AAPG Geoscience Technology Workshop

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Geology and petroleum systems of the East China Sea Basin

Abstract

The back-arc East China Sea Basin lies on the extended continental crust at the leading edge of the Eurasian plate. It comprises the East China Sea Shelf Basin in the west and the Okinawa Trough in the east, separated from each other by the Diaoyudao Paleo-uplift. Several oil and gas fields and oil- and gas-bearing structures have been discovered in the East China Sea Shelf Basin since the 1970s. Despite extensive petroleum exploration in the area, the geological evolution and petroleum geology of the basins remain poorly documented. The basin is also a politically sensitive region because China and Japan have been in stalemate over a maritime boundary dispute affecting potential oil and gas rights in the area. In this study, we describe and subdivide the basins according to their tectono-stratigraphic evolution. The basins are Cretaceous to Tertiary in age and pass through a number of typical tectonostratigraphic phases or basin cycles, which include extensional rifting, post-rift sag and local inversion. In order to distinguish between different phases of deformation in basin development, standard basin evolution patterns related to geodynamic drivers are identified as a first step. The basic unit, the basin cycle, consists of the sediments deposited during one tectonic episode. Most rifted basins evolve through more than one cycle, and include episodes of extension, subsidence and compression. The depositional sequences belonging to the cycles, by definition, represent distinct phases of the basin history, and are often bounded by regional unconformities triggered by tectonic events. Each sequence may contain one or more fully developed cycle of sedimentation, in which source rocks, reservoirs and seals may be present. On the basis of this we identify standard patterns in the sedimentary sequences that characterize the area and its tectonic evolution, and attempt to link them with the known petroleum systems. This we achieve by characterizing them and grouping them into basin cycle-related petroleum system families or types (PSTs). Finally, we examine the development of plays within the petroleum systems in the context of the tectono-stratigraphic evolution and identify groups of sub-basins with similar geological history and therefore potentially similar petroleum prospectivity (Figure 1). We believe that this process allows us to predict which petroleum systems and play types might be expected to occur in underexplored basins or parts of basins where subsurface data are limited. In particular, by comparing characteristic basin cycles and their characteristic PSTs (if present), we can make comparisons with similar basins elsewhere, potentially extending the analogues we can bring to bear.