Sedimentology of tidal braided-river deltas on slope: a case study of the Paleogene Pinghu Formation in the western slope zone of Xihu Sag

The Paleogene Pinghu Formation in the western slope zone of the Xihu Sag is a key area for oil and gas exploration in clastic rock reservoir. During the deposition of the Pinghu Formation, this area was in a continental-marine transitional zone, and the division of its sedimentary environment and facies types remain disputable. To clarify these issues, the distribution pattern of sand bodies in this area and the sedimentation characteristics of the Pinghu Formation were systematically analyzed by combining drilling, logging, core samples, and seismic surveys. Results show that the Pinghu Formation substantiates braided river deltas and tidal flats, from which 10 sedimentary facies were identified: distributary channels, inter-distributary channels, subaqueous distributary channels, inter-subaqueous distributary channels, mouth bars, distal bars, prodelta mud, sand flats, sandy-muddy mixed flats, and mud flats. In terms of space, in the northwestern part of the study area, braided river deltas were mainly developed; in the central part, the deltas were affected by tides, resulting in the development of subaqueous distributary channels that were modified by tides; in the southeastern part, tidal flats were developed. In terms of time, the early stage of the Pinghu Formation was transgressive and tidal action was dominant, which enhanced the tidal impact on the delta and formed relatively large tidal flats. The later stage was regressive, controlled by mainly fluvial action, thus the tidal impact on the braided river delta was relatively small, and the delta advanced towards the southeastern part of the study area. This study provided geological basis for oil and gas exploration in the Xihu Sag.