L25气田位于琼东南盆地深水西区,由上中新统黄流组早、晚2期限制型重力流水道岩性圈闭相关气藏纵向叠置而成,早期的Ⅲ、Ⅳ气组为强超压气藏(压力系数1.806~1.840),晚期的Ⅰ、Ⅱ气组为常压气藏(压力系数1.170~1.188).天然气组分、天然气碳同位素分析表明,L25气田天然气为有机成因煤型气.通过对天然气甲烷碳同位素与烃源岩镜质组反射率之间的关系、凝析油生物标志化合物特征、烃源岩埋藏史、热史等的综合分析,认为L25气田天然气来源于乐东凹陷下渐新统崖城组海相烃源岩生成的成熟—高成熟天然气.天然气成藏过程分析揭示,L25气田经历了早、晚2期天然气充注成藏:2.0Ma,成熟—高成熟天然气充注于晚期沉积的限制型重力流水道岩性圈闭,形成Ⅰ、Ⅱ气组常压气藏;近于现今,高成熟天然气充注于早期沉积的限制型重力流水道岩性圈闭,形成Ⅲ、Ⅳ气组强超压气藏.沉积与储层、压力演化及成藏特征的研究表明,早期天然气充注的规模化的储集体是琼东南盆地深水区新近系更有利的天然气勘探方向.

L25 gas field, locating in the western deepwater area, Qiongdongnan Basin, consists of vertically superimposed gas pools associated with two phases of confined gravity flow deposit in Upper Miocene Huangliu Formation. The early phase Ⅲ and Ⅳ gas units are strong overpressure gas pools with pore pressure coefficients from 1.806 to 1.840. The late phase Ⅰ and Ⅱ gas units are normal pressure gas pools with pore pressure coefficients from 1.170 to 1.188. The characteristics of natural gas component and carbon isotope of L25 gas field show that the natural gas was coal-type organic origin gas. The integrated study of the relationship between carbon isotope of methane and vitrinite reflection ratio of source rock, the biomarker of condensate oil, the burial history and thermal evolution history of source rock, indicates that the mature and high-mature gas dominantly come from marine source rock of Lower Oligocene Yacheng Formation in Ledong Sag. Analysis of natural gas accumulation process reveals that L25 gas field experience two stages of gas filling in the early and late periods. About 2.0Ma, mature and high-mature gas filled the lithological traps associated with the late phase confined gravity flow deposit, forming Ⅰ and Ⅱ gas units with normal pressures. About present, high-mature gas filled the lithological traps associated with the early phase confined gravity flow deposit, forming Ⅲ and Ⅳ gas units with strong overpressures. The comparative research on the features of deposit and reservoir, pressure evolution and accumulation, shows that the Neogene large-scale reservoir filled with gas in the early stage is more economical accumulation for exploration in the deepwater area, Qiongdongnan Basin.