Reevaluating the source and accumulation of tight oil in the middle Permian Lucaogou Formation of the Junggar Basin, China

The middle Permian Lucaogou Formation in the Jimusar Sag, Junggar Basin, NW China, is a world-class example of fight oil accumulations in lacustrine sedimentary sequences. Previous studies have indicated that the main oil accumulation mechanism in this system is near-source accumulation within upper and lower "sweet spots" (i.e., reservoirs). Here we reveal the complexity of oil source and accumulation in this tight oil system is greater than previously thought. A set of mudstones are developed between the upper and lower sweet spots of the Lucaogou Formation and are thus termed the middle section mudstones. They have previously been little studied as few intervened reservoir rocks are developed within the sequences. Here we report their oil generation and contribution for the first time. The mudstones are 97-182 m thick and have total organic carbon contents of 0.67-12.31 wt%. The kerogen is mainly type II, with small amounts of types I and III, and is in the mature stage. These data suggest that the middle section mudstones have good hydrocarbon generation potential. Oil-source correlations reveal a complex history of tight oil migration and accumulation. The oils generated in the upper part of the middle section mudstones migrated northward to the basin margins and southeastward to the basin center. In contrast, oil generated in the lower part of the middle section mudstones migrated westward to the west-central and central regions. The oil migration resulted in complex tight oil accumulation, which reflects the reservoir heterogeneity. Our data suggest that such reservoir heterogeneity appears to be typical of tight oil accumulations in continental lacustrine basins. In the study area, future evaluation, exploration, and exploitation of tight oil should also focus on the middle section mudstones and associated sweet spots in addition to the commonly-acknowledged upper and lower sweet spots.