Petrological and geochemical characteristics of coal and oil shale of Paleogene Lijiaya Formation in Huangxian Basin, Eastern China: Implication for evolution of symbiotic formation mechanism

Huangxian Basin is an important area for the production of coal and oil shale in China. The Paleogene Lijiaya Formation consists of five layers of oil shale and seven layers of coal interbedded within the basin. However, the research on the symbiotic metallogenic mechanism of coal and oil shale is relatively inadequate. Samples of oil shale and coal were selected and industrial quality, mineralogy, organic petrology, geochemistry, and biomarker tests were carried out to reveal the characteristics and formation mechanism of the symbiotic coal and oil shale. High-quality oil shale has the characteristics of high total organic carbon (TOC), high hydrocarbon generation potential (S 1 + S 2 ), and low quartz and high calcite content. Conversely, low-quality oil shale is characterised by low TOC, low S 1 + S 2 , high quartz and low calcite content, while coal has the characteristics of high TOC, high S 1 + S 2 , high quartz and low calcite content. The biomarker spectra of oil shale samples show an obvious single peak of C 27 , while coal samples show a multi peak pattern: C 23 , C 25 , C 27 and C 29 . The organic matter (OM) of high-quality oil shale comes mainly from bacteria and algae, mainly in the form of lamalginite, whereas the OM of low-quality oil shale is mainly derived from algae and terrestrial vitrinite, and the OM of coal originating from terrestrial higher plants vitrinite. Changes in paleoclimate, aquatic conditions and OM sources have helped form the symbiotic sequence of oil shale and coal. High-quality oil shale was formed in an anoxic-reducing environment during warm and humid paleoclimate with high lake productivity and salinity stratification during a seawater intrusion. Conversely, low-quality oil shale was formed during a warm and humid paleoclimate in a dysoxic environment, with a high terrigenous OM supply and weak salinity stratification. Coal was formed in cold-dry inclined paleoclimate and oxic freshwater swamp environment following seawater intrusion. These conclusions will improve the symbiotic metallogenic theory of coal and oil shale and contribute to the exploration and development of coal and oil shale in the Huangxian Basin and other similar basins.