Seismic geomorphology of shallow-water lacustrine deltas in the Paleocene Huanghua Depression, Bohai Bay Basin, eastern China

Abstract

The most important sedimentary facies comprising shallow-water lacustrine deltas are distributary channels, and these form abundant ribbon-shaped sand bodies that are excellent petroleum reservoirs. However, distributary channel sandstones are notoriously difficult to correlate and characterize in the subsurface based on well data alone. Here, we show how seismic geomorphology can be used to map lacustrine deltaic sedimentary successions in the First Member of the Kongdian Formation within the Zilaitun Oilfield, Huanghua Depression (China). Using core and wireline logs, this study identified distributary channels as the dominant sedimentary facies. Together with mottled gray-green or maroon mudstones and locally shingled progradational seismic reflection architecture, this indicates that deltas were built in a weakly-oxidized shallow-water lacustrine environment. Next, core observations were linked to seismic data to produce a successive series of stratal slices and root mean square amplitude attribute (RMSAA) maps. The RMSAA maps allowed distributary channel, mouth bar, and beach bar facies to be identified and mapped across the study area. Two patterns of distributary channels emerged from the data: (1) a distributive pattern with a lower concentration of distributary channels; and, (2) an anastomosing deltaic pattern with a high concentration of bifurcated distributary channels. Based on the seismic geomorphological analysis we show that the succession records three phases of delta evolution - retrogradation, aggradation, and progradation - which combine to form 3rd-order sequences. From a petroleum reservoir point of view, the distributive pattern yields narrower, thicker, and less connected sand bodies than the anastomosing pattern. This is consistent with observations from modern shallow-water lacustrine delta systems, providing independent support for the interpretation and highlighting the effectiveness of using seismic geomorphology.