川西南部地区雷口坡组天然气成藏条件及主控因素

Conditions and main controlling factors of natural gas accumulation of Leikoupo Formation in southwestern Sichuan Basin

四川盆地西部坳陷中三叠统雷口坡组储层规模发育,在川西坳陷的北部、中部均实现了天然气勘探的重大突破,但南部地区勘探效果较差。为了给下步勘探提供参考,综合分析了川西南部雷口坡组天然气成藏条件及主控因素。研究结果表明:1)雷口坡组储层主要分布于雷三段、雷四3亚段,雷三段发育滩相白云岩储层,雷四3亚段发育颗粒滩叠合表生岩溶储层,储层呈北东—南西向展布,向盆内方向逐渐减薄。2)雷口坡组天然气为典型干气,δ13C1和δ13C2同位素特征与典型的Ⅱ型干酪根生成天然气同位素分布接近,具有混源特征,二叠系海相烃源岩和须家河组烃源岩是主要天然气来源。3)雷口坡组发育构造-岩性和构造-地层圈闭气藏,构造-岩性气藏分布于龙门山断褶构造带,圈闭由挤压背斜结合储层横向相变控制,构造-地层气藏主要分布于斜坡区,由储层尖灭形成。4)雷四段岩溶储层发育程度、圈闭形成时间、后期保存及盖层条件是决定天然气是否成藏的关键。综合评价认为,平面上莲花山—高家场—雾中山—鸭子河一带为有利区,并进一步划分为西侧的构造-岩性圈闭有利带和东侧的构造-地层圈闭有利带。

The Middle Triassic Leikoupo Formation reservoir in the western depression of Sichuan Basin is developed at scale.Major breakthroughs in natural gas exploration have been achieved in the northern and central parts of western Sichuan Depression,but the exploration effect in the southern part is relatively poor.In order to provide reference for the next exploration,the conditions and main controlling factors of natural gas accumulation of Leikoupo Formation in southwestern Sichuan are comprehensively analyzed.The research results show that:(1)The reservoirs of Leikoupo Formation are mainly distributed in Lei 3 Member and sub-member 3 of Lei 4 Member.Lei 3 Member develops beach facies dolomite reservoirs,and sub-member 3 of Lei 4 Member develops granular-beach superimposed supergene karst reservoirs.The reservoirs are distributed in the northeast-southwest direction and gradually thins in the direction of the inner basin.(2)The natural gas of Leikoupo Formation is typical dry gas,whoseδ13C1 andδ13C2 isotope characteristics are close to the typical typeⅡkerogen-derived natural gas isotope distribution,with mixed source characteristics.The Permian marine source rocks and source rocks of Xujiahe Formation are the main sources of natural gas.(3)Leikoupo Formation develops structural-lithologic and structural-stratigraphic trap gas reservoirs.The structural-lithologic gas reservoirs are distributed in the Longmenshan fault-fold tectonic belt.The trap is controlled by the compression anticline combined with the lateral phase change of the reservoir.The structural-stratigraphic gas reservoirs are mainly distributed in the slope area and are formed by reservoir pinch-out.(4)The degree of karst reservoir development,trap formation time,later-preservation and cap rock conditions in Lei 4 Member are keys to determine whether natural gas is accumulated.The comprehensive evaluation shows that the Lianhuashan-Gaojiachang-Wuzhongshan-Yazihe area is a favorable area on the plane,and it is further divided into the favorable zone of structural-lithologic trap on the west side and the favorable zone of structural-stratigraphic trap on the east side.