Integrated fluid inclusion analysis and petrography constraints on the petroleum system evolution of the central and southern Biyang Sag, Nanxiang Basin, Eastern China

Abstract

We combine integrated fluid inclusion analysis, petrography, and burial-thermal-hydrocarbon generation history modeling, to delineate multiple generation-expulsion-charge episodes within the central and southern Biyang Sag, Nanxiang Basin, Eastern China. Basin models suggest the main phase of oil generation occurred between the late Eocene and early Miocene. This period generally corresponds to the three oil expulsion episodes (ca. 37.5 Ma, 33.1 Ma, and 23.0 Ma) recorded in petroleum inclusions that are hosted in fibrous calcite veins in organic-rich shales. Based on fluorescence spectrum parameters and the API gravities of a set of crude oils used as a calibration, the oils expelled during the three expulsion episodes were determined to have been heavy, black and light oils. Fluorescence correlations indicate that the three types of oil inclusions found in the source rocks correspond well with those detected in the reservoirs.The first episode of heavy oil emplacement occurred prior to quartz overgrowth whereas the second episode of black oil emplacement was synchronous with quartz overgrowth. Ferrodolomite and ankerite cements were deposited, this being accompanied by a second phase of quartz grain overgrowth dissolution. The last episode of light oil emplacement was coincident with the precipitation of ankerite. By integrating fluid inclusion Th, burial-thermal models, and constraints from oil expulsion timing, the timings of the three episodes of oil charge were estimated to be 35.3 Ma, 31.9 Ma, and 20.8 Ma. Tectonic deformation related to horizontal compression facilitated trap formation; this is thought to be the reason behind the observation that only light oil related to the third charging episode accumulated in the reservoirs.