四川盆地中部中二叠统热液白云岩储渗空间类型及成因

近年来,四川盆地川西北地区有多口井在中二叠统栖霞组、茅口组钻获高产气流,展现出该层系巨大的天然气勘探潜力,但有关该盆地中二叠统白云岩储层的成因机理则众说纷纭,一直未达成共识。为此,基于川中地区栖霞组、茅口组岩心及薄片等岩相学特征,以及碳、氧、锶同位素和包裹体均一化温度等地球化学特征,开展了中二叠统热液白云岩储渗空间类型及储渗相特征研究,分析了白云岩储层的成因及其控制因素。结果认为:(1)茅口组、栖霞组石灰岩地层经历了构造控制热液活动,发育萤石、鞍状白云石、天青石的热液矿物组合和膨胀角砾结构;(2)较之于宿主石灰岩,细—中晶云岩和鞍状白云石均呈现出δ18O负偏移、87Sr/86Sr正偏移;(3)中二叠统形成由热液溶孔、热液白云岩晶间孔、热液溶洞、热液扩溶缝等4种储渗空间构成的热液白云岩储渗体。结论认为:(1)热液白云岩储集相发育和分布受深大断裂的控制,呈"透镜状"分布在具"下凹"地震反射特征的区域;(2)中二叠统热液白云岩储集相与下伏烃源岩和上覆封堵层的有机组合可构成有利的天然气勘探目标。

Space types and origins of hydrothermal dolomite reservoirs in the Middle Permian strata,Central Sichuan Basin

Recently commercial gas flows have been obtained in several wells in the Middle Permian Qixia and Maokou strata in NW Sichuan Basin, where a great potential of natural gas exploration has been proven. However, opinions vary on the origins of the Permian dolomite reservoirs in this study area, no unanimous conclusion has ever been drawn. In view of this, based on core observation, microscopicthinsectionanalysis, and the corresponding geochemical indexes in carbon, oxygen, strontium isotopes, and reservoirsinclusionshomogenization temperature, an analysis was made on the hydrothermal dolomite reservoir types and facies characteristics, as well as such reservoir origins and their main controlling factors. The following findings were obtained. (1) The Middle Permian limestone strata such as Maokou, Qixia, etc. have undergone structurally controlled hydrothermal activities and eventually hydrothermal dolomite reservoir facies were developed including typical hydrothermal mineral assemblages like saddle dolomites, fluorite and celestites, and hydrothermal breccia structures. (2) Compared to host limestones like micritic limestones, etc., medium–fine crystalline dolomites and saddle dolomites were displayed as δ18O negative migration and 87Sr/86Sr positive migration. (3) The hydrothermal dolomite reservoir space is mainly composed of hydrothermal dissolved pores, hydrothermal intercrystalline pores, hydrothermal dissolved vugs, and hydrothermal enlarged fractures. In conclusion, it is demonstrated that the development and distribution of hydrothermal dolomite reservoir facies, controlled by large basement-rooted faults, are mainly distributed, from the seismic data, in a "lenticular form" in the area which is featured by "concave" seismic reflection; and that the organic combination of the Middle Permian hydrothermal dolomite reservoirs, underlying hydrocarbon source rocks and the overlying seal strata will be possibly a favorable exploration target in this study area.