The origin and migration mechanism of natural gas in the Yacheng Gas Field, Qiongdongnan Basin, South China Sea: Constraints from noble gas isotopes

Abstract

The Qiongdongnan Basin, located on the northern continental shelf of the South China Sea, is a prominent hydrocarbon-rich region. Among its core gas fields, the Yacheng Gas Field has drawn significant attention due to its complex tectonic framework and substantial natural gas reserves. However, the mechanisms governing the phase behavior (gas or liquid) and migration of natural gas in deep strata remain poorly understood and require further investigation. This study employs advanced mass spectrometry to analyze natural gas and noble gas components sampled from the Sanya and Lingshui formations in the Yacheng Gas Field. By integrating carbon isotope ratio measurements, noble gas isotopic analyzes, and PHREEQC-based solubility modeling, the research investigates the origin, migration mechanisms, and interactions of natural gas with subsurface fluid systems in this region. The results reveal that natural gas in the Yacheng Gas Field primarily originates from a mixed crust-mantle source and is closely associated with the basin's groundwater system, particularly through significant vertical interactions with formation water. In deep strata, natural gas predominantly migrates in the gaseous phase, with solubility decreasing progressively with depth.