Multiple-stage migration and accumulation of Permian lacustrine mixed oils in the central Junggar Basin (NW China)

Abstract

The Permian lacustrine petroleum system in the central Junggar Basin, northwest China, has recently obtained exploration success. These accumulations are characterized by mixed oils and multiple-stage migration and accumulation. This process, however, has not been well constrained, limiting the understanding of the general laws of hydrocarbon migration and accumulation and making it difficult to define exploration strategies. Here, we reconstruct this process mainly based on oil-bearing fluid inclusion analyses including petrography, homogenization temperature and sequential extraction. Inclusion petrography shows that petroleum inclusions dominantly display yellow fluorescence, while intergranular free oils mostly display strong blue green fluorescence. This implies that the reservoirs seem to have experienced at least two stages of hydrocarbon charge from different sources and/or maturities. The sources/maturities are determined through reservoir sequential extraction analyses. The free oils are mixed and sourced from the Lower Permian Fengcheng Formation (P1f) and the Middle Permian Lower Wuerhe Formation (P2w), while the inclusion oils are also mixed and derived mainly from the P1f source. Both oils are characterized by moderate maturity. Homogenization temperature measurements of oil-bearing fluid inclusions, combined with reconstruction of reservoir burial and thermal history, not only further verify multiple hydrocarbon charging events but also characterize the timing, which indicates a three-stage petroleum charging model. During the first stage of the Late Triassic, Carboniferous reservoirs were charged by P1f-sourced oils, which were dysmigrated or remigrated to Jurassic reservoirs during the second stage (i.e., the Early Cretaceous). In contrast, the third stage from the Late Cretaceous represents another hydrocarbon charge event that is the primary oil accumulation sourced from the P2w rocks. According to this model, both primary and secondary oil accumulations contribute to the current region's exploration success, and provide future exploration targets.