Abstract:

Source rock evaluation in offshore basins is crucial to the success of marine oil and gas exploration and also a key scientific problem to be solved urgently. Recently, a great breakthrough has been made in the Yangjiang Sag, Pearl River Mouth Basin revealing its excellent exploration potential in marine oil and gas. However, it is difficult to objectively evaluate the hydrocarbon generation potential of source rocks by seismic data alone when there are few exploratory wells in the study area. In order to determine the hydrocarbon generation potential of the Wenchang Formation-the primary hydrocarbon generating layer of the Yangjiang Sag, 4-dimensional sequence-stratigraphic simulation (Bandlands) is conducted to reconstruct the paleo-topography of the Yangjiang Sag during the Paleogene. The predicted paleoenvironmental characteristics better reveal the corresponding relationship between the paleo-topography and the sedimentary systems of the Wenchang Formation, and demonstrate the paleotopographic background of the Wenchang Formation has important impacts on the sedimentary development of source rocks. According to the analysis of sedimentary systems, the source-to-sink characteristics of the Wenchang Formation are summarized, which show that the sedimentary period of the Lower Wenchang Formation is the most favorable period for the source rock development. Combined with the paleo-topographic features, degree of source rock development (mainly the thickness and area of semi-deep lacustrine facies), connection status between source rocks and oil-source faults, and match between source rocks and shallow traps, the source rocks in each subsag are divided into three categories: high-quality source rocks (Enping sag 20, 21), relatively good source rocks (Enping subsag 27), and general source rocks (Enping subsag 19, Yangjiang subsag 24). On the basis of source rock distribution prediction and core analysis, combined with geological data and paleotopographic simulation results, the sedimentary burial history, thermal history and hydrocarbon generation history of source rocks in the Lower and Upper Wenchang Formations are analyzed, and the hydrocarbon generation potential of source rocks in the Wenchang Formation are simulated using PetroMod. The simulation results show that the source socks of the Lower Wenchang Formation has greater hydrocarbon generation strength than the Upper Wenchang Formation in the Enping subsag 20, 21, 27, indicating the Lower Wenchang Formation layer has the best hydrocarbon generation potential of source rocks, and the eastern and southern Yangjiang Sag (Enping subsag 20, 21, 27) have the best exploration potential. Source rock evaluation based on peleo-topographic analysis and source-to-sink characteristics is very important for guiding marine oil and gas exploration.

Key words: Yangjiang Sag, source-to-sink characteristics, 4-dimensional sequence-stratigraphic simulation, basin simulation, source rocks evaluation