Architecture and depositional processes of a submarine channel during the late Miocene in the Yinggehai basin, northwestern South China Sea

Abstract

Submarine channels are the most extensive sediment transport systems on Earth. They transport sediment over hundreds of kilometers and form sediment deposits in the deep-sea, which can act as hydrocarbon reservoirs. During the Late Miocene, a sinuous submarine channel with sediment processes was discovered in the Yinggehai Basin, northwestern South China Sea. However, process interpretations of the factors that influence the stacking patterns, distribution, and morphology of these sinuous channel deposits in this region are not well understood. Based on the interpretation of high-resolution 3D seismic data and exploration well data, and insights from flow dynamics, we analyzed the factors influencing channel morphology, the architecture of the channel, and its associated geomorphic evolution. The channel can be divided into three sections (Sections Ⅰ, Ⅱ, and Ⅲ) in terms of its planform, and into two seismic units (Unit 1 and Unit 2) in terms of its vertical structure. The sinuous submarine channel originated from a combination of inherent structural features from secondary faults and strong erosion from turbidity currents, which occurred due to a sharp decline in sea level during the Late Miocene. Changes in the channel's orientation led to erosion and channel-margin failures, causing an increase in channel width and a decrease in margin gradient. The distribution and stacking patterns of the deposits are greatly influenced by the provenance, sea level changes, and the morphology of the channel. In Sections Ⅰ and Ⅲ, a river-like secondary flow with a limited lateral density gradient leads to vertical deposits. In Section Ⅱ, changes in the channel's orientation altered its morphology and induced topographic forcing, producing a strong river-like secondary flow. This flow consistently caused sediment to accumulate on the inner bank and erode on the outer bank, leading to lateral stacking of deposits in Unit 2. During the filling of Unit 2, diapiric activity uplifted the strata, resulting in thinner deposition in Section Ⅲ, while deposition thickness increased in Section Ⅱ with sufficient provenance. Unit 1 of Section Ⅲ displays desirable properties for a hydrocarbon reservoir, including thick sandstone with good sorting and roundness. This study provides valuable insights into the diverse architectures of spatially and temporally linked submarine channels, and establishes a correlation between the evolutionary process of the channel and the stacking patterns of its constituent elements.