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 d13C₁ 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.

Keywords

Yinggehai Basin / Hydrocarbon accumulation / Noble gas geochemistry / Source tracing / Fluid leakage / Fluid migration

CRediT authorship contribution statement

Rui Liu: Writing - review & editing, Writing - original draft, Investigation, Formal analysis, Data curation, Conceptualization. Rui Xu: Writing - original draft, Investigation, Formal analysis. Tao Wen: Writing - review & editing, Writing - original draft, Methodology, Investigation, Conceptualization. Khi Atchinson: Writing - review & editing, Validation, Investigation. Ziqi Feng: Investigation, Visualization, Writing - review & editing. Fang Hao: Writing - review & editing, Visualization, Resources, Project administration. Lin Hu: Validation, Resources, Investigation, Data curation. Jinqiang Tian: Writing - review & editing, Visualization. Yazhen Zhang: Validation, Investigation, Data curation. Jianzhang Liu: Validation, Investigation. Lei Tuo: Validation, Formal analysis, Data curation.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This study was supported by National Science and Technology Major Project for Deep Earth (No. SQ2025AAA060128), CNOOC Ltd. (KJZH-2021-0003-00), and the Science and Technology Innovation Program of Laoshan Laboratory (No. LSKJ202203403).We appreciate the collaboration and enthusiastic support of Lixia Shen, Yangyu Wu, and Lei Shao at the Hainan Branch of CNOOC Ltd.

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