Geochemical characteristics of terrigenous saline lacustrine shale in the Qianjiang Depression, Jianghan Basin, China: implications for controls on the formation of the inter-salt organic-rich shale

ABSTRACT The inter-salt shale of the Qianjiang Formation in Jianghan Basin is a typical and important continental saline lake shale oil system in China. To achieve a better understanding of the formation of the inter-salt organic-rich shale, the mineral, major and trace elements and organic geochemical characteristics (total organic carbon (TOC) and biomarkers) of the Eq34-10 shale (high TOC) and the Eq40-5 shale (low TOC) were studied. The results suggest that (a) the organic matter in the inter-salt shale was predominantly derived from endogenous lower aquatic organisms, algae and bacteria; (b) there was a hot arid palaeoclimate and a low degree of chemical weathering conditions during the deposition of the inter-salt shale; (c) the inter-salt shale deposited in an extreme restriction hyper-saline lacustrine and reducing environment, and the salinity of the palaeolake decreases with the increase of the input of terrigenous clastic materials; and (d) the inter-salt shale deposited under an arid and semi-arid climate condition with an intermittent warm and humid climate. In summary, lake palaeoproductivity under the control of palaeoclimate change is the key and direct factor controlling the development of inter-salt organic-rich shale of Qianjiang Formation in Jianghan Basin.