Shallow gas and focused fluid flow systems in the Pearl River Mouth Basin, northern South China Sea

Abstract

Analysis of three-dimensional seismic data and multibeam data from the Pearl River Mouth Basin in the northern South China Sea, reveals numerous focused fluid flow features and associated widespread shallow gas from 500m to 2000m water depth. Shallow gas is usually indicated by acoustic anomalies, such as enhanced reflections, acoustic turbidity, and acoustic blanking. Two types of tectonically and stratigraphically controlled fluid flow related systems are identified. The first type can be observed in the deep strata, including gas chimneys, mud diapirs, mud volcanoes, pipes, and large normal faults, which are usually located over basement highs and are related to overpressure mainly caused by gas generation. The focused fluid flow structures serve as pathways for upward migration of thermogenic fluids from source rocks or gas reservoirs. The second type is a shallow fluid flow system consisting of minor normal faults, migrating canyons, mass transport deposits, contourite, and pockmarks. The minor faults and migrating canyons provide pathways for fluids of thermogenic origin transported by the deep fluid flow system and of biogenic origin generated in the shallow succession. The mass transport deposits and contourite mainly serve as caprock. Small-scale fluid seepage is observed at the modern seabed expressed as pockmarks located above shallow gas accumulations. The distribution of shallow gas is controlled by the combination of these two systems. The focused fluid flow features and shallow gas are poorly studied in the study area and we propose a 3D model of focused fluid flow and shallow gas distribution that can be used more widely in other passive and active continental margins. Our results are important to the understanding of resources (hydrocarbon and gas hydrate) exploration in such a petroliferous basin and they must be taken into account when assessing seabed stability.