Geochemistry, origin and accumulation of natural gas hydrates in the Qiongdongnan Basin, South China Sea: Implications from site GMGS5-W08

Abstract

The recent discovery of natural gas hydrates in the Qiongdongnan Basin (QDNB) during the fifth “China National Gas Hydrate Drilling Expedition” (GMGS5) confirms the great potential of gas hydrate resources within this basin. Based on the comprehensive geochemical investigation of the hydrate-related gas and hydrate-bearing sediments collected from the drilled site GMGS5-W08, this study determines the source and accumulation mechanism of gas hydrates. The hydrate-related gases from the site GMGS5-W08 consist of abundant methane (81.21%–97.69%) and some C2+ hydrocarbon gases (2.31%–18.79%). The stable carbon isotopic compositions of C1–C3 hydrocarbon gas components indicate that the methane has a mixed microbial and thermogenic origin (−59.5‰<δ13C1<−48.8‰), while the C2+ hydrocarbon gases are of thermogenic origin derived from coaly-type source rocks (−27.5‰<δ13C2<−22.4‰, −23.2‰<δ13C3<−20.2‰). The sediment samples collected at site GMGS5-W08 are characterized by low total organic carbon (TOC) contents (mostly < 0.4%), low hydrocarbon indices (mostly < 50 mg HC/g TOC), and low in situ temperatures (3.5°C-21°C), suggesting a poor microbial gas generation potential for the in situ uncompacted sediments. The extracts of the hydrate-bearing sediments are composed of in situ immature terrigenous organic matters and allochthonous mature hydrocarbons from the deeply buried strata. The allochthonous hydrocarbons (C14+ components) approximately originate from the low mature to mature marine mudstones (0.75%<calculated vitrinite reflectance (%Rc)<0.87%) of the Miocene Sanya and Meishan formations. The accumulation process of hydrate gas at site GMGS5-W08 can be subdivided into three stages: migration of gas and liquid hydrocarbons (stage I), gas accumulation, pressure increase, and microbial alteration (stage II), and pressure release, gas chimney and gas hydrate occurrence (stage III). The microbial methane within the gas hydrates is mainly derived from the secondary microbial methane generated in stage II and primary microbial methane generated from sediments within the zone of optimal temperature (35°C–75°C).