Formation and evolution mechanisms of coal and oil shale from the Middle Jurassic Shimengou Formation, northern Qaidam Basin, China

Abstract

Thick coal and oil shale layers are present in the Middle Jurassic Shimengou Formation of northern Qaidam Basin (NQB). However, the formation and evolution mechanisms of these coals and oil shales remain unclear. In this study, these mechanisms were revealed through sedimentological and geochemical analyses of the coal- and oil shale-bearing interval from Shimengou Formation of NQB. The results show that the coals were formed in a swamp with flourishing conifer plants under a humid climate, which greatly enriched the organic matter (OM) supply during coal formation. The restricted swamp environment with suboxic and brackish water favored OM preservation during peat accumulation. With rapid subsidence, the swamp-shallow lake was evolved into deep and semi-deep lake sedimentary system, in which the oil shales were deposited. Lake expansion resulted in a more reducing water column and more lake plankton input during oil shales deposition. Disturbance of arid climate triggered lake salinization along with water column stratification and salt-tolerant algal blooms during the late deposition of Shimengou Formation. The algal blooms and anoxic bottom water greatly promoted the formation of oil shale with high OM richness, hydrocarbon potential and excellent kerogen types. Therefore, the lake expansion caused by rapid subsidence of basin triggered the transformation from coal to oil shale formation, while the climate aridity during oil shale deposition further promoted the quality of oil shale. This study not only reveals the formation and evolution mechanisms of coal and oil shales from Shimengou Formation in QNB, but also presents a typical sample for the prediction of coal and oil shales in the other similar Mesozoic-Cenozoic faulted basins in China.