Abstract
The Yinggehai Basin is situated in the Yinggehai Sea, between Vietnam and the southwest of Hainan Island in China. This location is home to two significant shallow-gas field clusters, known as the Dongfang and Ledong Areas, where the actual controlled gas reserves have surpassed the anticipated resource amounts based on current knowledge of gas origins. It was traditionally thought that the gas reserves in the Dongfang Area were primarily derived from the Meishan Formation source rocks, whereas those in the Ledong Area came from source rocks in both the Meishan and Sanya Formations. Given that the carbon isotopes of kerogens in the Meishan Formation are heavier than those in the Sanya Formation, it was expected that natural gas in the Dongfang Area would exhibit heavier carbon isotopes compared with the Ledong Area. Nevertheless, this expectation was challenged by the measured carbon isotope values of natural gas in these areas. Thus, accurately determining the contributions of gas sources has emerged as a crucial aspect in the comprehensive investigation of natural gas accumulation and distribution patterns within these regions. This study performed thermal simulation experiments on mudstone samples from the Meishan and Sanya Formations to establish models for hydrocarbon generation dynamics and methane carbon isotope fractionation and to reconstruct the history of hydrocarbon generation and the evolution of methane carbon isotopes. The contributions of gas sources for the Dongfang and Ledong Areas were examined through the integration of data concerning the basin's fault system and the history of pressure evolution. The findings indicated that besides the diapir in both areas, a superimposition zone of faults from the Meishan and Sanya Formations in the Dongfang Area could act as a conduit for source reservoirs. Early natural gas from the Sanya Formation, prior to 5.5 Ma in the Dongfang Area, might first fill the Meishan reservoir before contributing to the formation of shallow-gas reservoirs, thus facilitating a "relay accumulation" process. This resulted in the presence of lighter carbon isotopes in hydrocarbon gases within the Dongfang Area. By evaluation of the principal hydrocarbon generation period of the source rocks, the effective period for source-reservoir channels, and the reservoir formation period, the optimum gas supply times from the source rocks were identified. Consequently, a map illustrating the gas source contribution patterns for the gas reservoirs in the Dongfang and Ledong Areas was created. Furthermore, the gas source contribution ratios for the Dongfang and Ledong gas fields were determined to be 65%:35% and 50%:50%, respectively, from the Meishan and Sanya source rocks, according to the methane carbon isotope fractionation model.