Characteristics and genesis of lacustrine laminar coal and oil shale: A case study in the Dachanggou Basin, Xinjiang, Northwest China

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 II1 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 II1. 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.