四川盆地西北部上古生界天然气勘探前景

近期,四川盆地西北部上古生界海相地层天然气勘探取得了重要的新进展,除证实中二叠统栖霞组孔隙型白云岩储层可能大面积连片分布外,还首次在井下发现了含气性较好的泥盆系厚层白云岩储层。最新研究成果表明:(1)区内沉积下寒武统巨厚优质烃源岩,烃源条件优越;(2)受加里东运动的影响,四川盆地西北部整体抬升,形成由南向北逐渐倾伏的西翼低陡东翼高缓的大型箱状隆起,构造顶部宽缓,为泥盆纪、石炭纪和二叠纪的继承性大规模台缘礁滩沉积和白云化作用奠定了基础;(3)上古生界存在多个不整合面,发生多期表生溶蚀作用,极大地改善了储层储渗性能;(4)龙门山推覆作用形成的构造形变特征为油气成藏创造了优越的烃源断裂及封盖条件,有利于大气田的形成。研究认为:该区(1)号隐伏断裂带以东的背冲背斜带具备形成大型构造—地层、构造—岩性复合圈闭气藏的有利条件,其中射箭河—中坝地区沉积古地貌地势相对较高、栖霞组白云化作用更强,加里东古隆起中轴线至东脊线附近是栖霞组气藏最有利的勘探区块,中轴线往西至(1)号隐伏断裂带是可同时兼探上古生界3套气层的最有利勘探区块。

Natural gas exploration prospect in the Upper Paleozoic strata,NW Sichuan Basin

Progress of natural gas exploration has recently been made in the Upper Paleozoic marine-facies reservoirs in the NW Sichuan Basin. Not only was being proved that the Middle Permian Qixia Fm pore-type dolomite reservoirs may be distributed in a great piece and in a large area, but for the first time favorable gas-bearing Devonian dolomite reservoirs in great thickness were discovered in the downhole there. The following achievements in the research are included: (1) The Lower Cambrian quality source rocks in huge thickness were found in the studied sedimentary area with advantageous hydrocarbon source conditions. (2) Influenced by the Caledonian movement, the whole NW Sichuan Basin was lifted up to form a massive box-like NS-trending plunging uplift low steep in the west but high sluggish in the east, laying robust foundation for the inheritance of Devonian, Carboniferous, and Permian platform margin reef sedimentation and dolomitization in a large scale. (3) In the Upper Paleozoic strata, there were multiple planes of unconformity and multi-stage supergene karst occurred, improving the preservation and seeping capacity of reservoirs. (4) Tectonic deformations resulted from the nappe structure in the Longmenshan mountain provided good hydrocarbon source fracture and sealing conditions for oil and gas reservoirs, which is conductive to the formation of a giant gas pool. The back-thrust anticline belt east to the east of the hidden fault Ⅰ in this study area is equipped with favorable conditions for large-scale structural–stratigraphic and structural–lithologic combination traps. As suggested in this study, relatively strong dolomitization was found in the Qixia Fm in the Shejianhe River–Zhongba higher palaeogeomorphic deposit area, so the most favorable exploration target of the Qixia Fm will be around the central axis of Caledonian paleo-uplift to the east ridge line, and the most favorable prospect of the Upper Palaeozoic will be the central axis to the west of the hidden fault Ⅰ where three gas bearing reservoirs will be possibly explored at the same time.