Factors controlling organic-rich shale development in the Liushagang Formation, Weixinan Sag, Beibu Gulf Basin: Implications of structural activity and the depositional environment

Abstract

The mechanisms of lacustrine organic-rich shale formation have attracted attention due to its association with global shale oil and shale gas exploration. Samples of general-quality and excellent-quality source rocks, and oil shale from the Beibu Gulf Basin were analyzed to investigate their organic geochemistry, palynofacies, and trace elements. Hydrocarbon potential was higher in the oil shale (29.79 mg/g) than in the general-quality source rock (3.82 mg/g), and its kerogen type was I-II2. Hydrogen-rich liptinite (cutinite and sporinite) components derived from terrigenous higher plants provided most of the hydrocarbon potential of excellent-quality source rock and oil shale. Under the influence of depression-controlling fault activity, a deeper subsidence center promotes the deposition of excellent-quality source rock and oil shale in brackish-hypoxic bottom water. A shallower subsidence center, due to subsag-controlling fault activity, promotes the formation of excellent-quality source rock under fresh-brackish and weak oxidation-weak reducing conditions. The local uplift and shallow-slope led to the formation of general-quality source rock, under freshwater weak-oxidation conditions. A model was established for organic matter (OM) accumulation in organic-rich shales, accounting for fault activity, terrigenous hydrogen-rich OM, and the preservation conditions, to predict the development of excellent-quality source rock from areas with low levels of exploration.