Abstract:

To address the unclear sedimentary microfacies types and sandbody distribution characteristics in the Baoshi Formation-lower member of Pinghu Formation (hereinafter referred to as lower Pinghu member) in the Pingbei area of the Baochu slope belt, Xihu Sag, East China Sea Shelf Basin, the study employed a comprehensive approach combining paleogeomorphology, biological traces, trace element analysis, well-seismic integration, and modern depositional analogs to analyze the sedimentary microfacies types, spatiotemporal evolution, and trap models. The research indicated that the Baoshi Formation-lower Pinghu member in the Pingbei area was in the intense rift period, with a paleo-geomorphologic pattern characterized by deep depressions and high uplifts, exerting a strong control on the sedimentary system. The Baoshi Formation was sourced from magmatic rocks in the northern Hupijiao uplift, while sediment supply from the Haijiao uplift in the western lower Pinghu member gradually increased, forming a dual-provenance system. Trace element analysis indicated that the deposition of the Pinghu and Baoshi Formations occurred under an arid and hot paleoclimate in a generally suboxic, marine-continental transitional environment. Four third-order sequences were developed in the Baoshi Formation-lower Pinghu member, representing a progressive marine transgression. Integrated analysis of core facies, logging facies, biofacies, and ichnofacies revealed that the Baoshi Formation-lower Pinghu member developed three sedimentary facies (tidal delta, tidal flat facies, and marine facies) and nine microfacies (subaqueous distributary channels, sheet sands, mouth bars, tidal channels, sand flats, mixed flats, mud flats, interdistributary bays, and bay mud). The extensively developed tidal flat facies was the dominant sedimentary facies type. Dendritic flood and ebb tidal deltas, tidal channels, and tidal sand bars developed near the slope depression belt. Five sand-controlling models were identified in the Baoshi Formation-lower Pinghu member: graben-horst type, uplift-fault slope type, multiple fault slope type, transfer zone type, and flexural slope break type. Five trap types were summarized: graben-horst structural traps, uplift-fault slope structural traps, multiple fault slope structural-lithological composite traps, transfer zone structural-lithological composite traps, and flexural slope break lithological traps. Within the inner slope zone, the Pinghu and Baoshi Formations developed large-scale sand bodies under bay environments, which were prone to lithologic pinch-out controlled by flexural slope breaks. Tidal and wave reworking produced clean fine sandstones with strong compaction resistance, forming favorable “sweet spot” reservoirs. Overall, the inner slope zone possesses excellent accumulation conditions. The research findings provide clear directions for future exploration.

Key words: Xihu Sag, Pingbei area, rift period, source-sink system, exploration direction