Abstract:

Xihu Sag in the East China Sea Shelf Basin is an important petroliferous area along the coast of China. Exploration activities in the sag reveal complex distribution of (syndepositional) faults and sand bodies but the controlling factors on the distribution of the sand bodies are still unclear. In this study, core observation, logging and 3D seismic data are combined with fine sand body interpretation based on paleogeomorphology and seismic attributes to study the effect of synsedimentary faults on the sand bodies in the Pinghu Formation of the sag. According to the different development scale, the syndepositional faults can be divided into three levels, which serve to control the sag pattern, slope characteristics and sand body distribution, respectively. In addition, the controlling effect of the three levels of the syndepositional fault distribution patterns on the vertical sedimentation and plane extension of the sand bodies is analyzed. The results show that there are six fault combination patterns controlling the sandstone development in the study area, namely the forward fault-step type, forward fault-step-tilt type, forward fault-step overlying type, corner type, reverse fault step type, and composite type with both forward and reverse faults and steps. Of which, forward types facilitated the transportation of sand bodies to the center of the sag and enlarged the distribution of reservoirs within the sag, and reverse types had significant sand-rich effect and provided superior conditions for hydrocarbon accumulation. The six combination patterns are helpful in guiding the finding of vertical and horizontal enrichment zones of sand bodies, determining the distribution of sand bodies and predicting the locations of reservoirs.

Key words: syndepositional fault, reverse fault, sand body control mechanism, sand body distribution, Pinghu Formation, Xihu Sag, East China Sea Shelf Basin