Noble gas constraints on fluid flow and hydrocarbon accumulation in the Yinggehai Basin, Northwestern South China Sea

Abstract

Multiple physicochemical processes involving organic and inorganic components may alter hydrocarbon composition and isotopic signatures, posing a challenge in accurately tracing natural gas accumulation. In contrast, noble gases are chemically inert and highly sensitive to fluid flow processes, offering a powerful tool for precisely tracing natural gas accumulation. By analyzing and modeling noble gas geochemistry data of gas samples from gas fields in the Yinggehai Basin, South China Sea, we constrained fluid flow patterns and traced the natural gas accumulation process. In particular, the low ³He/⁴He and high ⁴⁰Ar/³⁶Ar values of gas samples suggested atmospheric-crustal mixing, with the suspected central fault significantly influencing the ⁴⁰Ar* (* denotes crustal noble gas) proportion and ⁴⁰Ar/³⁶Ar value in charging fluids. Binary mixing of atmospheric and crustal noble gases elevated the ⁴⁰Ar*/⁴He* value in well-preserved gas fields. Diapir activity and/or long-term artificial extraction had likely promoted noble gas leakage which further elevated the ⁴⁰Ar*/⁴He* to abnormally high levels. Three key time windows for ⁴He* accumulation, i.e., 4–4.5 Ma, 1–2 Ma, and 0–0.5 Ma, were identified in well-preserved gas fields. The suspected central fault facilitated the migration of both high ⁴⁰Ar/³⁶Ar fluids and highly mature hydrocarbons characterized by heavier δ¹³C₁ and high C₁/C_1-5 ratios. In most gas fields, methane (C₁) migration was dominated by the gas phase, as indicated by the high C₁/³⁶Ar value. However, in a few leaked or shallow-buried gas fields, low C₁/³⁶Ar ratios suggest that C₁ also migrated with water. The duration of trap sealing and the depth of the transport system played critical roles in hydrocarbon accumulation. Longer trap sealing and greater transport system depth favored hydrocarbons derived from the Lower Miocene Sanya Formation. In contrast, shorter trap sealing durations and limited transport system depth led to the accumulation of hydrocarbons sourced from the Middle Miocene Meishan Formation.