Shallow gas and gas hydrate accumulations influenced by magmatic complexes in the Pearl River Mouth Basin, South China Sea

Abstract

We analyse occurrences of gas hydrates and free gases above a 1200 km2 complex of igneous volcanoes and sills using newly acquired three-dimensional (3D) seismic data in the Pearl River Mouth Basin of the South China Sea. In strata above volcano summits, we recognize three enhanced reflection layers at different stratigraphic levels, including discontinuous Bottom Simulating Reflectors (BSRs) and pluming BSRs. We also show that the lateral tips of eight sills, which at times appear to terminate against supra-sill faults, underlie gas chimneys linked to gas hydrate and free gas in shallower strata. These supra-sill faults and gas chimneys, as well as supra-volcano faults, provided pathways for upward fluid (gas) migration from deep sources. Given the spatial relationship between volcanoes, sills, free gas, and gas hydrates, we suggest that the igneous structures focused fluid migration into overlying faults and gas chimneys. We also explore whether higher geothermal gradient at the summits of the volcanoes caused the base of gas hydrate stability zone to be shallower, forming a pluming BSR. Our study highlights how volcanic systems can influence the formation and distribution of later gas hydrate and free gas.