Sequence stratigraphy and seismic geomorphology application of facies architecture and sediment-dispersal patterns analysis in the third member of Eocene Shahejie Formation, slope system of Zhanhua Sag, Bohai Bay Basin, China

Abstract

An important hydrocarbon reservoir is hosted by the third member of the Shahejie Formation (Es3) in the Zhanhua Sag of the Bohai Bay Basin. Using lithology, wire-line logs and three-dimensional (3-D) seismic data in the southern slope of Zhanhua Sag, we demonstrated the utility of stratal slice images for analyzing facies architecture and sediment-dispersal characters of complex depositional systems in Es3. The Es3, a para-second-order sequence, can be subdivided into three third-order sequences (from base to top: SQ1, SQ2, and SQ3). The facies architecture was analyzed by using the seismic geomorphology approach based on 3-D seismic data. Sediments of the Es3 sequences were derived from the southern Chenjiazhuang Uplift via six major incised valleys, four of them associated with slope-break belt in the southern zone, and others in the southwestern zone. Seismic stratal slices reveal different characteristics of the channels and lobes between south and southwest from source to sink. On the basis of an integrated analysis of well log, core data, seismic facies based on multi-seismic attributes, three depositional environments (e.g., “fan-delta”, “shallow lacustrine” and “turbidite” facies) have been recognized. Stratal slices indicate that the depositional environments of these sequences evolved from gravel- or sand-rich fan delta and turbidite to lacustrine mud, and lastly to mixed sand-mud fan delta systems. The results of high resolution 3-D depositional systems analysis showed that the third-order sequence located in different systems tracts of the para-second-order sequence would contribute to their systems tracts with the different types and scales of depositional systems. Types of slope belts, subsidence rate, sediment provenance and their evolution jointly control the sediment-dispersal characters in deposition area. The proposed sediment dispersal patterns may aid in the prediction of potential reservoir distribution.