对中坝须二段气藏圈闭分析的思考

AN IDEA ABOUT THE TRAP ANALYSIS OF T_3x^2 GAS RESERVOIR IN ZHONGBA FIELD

拥有压降法储量 10 0× 10 8m3 的中坝须二段气藏发现于 1972年 ,虽然地质研究在勘探初期提出过该气藏为印支期古构造圈闭的认识 ,但随后在整个勘探开发进程中 ,圈闭被视为构造或断封圈闭。笔者通过圈闭分析认为 ,中坝须二段气藏为古构造—成岩圈闭 ,成藏关键时刻是晚侏罗世早期。随着油气对燕山期古构造的充注 ,成岩作用出现差异 ,胶结和充填作用在含油气部位受到抑制甚至停顿 ,而在气水过渡带以外则持续进行 ,大量碳酸盐胶结物使岩石孔渗性急剧降低 ,形成沿古构造边界的环状致密岩带 ,进而封堵已聚集的油气 ,展现了古构造基础上的成岩圈闭特征。统计表明 ,中坝须二段气藏勘探成功率为 5 3% ,工业气井集中于古今构造叠合部位的相对高孔渗带 ,圈闭以外的钻井 ,即使构造位置高 ,也基本失利。因此认为 ,聚集期古构造是形成油气藏的最佳部位 ,由于差异成岩作用将油气“冻结”定位 ,故晚期构造的改造调整作用有限。进而提出 ,圈闭分析是成藏研究的基础 ,为提高气井命中率 ,在勘探布井工作中 ,圈闭分析应列为关键技术。

With pressure drawdown method reserves of 10 000×106m3,T 3x2 (the second member of Xujiahe Formation,Upper Triassic) gas reservoir in Zhongba field was found in 1972.Although a thought that the gas reservoir belonged to a palaeostructural trap in Indo-Chinese epoch was put forward in the initial stage of exploration,it was gradually considered as a structural trap or a fault-sealed trap in the process of exploration and development.Through trap analysis it is thought that the T 3x2 gas reservoir in Zhongba field is a palaeostructural-diagenetic trap and the key period of gas reservior formation was the Early Jurassic.Along with that the palaeostructure in Yanshan epoch was charged with hydrocarbon,the difference between diageneses occurred:the cementation and filling were restrained and even stopped at the hydrocarbon-bearing portions but they had been still going on beyond the water-gas transition zone.A lot of carbonate cement made the rock porosity and permeability be rapidly reduced and a looping tight rock zone was formed along the palaeostructural boundary to seal the accumulated oil and gas,which shows a diagenetic trap character on the basis of palaeostructure.According to statistical data it is indicated that the exploration well success ratio of T 3x2 gas reservoir in Zhongba field was 53% and the commercial gas wells were concentrated at the relatively high porosity-permeability zones of superposition portions of the ancient and modern structures and the wells drilled beyond the trap,even at the high structural portions,were barren holes on the whole.It is considered that the accumulative phase palaeostructure was the optimal portion of forming reservoir and the hydrocarbon was positioned by “freezing-up” because of differential diagenesis,so the structure's being reformed and adjusted in the late stage were limited.Finally,it is proposed that the trap analysis is the basis of studying reservoir formation and it should be listed at the key technique in well-spacing for raising exploration well success ratio.