琼东南盆地深水区宝岛21-1气田天然气来源及输导体系

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

宝岛21-1气田的发现是琼东南盆地宝岛凹陷近30年以来油气勘探取得的最大突破,打开了勘探新局面,有关其天然气来源和输导体系的认识,对宝岛凹陷进一步勘探具有重要意义。以宝岛21-1气田为主要研究对象,采用油气地球化学及断裂体系刻画等手段,详细分析了天然气组成、碳同位素特征和凝析油生物标志化合物特征,精细刻画了主控断层几何形态和运动学特征,并分析了其对天然气富集的贡献。结果显示,宝岛21-1气田具有重烃气含量高、干燥系数低及碳同位素轻的特点,与Y13气田和L17气田具有明显差异,揭示除陆源有机质的贡献外,还有明显的腐泥型有机质贡献,主要为海相藻类的贡献。宝岛21-1构造经历了3期油气充注,断裂与陵水组三段三角洲砂体构成了高效输导体系,断裂形态与差异活动性控制油气汇聚-输导,使该构造所在位置成为优势聚集区。建立了紧邻生烃主洼,沿“大型构造脊+断面脊汇聚,长期活动的大型沟源断裂与三角洲砂岩配置高效输导”的天然气富集模式,为宝岛凹陷及琼东南盆地勘探提供重要依据。

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.