Deep-water gravity flow deposits in a lacustrine rift basin and their oil and gas geological significance in eastern China

Abstract

The types, evolution processes, formation mechanisms, and depositional models of deep-water gravity flow deposits in a lacustrine rift basin are studied through core observation and systematic analysis. Massive transport of slide and slump, fluid transport of debris flow and turbidity currents are driven by gravity in deep-water lacustrine environment. The transformation between debris flow and turbidity current, and the transformation of turbidity current between supercritical and subcritical conditions are the main dynamic mechanisms of gravity flow deposits in a lake basin. The erosion of supercritical turbidity current controls the formation of gravity-flow channel. Debris flow deposition gives rise to tongue shape lobe rather than channel. Deep-water gravity flow deposits are of two origins, intrabasinal and extrabasinal. Intrabasinal gravity flow deposits occur as single tongue-shape lobe or fan of stacking multiple lobes. Extrabasinal gravity-flow deposits occur as sublacustrine fan with channel or single channel sand body. However, the nearshore subaqueous fan is characterized by fan of stacking multiple tongue shape lobes without channel. The differential diagenesis caused by differentiation in the nearshore subaqueous fan facies belt results in the formation of diagenetic trap. The extrabasinal gravity flow deposits are one of the important reasons for the abundant deep-water sand bodies in a lake basin. Slide mass-transport deposits form a very important type of lithologic trap near the delta front often ignored. The fine-grained sediment caused by flow transformation is the potential “sweet spot” of shale oil and gas.