四川盆地二叠系火山碎屑岩气藏勘探重大发现及其启示

近期,中国石油天然气股份有限公司风险探井——永探1井测试获得22.5×10~4 m~3/d的高产气流,首次在四川盆地发现了喷溢相火山碎屑岩气藏,实现了火山岩气藏勘探的重大突破。为了评价该盆地二叠系火山岩气藏的勘探前景,系统总结了该区火山岩气藏的勘探历程、永探1井区气藏的特征、天然气成藏的控制因素与模式。研究结果表明:①在经历了长达50余年的火山岩持续探索后,得益于勘探思路的转变和地震预测技术的进步,得出了简阳地区二叠系发育爆发相火山岩异常体、天然气成藏条件好,是该盆地又一个重要的天然气勘探新领域和新对象的认识,指导了风险探井——永探1井的部署及重大勘探发现;②永探1井二叠系发育厚逾100 m的优质火山岩孔隙性储层,其储集岩石类型以喷溢相角砾熔岩、含凝灰角砾熔岩为主,储集空间主要为溶蚀微孔、角砾间溶孔及气孔,孔隙度介于6.68%～13.22%、平均为10.26%,渗透率介于0.01～4.43 mD、平均为2.35 mD;③该气藏埋深介于4 500～6 000 m,产层中部地层压力为125.625 MPa,压力系数达2.22,为异常高压气藏,天然气组成中甲烷含量为99.03%,微含硫化氢;④简阳—三台地区发育爆发相火山岩且紧邻德阳—安岳裂陷寒武系优质烃源岩,早期张性深断裂沟通源储,上部发育上二叠统龙潭组泥岩、下三叠统厚层膏岩等多套盖层,具备形成大型构造—岩性复合圈闭气藏的有利条件,有利勘探面积达6 000 km~2。结论认为,永探1井火山岩气藏的发现,证明了该盆地二叠系火山岩具有较大的油气勘探潜力,坚定了转变勘探思路、加快进军新区新领域、寻找更多更大勘探发现的信心。

A major discovery in Permian volcanic rock gas reservoir exploration in the Sichuan Basin and its implications

Recently, a high-yield gas flow of 22.5×104 m3/d was achieved in the wildcat well Yongtan 1, deployed by the PetroChina Southwest Oil and Gasfield Company, and an extrusive facies vocaniclastic gas reservoir was first discovered in the Sichuan Basin, which is a major breakthrough in the exploration of volcanic rock gas reservoirs. In order to further evaluate the exploration potential in Permian volcanic rock gas reservoirs in this basin, we systematically reviewed and summarized the exploration process of volcanic rocks in this study area, as well as the characteristics of gas reservoirs of well Yongtan 1, the controlling factors and modes of hydrocarbon accumulation. The following findings were obtained. (1) Over the past five decades of gas exploration of volcanic rocks, the change of exploration thought and progress in seismic prediction contribute to a major discovery in this study area, i.e., the well location deployment of Yongtan 1 was determined in the Jianyan area, where the volcanic anomalies of the Permian developed outbursts are well developed and the gas reservoir conditions are favorable. This also proves that another new gas exploration area in this basin was found. (2) From the data of Yongtan 1, we found high-quality volcanic porous reservoirs with a thickness of over 100 m developed in the Permian strata, the effusive volcanic rock types dominated by breccia and tuffaceous breccia, reservoir space composed of dissolution pores, interbreccia dissolution pores, and other pores, the porosity of which is between 6.68% and 13.22% and 10.26% on average, the permeability between 0.01 and 4.43 mD and 2.35 mD on average. (3) This abnormally high pressure gas reservoir was buried 4 500–6 000 m deep underground with the middle formation pressure being 125.625 MPa and the pressure coefficient being up to 2.22. Methane content in the minor H2S-bearing produced gas flow reached up to 99.03%. (4) Volcanic rocks in the Jianyang–Zhongjiang–Santai area are developed, and they are adjacent to the Cambrian high-quality source rocks in the Deyang–Anyue rifting. The early extensional deep faults play a role in connecting source and storage. There are many sets of caprocks, such as the Upper Permian Longtan Formation and Lower Triassic layered thick paste rocks, providing conditions for forming large-scale tectonic–lithologic composite trap gas reservoirs, and the favorable exploration area reaches up to 6 000 km2. In conclusion, this major discovery not only proves that Permian volcanic rocks in the Sichuan Basin has large potential for oil and gas exploration, but reaffirms the confidence in changing our exploration thought, speeding up new exploration zones and areas, and discovering more and bigger gas fields.