Hydrocarbon generation and expulsion of the Fengcheng Formation in the Mahu sag, Junggar Basin, China: Implications for shale oil resource potential

The Permian source rocks in the Junggar Basin are widely developed, especially the Fengcheng Formation, which is the most significant source rock in the basin. However, due to insufficient research on the hydrocarbon generation (HG) and hydrocarbon expulsion (HE) characteristics of the source rocks, it is unclear whether a significant amount of retained hydrocarbons remain within shales. In general, the original organic matter abundance and kerogen type control hydrocarbon generation potential (HGP) and HE capacity in lacustrine shales. Therefore, the degradation rate method was used to establish the original organic carbon recovery model for different types of kerogen. Combined with the geologic and geochemical characteristics of the source rock, the HG, HE, and shale oil resource potential of the Fengcheng shale have been evaluated. We have found that the Fengcheng shale is mainly carbonate-type mudstone widely distributed with an average thickness greater than 100 m. The Fengcheng shale is composed of type II kerogen and reached the mature to high-mature thermal maturity stage, with the maximum original organic carbon exceeding 4.0 wt%. Meanwhile, the amount of retained hydrocarbons within shales is abundant according to the HGP model. Monte Carlo simulation finds that the shale oil resources of the Fengcheng shale are 23.30 × 108 t. Free oil resources account for 60%, reaching 13.75 × 108 t, indicating tremendous shale oil exploration potential.