Thermal maturation, hydrocarbon generation and expulsion modeling of the source rocks in the Baiyun Sag, Pearl River Mouth Basin, South China Sea

ABSTRACT The Baiyun Sag plays a role as the most important, largest deepwater target in petroleum exploration across the northern side of the South China Sea. Three sets of primary source rocks are developed in the Baiyun Sag: Early Eocene middle-deep lacustrine source rock (EMD), Late Eocene-Early Oligocene inshore-shallow lacustrine source rock (OIS), and Late Oligocene transitional source rock (LOT). A quantitative 3D model of the Baiyun Sag was constructed by using a stratigraphic framework based on 3D seismic interpretations and lithology logging data of 66 wells. The geothermal parameters of the 3D model were determined and calibrated based on the measurements of seafloor temperature, formation temperature, and vitrinite reflectance from the wells. The source rock property parameters of the EMD, OIS, and LOT in the 3D model were set according to the results of the total organic carbon test and Rock-Eval pyrolysis analysis of the well samples. The thermal maturation, hydrocarbon generation and expulsion histories of the EMD, OIS, and LOT were reconstructed to study how the evolution of source rocks influenced the current oil accumulation. Modeling results show significant differences among evolutions of the EMD, OIS, and LOT in both vertical and plane directions. The majority of the EMD reached the threshold of hydrocarbon generation (Ro = 0.5 %) at about 45 Ma and kept in the oil window (Ro = 0.5-1.3 %) from 45 to 15 Ma. The majority of the OIS and LOT was not capable of generating hydrocarbon until 35 Ma and 20 Ma, respectively, and have not entirely passed the oil window at present. The EMD, OIS, and LOT in the south and east region of the Baiyun Sag matured faster than the ones in the north and west region. The total oil generation quantities of the EMD, OIS, and LOT are 3772.51 × 108 t, 585.93 × 108 t, and 517.83 × 108 t, respectively; and the total oil expulsion quantities of the EMD, OIS, and LOT are 1246.04 × 108 t, 47.85 × 108 t, and 18.49 × 108 t, respectively. The oil generated and expelled in the Sub-sag B1 accounts for 82 % of the total oil generation and 96 % of the total oil expulsion in the study area, respectively. However, only 4 % of the oil originated from the EMD, and 3 % of the oil generated in the Sub-sag B1, was expelled after the deposition of primary reservoir rocks (23.8-0 Ma). On the contrary, 69 % of the oil expulsion of the OIS and all of the oil expelled from the LOT took place after 23.8 Ma. The EMD in the Sub-sag B2, the OIS in the Sub-sag B1 and B4, and the LOT in the Sub-sag B1 are favorable to current oil accumulation due to their high proportions and large quantities of the oil expulsion after the deposition of primary reservoir rocks.