Mudstone dominated lacustrine deltaic: Lower Cretaceous Damoguaihe Formation, Hailaer Basin, China

Abstract

Classic Gilbert-type deltas (GTDs) of both lacustrine and marginal marine settings are usually dominated by coarse-grained sandstone and conglomerate, along with gravity-follow deposits. Like other types of deltas, the sediments of GTDs have been traditionally divided into delta plain, delta front and prodelta belts that transit basin-ward from flat-lying topset through inclined foreset to tangentially-flat bottomset accumulations, respectively. The major difference between GTDs and other types of river-, tide- or wave-dominated deltas is the relative coarser and steeper foreset accumulations of the GTDs. The Early Cretaceous Damoguaihe Formation of the Hailaer Basin was deposited in a lacustro-deltaic system. Outcrop, seismic and well-data analyses provide stratal geometric pattern that resembles Gilbert-type deltaic setting but with foresets atypical to the GTDs. The formation preserves two fundamental characteristics: oblique-tangential stratigraphic pattern depositional setting and mudstone-dominated foreset lithology which does not fit the description of foresets of common Gilbert-type deltas. Thus, we envisage that the Lacustro-deltaic successions of the Damoguaihe Formation represent atypical Gilbert-type accumulations. We present here depositional properties that are key to hydrocarbon exploration, and whether volumetrically-significant sand bodies occur within the Damoguaihe Formation as it has been discussed in previous studies. This study endeavors to provide detailed lithologic description of the Damoguaihe Formation and its depositional setting as atypical Gilbert-type delta dominated by mudstone instead of the relatively well-documented sandstone dominated types. This study also sheds light on potential exploration risks that can be posed by fine-grained lithologic aspects of the formation, particularly in frontier areas where little data are available.