四川盆地中二叠统天然气地质特征与勘探方向

四川盆地中二叠统天然气资源探明程度低,勘探潜力巨大。为加快勘探进程,在分析该盆地中二叠统天然气沉积特征、储层特征的基础上,开展了烃源供给、储层与圈闭类型、保存条件、构造与成藏期次以及成藏模式等方面的研究,探究了该盆地中二叠统天然气富集的主控因素并明确了下一步的勘探方向。研究结果表明:(1)四川盆地中二叠统天然气成藏具多层系供烃、多类型储层、多类型圈闭、多期成藏和多类型成藏模式等特点;(2)烃源岩以二叠系自身为主,其次为下寒武统筇竹寺组和下志留统龙马溪组,中二叠统生气中心位于该盆地西北部和中部—川南地区,生气强度介于26×10~8~44×10~8 m~3/km~2,具备形成大中型气田的物质基础;(3)中二叠统发育白云岩和岩溶缝洞灰岩两类储层,前者主要分布在颗粒滩中或基底断裂附近,后者广泛分布于茅口组中上部;(4)优质烃源岩、储层的白云石化和表生岩溶作用控制了大中型气田的分布;(5)印支期古隆起是油气聚集的有利指向区,喜马拉雅期是油气调整的关键时期。结论认为:(1)中二叠统栖霞组台缘滩最有利勘探区为川西北广元—江油及都江堰一带,其次为川西南台缘带,台内滩有利勘探区则主要分布在川中高石梯—磨溪地区和蜀南地区;(2)中二叠统茅口组最有利勘探区为泸州—内江地区,其次为双鱼石—南充、卧龙河—石柱、高石梯—磨溪、达州—开江、九龙山等地区。

Geological characteristics and exploration orientation of Mid-Permian natural gas in the Sichuan Basin

In the Sichuan Basin, the less-proved natural gas resources in the Mid-Permian strata are highly potential for exploration. In this paper, sedimentary and reservoir characteristics of Mid-Permian natural gas in the basin were analyzed. On this basis, researches were conducted on hydrocarbon supply, reservoir and trap types, preservation conditions, structure, and phases and patterns of hydrocarbon accumulation to highlight the controlling factors for the enrichment of Mid-Permian natural gas in the basin and the orientation of future exploration operations. Research results are as follows. First, the Mid-Permian natural gas reservoirs in the Sichuan Basin are characterized by multiple sources from different layers, existence of reservoirs and traps of various types, formation of different reservoirs at different stages and other features. Second, hydrocarbon source rocks are predominantly in the Permian strata. There are also some hydrocarbon source rocks in the Lower Cambrian Qiongzhusi and Lower Silurian Longmaxi Fms. The gas-generation center of Mid-Permian Fms lies in the northwestern and central–southern parts of the Sichuan Basin. With a gas-generation intensity of 26×10⁸～44×10⁸ m³/km², it has the necessary material basis available for the formation of large- or medium-sized gas fields. Third, the Mid-Permian strata contain dolomite and karst fractured-vuggy limestone reservoirs, with the former distributed predominantly in grain beach or around basement faults, and the latter distributed extensively in middle and upper parts of the Maokou Fm. Fourth, the presence of high-quality hydrocarbon source rocks, and dolomitization and epigenetic karstification of reservoirs are controlling factors for the distribution of large- and medium-sized gas fields. Fifth, the Indosinian paleo-uplifts are favorable zones for the accumulation of hydrocarbons, and the Himalayan is the key period of oil and gas adjustment. In conclusion, in the Mid-Permian Qixia Fm, the favorable exploration zones are represented by platform margin beaches mainly along the Guanyuan–Jiangyou and Dujiangyan areas in the NW Sichuan Basin, followed by platform margins in the SW Sichuan Basin, and intraplatform beaches predominantly in the Gaoshiti–Moxi area in the central and southern Sichuan Basin. Besides, in the Mid-Permian Maokou Fm, the favorable exploration zones are mainly the Luzhou–Neijiang area, followed by the Shuangyushi–Nanchong, Wolonghe–Shizhu, Gaoshiti–Moxi, Dazhou–Kaijiang, Jiulongshan and other areas.