Potential of subtle hydrocarbon reservoirs in paleo-valleys in the Pingxi area of Xihu Sag

The Pinghu Oil and Gas Field in the Xihu Sag, after many years of development, has made great contributions to the oil industry in offshore China. However, it now faces severe challenges of insufficient reserves. Pingxi area, the west part of the oil and gas field, as a new territory with large area and low exploration degree, is expected to be a potential area for discovery of new resources. Based on 3D seismic data and paleogeomorphological studies, several paleo-valleys have been found from south to north in the area, which have favorable background for the formation of subtle oil and gas reservoirs. Comprehensive analysis of paleontology, trace elements, logging and cores in addition to seismic survey data suggest that there developed multi-stage lowstand fan deltaic systems in the early Eocene Pinghu Formation, which may gradually fill up the basement from bottom to top to become favorable reservoir facies belts for oil and gas accumulation. With the rise of sea level, transgressive tidal flat mainly composed of stable mudstone deposits settled down on the fan delta to form good reservoir-seal assemblages. The Pingxi area is fortunately located on the path of fluid migration due to its relatively high landform. The lower part of the fan delta is characterized by well-developed oil source faults and sufficient oil and gas supply, while the root of the fan delta above the ancient shoreline is close to source rocks which is high in argillaceous materials, poor in sorting, and strong in compaction. It is obviously different from that of the fan delta front, and may provide excellent sealing conditions. The valley-fan-fault coupling is favorable for the formation of fan type of subtle reservoirs, which are of significance to the sustainable development of Pinghu Oil and Gas Field in the future.