Defining stratigraphic oil and gas plays by modifying structural plays: A case study from the Xihu Sag, east China Sea Shelf Basin

Abstract

The study focuses on an area of relatively mature oil and gas exploration located in the western slope of the Xihu Sag, East China Sea Shelf Basin. Three stratigraphic oil and gas plays are introduced to replenish the shrinking prospect inventory in the area. The reservoirs of these plays are traditional pay intervals of the Eocene Pinghu Formation and Baoshi Formation composed of tide-dominated delta front, tidal flat and delta front sand bar facies. By investigating conventional seismic data, seismic inversion volumes, and well log data, multiple tidal-influenced delta front channels, beach sand packages have been mapped out in different reservoir sand intervals of the Pinghu Formation and Baoshi Formation. We recognize four types of stratigraphic terminations: channel flank pinch-outs, axial pinch-outs of sand-fills, channel mouth sand bar pinch-outs, and onlap pinch-outs for the Baoshi Formation. Both the depositional facies and relatively low sand-shale ratio of the reservoir formations favor the occurrence of stratigraphic terminations. The closure mechanism of all these plays are similar to the traditional structure plays in the area: fault blocks or fault-bounded rollover anticlines, except that one or more closing faults are replaced by these stratigraphic terminations. Several small-sized closures should exist but the relatively large ones are centered around a major intra-slope rise, raising the exploration potential in this maturely explored area. The new exploration drilling campaign has so far tested the play of delta front channel facies, with three out of four wells commercially successful. These play concepts open up a novel approach to identify new traps in the study area and many other rift basins with similar settings in eastern China.