--> Seismic-Based Sedimentary Episode Identification for Overlapping Compound Sedimentary Bodies in Continental Lacustrine Rift Basins: A Case Study from the Bohai Bay Basin, China

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Seismic-Based Sedimentary Episode Identification for Overlapping Compound Sedimentary Bodies in Continental Lacustrine Rift Basins: A Case Study from the Bohai Bay Basin, China

Abstract

Abstract

Stratigraphic reservoirs, differing from structural reservoirs, are characterized by rapid variations in sedimentary facies, thinner beds and highly scattered sediment-dispersal patterns. Therefore, it is vital to identify independently distributed sedimentary bodies and clarify their contact relationships from the overlapping compound sedimentary bodies. In this study, we took the Offshore Bohai Bay Basin as a study case, to investigate the sedimentary episodes of the overlapping compound bodies using established seismic-based methods. The maximum provenance direction (MPD) of the independently distributed sedimentary body is always characterized by the direction of the fastest water flow velocity, accompanied with the highest sediment transport and accumulation rates. The sedimentary body along the MPD should thus possess: a) the largest progradational reflection angle; b) the furthest transport distance; c) the thickest sediment bed; d) mound seismic reflection configuration perpendicular to the MPD. For overlapping compound sedimentary bodies, there exists differences both in external form and internal reflection configuration of the seismic facies for superimposed sedimentary bodies since the provenance orientation and cycles differ at each depositional stage. Individual sedimentary bodies can be extracted from the compound bodies through the changes of: a) superimposed relationships along the strike cross sections; b) prograding types along the dip cross sections. Based on the established seismic-based methods for sedimentary episode identification, we recognized a total of 7 sedimentary bodies and delineated each depositional stage. Four evolution phases were recognized for the Second Member of the Dongying Formation in the study area. During the first phase, at the early stage of HST, four restricted deltas were deposited in the first slope-break area of the study area. The depositions (BD1, BD2, and BD3) of other three stages were deposited in the second slope-break area of the study area. For the second phase, the lake level fell gradually, forming a braided delta named BD1. Afterwards, BD2 and BD3 were developed successively in the third and fourth phases. The established episode identification methods based on 3-D seismic data complement the methodology based on well data which has an extremely-high vertical resolution but barely any lateral resolution, and should serve as a useful reference for similar studies.