A comparative study of organic-rich shale from turbidite and lake facies in the Paleogene Qikou Sag (Bohai Bay Basin, East China): Organic matter accumulation, hydrocarbon potential and reservoir characterization

Organic-rich shale (ORS) was developed in various sedimentary facies in lacustrine basins, displaying different organic matter (OM) accumulation processes, hydrocarbon potential and reservoir characterizations. Taking the Paleogene Qikou Sag (Bohai Bay Basin, East China) as an example, the ORS from two typical sedimentary facies in lacustrine basins, i.e., turbidite and deep to semi-deep lake were comparatively studied, based on the integration of mineralogical, bulk geochemical, organic petrographic, isotopic and elemental geochemical, and shale reservoir analyses. Under the influence of turbidity currents, the Es32 ORS was formed in the paleo-lake characterized by low paleoproductivity, suboxic-oxic and freshwater conditions, and high detrital influx. However, high terrigenous OM input led to moderate TOC contents (avg. 1.58 wt%) and low HI values (avg. 193 mg/g TOC) in the Es32 ORS of turbidite facies. In contrast, the Es1z ORS of deep to semi-deep lake facies was deposited in the paleo-lake with high paleoproductivity, suboxic and brackish water conditions, and low detrital influx, which resulted in high TOC contents (avg. 2.77 wt%) and HI values (avg. 335 mg/g TOC). Combined with the analysis of hydrocarbon potential and reservoir characterizations, the Es1z ORS of deep to semi-deep lake facies could serves as main source rocks for crude oil, whereas a significant shale oil potential is likely to observe in the Es32 ORS of turbidite facies in areas with favorable maturity and high brittleness. Our results are conductive to the formation mechanisms of ORS from various sedimentary facies in lacustrine basins, and provide scientific guidance for hydrocarbon exploration in similar areas.