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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]

Lacustrine laminar coal is an unusual and rare sedimentary deposit, especially when it coexists with oil shale; this combination has special significance for indicating the evolution of the paleoenvironment. The coal-bearing and oil shale strata of the Early Jurassic Badaowan Formation in the Dachanggou Basin, Xinjiang Uygur Autonomous Region, present an ideal example to reveal the evolution of high-frequency environmental changes in the Early Jurassic. Based on bulk geochemical data, the paleoenvironmental, paleoclimatic and genetic differences between lacustrine laminar coal and oil shale were studied. The Badaowan Formation oil shale has high organic matter abundance and hydrocarbon-generating potential and contains type I and type II 1 organic matter. However, compared with oil shale, interlayered laminar coal has higher organic matter abundance and hydrocarbon-generating potential, and the type of organic matter is II 1. Various environmental geochemical indicators suggest that oil shale formed in a warm and humid climate and was deposited in fresh water under oxic-suboxic conditions and that algae were the main source of organic matter, followed by terrigenous organic matter. Laminar coal formed in relatively high-temperature periods and accumulated in suboxic-euxinic saline lakes. The source of organic matter was mainly higher plants or hydrophilic plants around the basin, followed by algae. Short-term thermal events swamped the lake margins and increased the input of terrigenous organic matter, which promoted the formation of thin laminar coal interbedded with oil shale. • The geochemical characteristics and sedimentary environment of silty mudstone, lacustrine oil shale, and laminar coal are compared. • The source of organic matter of lacustrine oil shale and laminar coal is revealed. • The genetics of lacustrine oil shale and laminar coal are analysed. • The Lacustrine laminar coal is formed in a drier and hotter period, which is indicative of short-term thermal events. [ABSTRACT FROM AUTHOR]

Abstract The Eocene was an interval of climate evolution, when the extreme high temperatures of the earliest Cenozoic gave way to a global cooling trend. Fine-grained lacustrine sediments of Eocene age from northeastern China represent an excellent archive to study long-term environmental and climatic changes at high-resolution. The Huadian Basin is a Cenozoic, fault-bounded basin in Jilin Province, northeastern China. Though small in area, it contains numerous oil shale- and coal-bearing layers, which provide an abundant Middle to Late Eocene palynoflora. Eighty-five palynomorph genera and one hundred palynomorph species were identified in the Huadian Formation, allowing to distinguish 4 palynozones. The nearest living relatives of the recovered palynomorph taxa suggest the presence of subtropical evergreen and deciduous broad-leaved forests during deposition of the Huadian Formation. Using the coexistence approach (CA), we calculated seven paleoclimatic parameters, which indicate a subtropical warm and humid climate with a mean annual temperature above 13.6 °C and a mean annual precipitation of at least 887 mm. However, the climate became cooler and drier during the transition from the lower Oil Shale Member to the upper Carbonaceous Shale Member. Palaeoclimatic data from the various Middle and Late Eocene basins of northeastern China (i.e., the Huadian, Fushun, Yilan, Hunchun and Jishu basins) were compared with data from North American and European basins. Middle Eocene climate in the Northern Hemisphere was generally warm and humid, and the mean annual temperature and precipitation in northeastern China were similar to those in North America, though slightly lower than those of Europe. Similar trends in temperature and precipitation on different continents support the existence of a global Mid-Eocene cooling event. This paper is the first systematic study of Middle to Late Eocene seasonality in the Huadian Basin. Our results show an apparent differentiation in both temperature and precipitation between summer and winter, supporting the view that the East Asian Monsoon was established in northeastern China already during the Middle to Late Eocene. This Middle and Late Eocene monsoon was likely driven by the development of Pacific trade winds and an alternating, seasonal high- and low-pressure system over Siberia. The climatic changes observed in our dataset, especially the precipitation changes, appear to have played an important role in controlling oil shale formation in the Huadian Basin. A warmer and more humid climate, with abundant precipitation and high subsidence rates, formed a deeper Huadian depositional lake. High productivity and stable stratification provided the abundant organic matter and favorable preservation conditions necessary for oil shale accumulation. Highlights • The Middle-Late Eocene palynoflora were firstly systematically studied in Huadian Basin. • We quantitatively estimated the Middle-Late Eocene climate based on CA method. • A globally cooler phase was recognized in the Middle Eocene. • Climate seasonality in the Middle-Late Eocene supports the EAM was already prevalent in NE China. • Changes in climate, especially precipitation, play an important role in oil shale formation. [ABSTRACT FROM AUTHOR]