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

•Massive coal and oil shales were developed in Middle Jurassic Qaidam Basin.•Flourishing conifer and restricted swamp environment controlled coal deposition.•Moderate climate aridification promoted the formation of high-quality oil shale.•Sedimentary evolution is the trigger for the transition from coal to oil shale.