Geomorphology, Sedimentology and Architecture of the Ancient Pearl River Delta System, Northern South China Sea

Abstract

Three-dimensional seismic slices, including the horizon or stratal ones, over one-hundred well logs and a small amount of drilling cores have been used to characterize the depositional elements that comprise the ancient Pearl River Delta System (PRDS). In total, four major types of sandy depositional elements in the hydrocarbon target interval Miocene Formation have been recognized, including the distributary channels, mouth bars, beach ridges, shelf sand sheets.

According to the dimension and internal architecture, three major types of distributary channels have been identified:(1)Trunk channels is typically wide, with an average width over 5 km. In seismic slice trunk channels usually show channelized feature and often cross-cut each other. Wells penetrating the channel belts prove that they have obvious blocky well log motifs. Medium channels are usually anabranch from the trunk channels, and the width ranges from 0.5 km to less than 2 km. The most prominent characteristic of medium channels is the presence of the lateral accretion deposits or point bars . The sinuosity of medium channels is obviously higher compared with the trunk channels. Tributary channels is relatively small in scale, commonly 100 m to 300 m in channel width. They display medium to high sinuosity, without evidence of lateral migration.

Unlike the fluvial channels which have sharp contact with adjacent strata, the recognition of mouth bars is especially difficult, primarily due to their interfingering with juxtaposed facies within the delta front environment. However, the determination of the location of the channels would greatly help to locate the mouth bars which typically form at the end of the terminal channels. Also, the dimension of the mouth bars would be closely related with the corresponding channels.

Although the beach ridges resemble the shelf sand ridges in plan view, they also have obvious differences. Beach ridges tend to form adjacent to fluvial deposits while shelf ridges are typically isolate in the shelf muds. Also, beach ridges are much more continuous in map view, which could reach hundreds of kilometers along the shoreline directions.

Shelf sand sheet is one of the most distal sandy elements parallel to shallow marine systems, and is mainly distributed in the outer shelf region. On seismic sections, it displays medium to high amplitude with a high continuity. In the map view, they tend to have irregular and sheeted shapes.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018