Influence of Gas Composition on Gas Hydrate Stability Zones in the Northern South China Sea

Evaluation of gas hydrate stability in marine sediments is commonly conducted assuming pure methane systems, although increasing drilling and logging evidence indicates that natural gas hydrates frequently contain minor amounts of heavier hydrocarbons. In the northern South China Sea, the presence of ethane has been widely reported, yet its influence on hydrate phase equilibrium and the distribution of the gas hydrate stability zone (GHSZ) remains insufficiently quantified. The results show that ethane is preferentially incorporated into large cages and promotes structure II hydrate stability, leading to lower dissociation pressures and higher stability temperatures compared with pure methane hydrates. Incorporation of as little as 1 mol% ethane systematically deepens the predicted base of the GHSZ and enlarges the hydrate-free gas coexistence interval beneath the bottom-simulating reflector (BSR). These effects indicate that conventional pure CH4 models underestimate both the thickness of the hydrate stability zone and the potential extent of hydrate occurrence. At the regional scale, composition-dependent stability provides a coherent explanation for discrepancies between seismic BSR depths and hydrate predictions. This study establishes a composition-sensitive framework for regional GHSZ evaluation, demonstrating that even trace hydrocarbons must be considered to reliably assess hydrate occurrence, resource potential, and associated geohazards in continental margin settings.