The sedimentary characteristics of tidal delta and tidal flat in transgressive system tract of Pinghu Formation in Pinghu area, Xihu Sag

The Pinghu Formation of Eocene in the Pinghu area of Xihu Sag is a set of transitional deposits consisting of alternative marine and non-marine depositional systems, which are characterized by thinly interbedded sand, mud and coal seams of deltaic-tidal transitional facies. Due to the complexity of sedimentary structures and the diversity of sedimentary microfacies, it has remained controversial among scholars about their origin and depositional systems. The uncertainty of sedimentary system restricted oil and gas exploration and development. Based on the core, logging and other analysis data from the Pinghu Formation, this paper defined the typical facies types and described their sedimentary characteristics for the P7 layer formed in the transgressive domain in the first and second stages of Pinghu Period. By means of seismic sedimentology, typical tidal delta is recognized and the depositional characteristics of tidal delta in the transgressive summarized. The sandbodies of tide-dominated delta are strongly reformed by tide. There develop such typical sedimentary structures as double clay layers, reaction surface, rhythmic bedding and tidal bundles. The tide dominated delta is mainly composed of the subfacies of inner and outer deltaic fronts, with deltaic plain locally. During the transgression period, the sea level rose rapidly, the delta front was strongly reworked by tide, and as the result, the sand bodies dominated by tidal bars and tidal channels were formed. It is believed that the establishment of tide-dominated deltaic sedimentary model in this paper may provided a clue and guideline for further exploration and development of oil and gas in the Pinghu Formation of the Xihu Sag.