Abstract:

The Xihu Sag is a hydrocarbon-rich sag with high oil and gas exploration value in East China Sea Shelf Basin. In recent years, continuous breakthroughs have been achieved in the exploration efforts of the Eocene Pinghu Formation of Xihu Sag, making it an important target for oil and gas exploration and development. However, previous studies were mostly carried out on a macroscopic regional scale with scattered data, resulting in many controversies in the academic community regarding the depositional system of the Pinghu Formation, which seriously affects the accuracy and efficiency of subsequent exploration and development. Focusing on the early deposition of the Pinghu Formation in the Pingbei area of Xihu Sag, this study comprehensively utilized data including core, well logging, and seismic data to conduct an in-depth investigation of the depositional system, aiming to clarify depositional characteristics and establish a depositional model. This study divided the lower Pinghu Formation into two third-order sequences: Sequence 1 (SQ1) and Sequence 2 (SQ2). Both exhibited significant tidal depositional characteristics, with diagnostic depositional structures including bidirectional cross-bedding, reactivation surfaces, double mud layers, and flaser-wavy-lenticular tidal composite bedding. During the SQ1 period, the study area exhibited a typical barrier coastal depositional environment. The NE-SW-trending barrier sand bars divided the area into two parts. A lagoon-tidal flat system was formed on the west side, where depositional microfacies such as tidal channels and delta fronts developed. On the east side, there was an open coastal system, where strip-shaped longshore sand bars developed parallel to the barrier island. During the SQ2 period, affected by regional transgression, the development of the barrier island-lagoon system was suppressed, and the depositional environment transformed to a tide-dominated open tidal flat, completely developing depositional sequence of sand flat-mixed flat-mud flat. Affected by tidal action, NW-SE trending tidal channels developed in the intertidal and subtidal zones, and tidal sand bars and sand flats also developed in the subtidal zone. Furthermore, through a detailed analysis of sand body types, shapes, and distribution patterns, combined with the depositional environment and classic depositional models, a barrier coastal-tidal flat composite depositional model was established. The establishment of this model not only provides a basis for understanding the depositional evolution process of the Xihu Sag, but also offers guidance for oil and gas exploration and development from the slope zone to the sag area in Xihu Sag.

Key words: Xihu Sag, Pinghu Formation, tidal flat, barrier island, depositional model