四川盆地中二叠统茅口组天然气大面积成藏的地质条件

近年来在四川盆地西北部(川西北)、中部(川中)等地区中二叠统茅口组钻获了一批高产气井,展现出该层系良好的天然气勘探前景,但其能否成为该盆地天然气规模勘探的重要层系,还取决于是否具备天然气大面积成藏的地质条件。为回答上述疑问,从烃源条件、颗粒滩分布、风化壳型岩溶储层分布及成因等3个方面开展了研究。结果表明:(1)茅口组缓坡颗粒滩体在广元—广安—重庆以西地区大面积分布,为储层形成奠定了地质基础;(2)全球海平面下降导致的区域性侵蚀面有利于大面积岩溶型储层的形成;(3)下志留统龙马溪组、茅一段—茅二c层及上二叠统龙潭组等3套主力烃源岩与茅口组风化壳岩溶储层构成"三明治式"源—储成藏组合,是天然气大面积成藏的关键。进而分析了茅口组天然气的富集条件,提出了有利勘探方向与目标。结论认为:(1)该盆地茅口组具备大面积天然气成藏的地质条件;(2)侵蚀微古地貌及后期走滑断裂改造控制了该区大型缝洞体的分布,颗粒滩—风化壳岩溶—走滑断裂"三位一体"控制了该区天然气富集高产的有利区带;(3)川中高石梯—磨溪地区走滑断裂发育,茅口组天然气成藏条件良好,是天然气规模勘探的有利地区。

Geological conditions for massive accumulation of natural gas in the Mid-Permian Maokou Fm of the Sichuan Basin

In the past few years, a number of high-productivity gas wells were drilled in the Mid-Permian Maokou Fm in the northwestern and central parts of the Sichuan Basin. This set of strata appears to have bright prospects in natural gas exploration. To determine if it has necessary geological conditions for the massive accumulation of natural gas, we conducted a study in three aspects, i.e. conditions of hydrocarbon source rocks, distribution of grain beaches, and distribution and origin of weathered crust karst reservoirs. The following findings were obtained. First, the Maokou Fm gentle-slope grain beaches are distributed extensively in the areas to the west of Guangyuan–Guang'an–Chongqing, laying a reliable geological foundation for the formation of reservoirs. Second, the global sea-level fall led to the formation of regional erosion surfaces, which eventually promoted the formation of massive karst reservoirs. Third, three major sets of hydrocarbon source rocks (Lower Silurian Longmaxi Fm, Mao-1–Mao-2c, and Upper Permian Longtan Fm) and the weathered crust karst reservoir of Maokou Fm formed a "sandwich-type" source–reservoir assemblage, which is the key to the massive accumulation of natural gas. On this basis, the gas enrichment conditions in the Maokou Fm were analyzed, and favorable exploration orientation and targets were proposed. In conclusion, the Maokou Fm in the basin has necessary geological conditions for the massive accumulation of natural gas. Besides,the erosion palaeogeomorphology and later strike-slip faults reworked and controlled the distribution of large fractures and cavities in the study area, and the combination of grain beaches, weathered crust karsts, and strike-slip faults controlled the favorable zones for the enrichment and high-productivity of natural gas. Moreover, the Gaoshiti–Moxi area in the central Sichuan Basin, where strike-slip faults are developed and there are favorable gas accumulation conditions in the Maokou Fm, is an ideal option for scale natural gas exploration.