--> Shallow-Water Deltaic Systems in Lacustrine Raoyang Depression, Bohai Bay Basin, China

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Shallow-Water Deltaic Systems in Lacustrine Raoyang Depression, Bohai Bay Basin, China


Thin, shallow-water deltaic sandstones are important reservoirs for future reserve growth in the Bohai Bay Basin and elsewhere. In comparison with well-known thick deltaic systems that are associated with clinoformal seismic configurations, these deltaic systems have been less recognized because of the lack of visible seismic clinoforms. In this study, we identified high-order (< 20 m thick) shallow-water deltaic depositional sequences by integrating sequence-stratigraphic correlation, core and wireline-log analysis, and seismic-amplitude stratal slices in the context of depositional facies and depositional history. In the Raoyang Depression, Sha-1 to Dong-3 members of the Tertiary were recognized as third-order LST, TST, and HST deposits in a late postrift lacustrine environment. Conventional core and wireline-log analysis revealed predominantly deltaic sediments (distributary channel sands, mouth sand bars, delta-front sand sheets, and bay muds and silts) followed by meandering fluvial deposits (point-bar sands and floodplain muds). The color of interbed shales in wells grades from gray at the base to red at the top, suggesting a relative change in water depth from deep to shallow. In dip seismic sections, shingled clinoforms were observed at the base, followed by discontinuous, subparallel reflections at the top, indicating a water-depth variation from at least 50 m to less than 5 m. On amplitude stratal slices, observed seismic geomorphologic patterns range from shingled, separate channel-form/lobate systems at the base, to amplitude-zoned but continuous channel-form/lobate systems in the middle section, to meandering and arch-shaped channel-form complexes at the top. As a tentative conclusion, although shallow-water deltas with subseismic clinoforms are difficult to interpret in conventional seismic correlation, they can be expressed as characteristic amplitude-zoning patterns on stratal slices. This observation has been applied to hydrocarbon exploration in the area, with a greatly improved, successful rate of drilling.