Thermogenic gas controls high saturation gas hydrate distribution in the Pearl River Mouth Basin: Evidence from numerical modeling and seismic anomalies

Abstract

We integrate three-dimensional (3D) seismic data, well log and two-dimensional (2D) modeling to understand the accumulation and saturation of gas hydrates and their relationships with biogenic and thermogenic gases in the Pearl River Mouth Basin, South China Sea (Sites SH2, SH5, W17, and W19). The model based on physical, geochemical and geological parameters from shallow gas hydrate sites, deep oil sites and seismic interpretation, and it is tied to well-log and seismic quantitative interpretation, to predict the main controlling factors for gas hydrate distribution. The modeling results show that the biogenic methane generation is distributed within the strata shallower than 1500 mbsf (meters below the seafloor), whereas thermogenic gas generation ranges from 2300 mbsf to 6000 mbsf. The erosion surfaces of migrating canyons represent important pathways for upward migration of biogenic gas resulting in average hydrate saturation <20% below the ridges of canyons. The total organic carbon content is an important factor related to biogenically-sourced gas hydrate distribution. Higher gas saturation (>40%) at the base of gas hydrate stability zone (BGHSZ) is related to the focused migration of thermogenic gas along faults and throughout permeable carrier beds. Although gas chimneys are widely identified from the coherence attribute extracted from 3D seismic data, their impact on the obviously higher (locally up to 60%) gas hydrate saturations in the fine-grained sediments appears site-dependent and associated with fault activity. By comparing well log-derived gas hydrate saturations, our modeling shows that about 80% of the methane-forming hydrate is thermogenically-sourced at Sites W19 and W17. The integrated modeling effort presented in this work, based on provides a better understanding of the different contributions from different gas sources for gas hydrate accumulation along the northern slope of the South China Sea.