phase of lacustrine basin which was under reduction condition with alternating humid and drought climate and salinity stratification developed semi-deep lake and deep lake facies which formed oil shale; the shrinkage phase of lacustrine basin with warm and humid climate developed delta plain with peat swamp which formed coal. [ABSTRACT FROM AUTHOR]