川东北元坝地区长兴组与飞仙关组天然气成藏差异性成因

Causes of differences of natural gas pooling between the Changxing Formation and the Feixianguan Formation in Yuanba,Sichuan Basin

四川盆地川东北元坝气田的气藏类型为我国埋藏最深、以台地边缘礁滩相储层为主的大型礁滩岩性气藏,勘探形势整体较好,然而其海相主要产气层上二叠统长兴组与下三叠统飞仙关组的气藏规模差异较大。为找出二者产能差异性的成因,对其储层、输导体系、压力系统及储源关系等特征进行了地质与成像测井相结合的综合分析。结论认为:该区下三叠统嘉陵江组上部—中三叠统雷口坡组膏盐岩盖层分布稳定,长兴组与飞仙关组气藏气源均主要来自上三叠统吴家坪组(P2w)和龙潭组(P2l)烃源岩;储层特征上的差异导致长兴组主要为中高产气藏,飞仙关组主要为低产气藏;而输导体系、压力系统、储源关系的不同,也对富集程度产生了一定影响。

The Yuanba Gas Field in the northeastern Sichuan Basin is the deepest large lithologic gas field dominated by reservoirs of reef-bank facies on the platform margin.The major pay zones in the Upper Permian Changxing Formation and in the Lower Triassic Feixianguan Formation are quite different in the scale of gas reservoirs.In order to find out the causes of this difference,we performed a comprehensive analysis of geological data and image logging data in respect of reservoir,carrier system,pressure system,reservoir-source correlation,etc.The following conclusions were obtained.First,the salt layer in the Lower Triassic Upper Jialingjiang Formation-the Middle Triassic Leikoupo Formation is a quality seal with stable distribution,and the Changxing and Feixianguan formation gas reservoirs share the same source rock,i.e.the Wujiaping and Longtan formations.Second,it is the differences in reservoir poroperm characteristics that result in the Changxing Formation gas reservoir being highly productive while the Feixianguan Formation gas reservoir being lowly productive.Finally,the differences in carrier system,pressure system and reservoir-source relationship also play their part in influencing the degree of gas accumulation.