Natural gas sources and migration pathways of the Baodao 21-1 gas field in the deep-water area of the Qiongdongnan Basin

The discovery of Baodao 21-1 gas field highlights a landmark achievement in the hydrocarbon exploration efforts in the Baodao Sag of the Qiongdongnan Basin over the past three decades, opening up new exploration prospects. To further explore the gas potential in the sag, understanding the sources and migration pathways of natural gas is paramount. This study delves into the exploration of the Baodao 21-1 gas field, offering detailed insights into the composition and carbon isotopes of natural gas, as well as condensate biomarkers, based on petroleum geochemistry and fault system characterization. Furthermore, this study presents a fine-scale characterization of the geometrical morphologies and kinematic properties of dominant faults and evaluates their role in natural gas enrichment. Key findings include. (1) The Baodao 21-1 gas field showcases high concentrations of heavy hydrocarbon gases, low drying coefficients, and light carbon isotopes, distinguishing itself from Y13 and L17 gas fields. This suggests significant contributions from marine algae-derived sapropelic organic matter besides terrigenous organic matter. (2) The Baodao 21-1 structure experienced three stages of hydrocarbon charging. The structure’s faults and the deltaic sand bodies in the third member of the Lingshui Formation serve as effective pathways for hydrocarbon migration. The fault morphology and differential activity dictate the hydrocarbon accumulation and migration. All these make the Baodao 21-1 gas field become a major area for hydrocarbon accumulation. (3) The natural gas enrichment mode for the gas field is established. This mode includes proximity to major hydrocarbon-generating sags, gas accumulation along a large-scale structural ridge and a fault-plane ridge, and efficient migration pathways via long-term active faults in large scale connecting source rocks, and deltaic sandstones. This study lays the groundwork for forthcoming exploration in both the Baodao Sag and even the Qiongdongnan Basin as a whole.