四川盆地东部地区二叠系茅口组孤立浅滩的发现及天然气勘探潜力

Discovery of isolated shoals in the Permian Maokou Formation of eastern Sichuan Basin and their natural gas exploration potential

中二叠统茅口组长期以来一直是四川盆地天然气勘探的重点层系之一,茅口组天然气勘探开发先后以四川盆地南部地区岩溶缝洞型石灰岩储层和四川盆地中部地区滩相白云化储层为主要目的层系。为了寻找并落实茅口组下一个天然气勘探方向,在勘探程度相对较低的四川盆地东部(以下简称川东)地区开展了大量基础研究工作,在重新梳理地层格架的基础上,对茅口组沉积、储层及成藏条件进行了系统分析,指出了该区孤峰段硅质岩下茅二下亚段孤立浅滩的存在,最后明确了茅口组天然气的勘探潜力。研究结果表明:(1)川东地区在茅口组孤峰段事件性硅质岩沉积之前,宣汉—万州地区茅二下亚段发育碳酸盐岩孤立浅滩,面积可达6500 km^(2);(2)在宣汉—万州地区茅二下亚段发现了滩相基质孔储层,其中发育于茅二下亚段底部的孔隙型白云岩储层厚3~12 m,发育于茅二下亚段中上部的孔隙型石灰岩储层厚5~18 m;(3)川东地区茅口组滩相储层成藏条件优越,其中烃源岩主要为下伏上奥陶统五峰组—下志留统龙马溪组厚层页岩,且与烃源岩沟通的断裂发育,区域盖层主要为孤峰段硅质岩及吴家坪组海相泥岩、页岩,保存条件良好。结论认为,川东地区茅口组孤立浅滩的发现和良好的成藏组合条件,将进一步加快该区天然气勘探开发进程,茅口组大面积分布的高能浅滩有望成为下一个天然气勘探开发重要领域。

The Middle Permian Maokou Formation has long been one of the key targets for natural gas exploration in the Sichuan Basin,and the exploration and development of natural gas in the Maokou Formation have mainly focused on karstic fractured-vuggy limestone reservoirs in southern Sichuan Basin and dolomitized shoal facies reservoirs in central Sichuan Basin successively.In order to ascertain the prospects of natural gas exploration of the Maokou Formation,a lot of basic research efforts have been taken in eastern Sichuan Basin where the exploration maturity is relatively low.On the basis of further studying the stratigraphic framework,a systematic analysis is conducted on the sedimentation,reservoir and hydrocarbon accumulation of the Maokou Formation,the existence of isolated shoals of lower Mao-2 Member below the silicite of Gufeng Member is pointed out.Finally,the exploration potential of natural gas in the Maokou Formation is clarified.The results show that before the sedimentation of event silicite in Gufeng Member of Maokou Formation in eastern Sichuan Basin,isolated carbonate shoals had been developed in the lower part of the second member of Maokou Formation(lower Mao-2 Member)in the Xuanhan-Wanzhou area,with an area of up to 6500 km^(2).Shoal facies matrix pore reservoirs have been discovered in the lower Mao-2 Member of the Xuanhan-Wanzhou area:the porous dolomite reservoir developed at the bottom of lower Mao-2 Member is 3.0–12.0 m thick,and the porous limestone reservoir developed in the middle-upper part of lower Mao-2 Member is 5–18 m thick.The hydrocarbon accumulation conditions of shoal facies reservoirs in the Maokou Formation are excellent in eastern Sichuan Basin:the source rock mainly consists of thick shale of underlying Upper Ordovician Wufeng Formation–Lower Silurian Longmaxi Formation,and the faults connecting the source rocks are developed,while the regional caprocks are mainly composed of silicite of Gufeng Member and marine mudstone and shale of Wujiaping Formation,the preservative conditions are good.It is concluded that the discovery of isolated shoals and favorable hydrocarbon accumulation conditions of Maokou Formation in eastern Sichuan Basin will further accelerate the process of natural gas exploration and development in the region,and the high-energy shoals widely distributed in the Maokou Formation are expected to become the next replacement option of natural gas exploration and development.