Paleocene sequence stratigraphy and depositional systems in the Lishui Sag, East China Sea Shelf Basin

Abstract

The East China Sea Shelf Basin (ECSSB) is a typical back-arc rift basin and is divided further by a series of sags and basement ridges. The Lishui Sag on the southwest margin of the ECSSB, is underlain by a productive, gas-prone, marine Paleocene section. In combination with sequence stratigraphy and seismic geomorphology, an integrated approach was applied to map the high-frequency sequences and systems tracts of the Lower E1m (57.2–56.5 Ma) and Upper E1l strata (60–57.2 Ma) by using well, two-dimensional and three-dimensional seismic data. Our approach involves documentation of seismic stratal geometries, seismic facies and core lithofacies. Seismic truncation, offlap, onlap and downlap geometries were used for identifying two depositional sequences with four systems tracts in the Upper E1l and Lower E1m strata. Two sedimentary facies were recognized from core and log data and identified as tidal sand ridges and delta mouth bar deposits. The depositional facies were imaged from a series of strata slices obtained by 3D survey. Five types of depositional systems were recognized based on the geometry change expressed on strata slices. In stratigraphic order these are (a) incised channels on the inner shelf and subaqueous fans on the middle shelf, (b) diffuse, elongated linear tidal sand ridges on the continental shelf and turbidity flows on the slope, (c) incised valley, prograding shelf edge delta on the continental shelf and shelf edge and delta front turbidites on the slope, (d) incised valley fills, fluvial aggradation and fluvial dominated mouth bar on the continental shelf, and (e) shoreface barrier bar and sand ridges that are parallel to the shoreline. This integrated analysis provides us an opportunity to evaluate the exploration significance in the stratigraphic framework and improve the accuracy of petroleum plays prediction.