Quantitative characterization of the depositional system in Gas field A, Pinghu slope belt, Xihu Sag and its bearing on periodicity of sea level changes

The Pinghu Formation in the Pinghu slope zone is one of the most important oil and gas exploration targets and gas producers in the Xihu Depression of the East China Sea. The key to improve the accuracy of reservoir prediction should be put on the right clarification of sedimentary background, evolution pattern and the mechanism of deep reservoir preservation. In this paper, the depositional systems of the Gas field A are quantitatively characterized with coring and well logging data. Our results suggest that the Pinghu Formation of the Gas field A consists of three depositional systems, i.e. the tidal delta system, tidal-river delta system and river delta system in ascending order. In addition to it, the periodicity of sea level fluctuation of the Pinghu Formation is analyzed by means of wavelet transform and spectrum analysis upon the global sea level changes or so-called Milankovitch cycles. Seven transgression events are identified, among which the P7 is the largest, and each cycle lasted for a time span of about 405 ka. Based on this, the isochronal sequence stratigraphic frameworks of the gas field are established. Finally, on the basis of high-frequency sequence framework and evolutionary pattern, the distribution characteristics of sand bodies in the layers of P7-P5 are carefully analyzed. Combined with the vertical variation in reservoir physical properties, it is considered that the reworking of sand bodies by tidal activities is critical important to the enhancement of the physical properties of the sandstone reservoirs. Therefore, the tidal sand bodies in the subtidal zone are the most favorable targets for oil and gas exploration.