鄂尔多斯盆地膏质白云岩有利储层的成因及演化

Origin and evolution of gypsum dolomite as a favorable reservoir in the Ordos Basin,China

膏质白云岩是鄂尔多斯盆地中东部下古生界马家沟组M51最有利储层,影响膏质白云岩储层性质的最大因素在于溶蚀作用。本文在已有的研究基础上,应用岩心资料及阴极发光、地球化学等方法,测定M51膏质白云岩及其特殊的示底构造的化学成分,并分析其成因模式。岩心、薄片以及物性资料显示:石膏溶蚀形成的大量膏模孔及伴生的次生孔是储层的主要储集空间,示底构造规模性出现于膏质云岩中。阴极发光、常微量元素分析结果显示:示底构造上部充填埋藏期方解石;下部充填碎屑白云石,与示底构造外围的准同生期泥粉晶白云石具有相近的地球化学特征,与深部埋藏期形成的细中晶白云石差异明显。示底构造的演化是膏质白云岩有利储层形成的重要依据,本文依据化学离子平衡原理,对不同成岩阶段发生的溶蚀作用、填充作用进行离子间相互置换的重演,分析得知不稳定的化学组成是石膏易被溶蚀的内在原因,石膏的规模性溶蚀是膏质云岩有利储层形成的关键。

Gypsum dolomite is the most favorable natural gas reservoir in M51 of the Paleozoic Majiagou Formation in the middle eastern Ordos basin while its properties are mostly affected by dissolution.On the basis of past studies,the composition and genesis of gypsum dolomite and geopetal structure of M51 were determined by core data,cathodoluminescence and geochemical analyses.Thin section and physical property studies show that the numerous cast holes and other secondary pores from gypsum dissolution form the main reservoir space.In addition,a large number of geopetal structures are found in gypsum dolomite.Cathodoluminescence and elemental analysis show that the upper part of the geopetal structure is filled with calcite formed during the burial period;while the lower clastic dolomite is filled with clastic dolomite that has similar geochemical characteristics with the penecontemporaneous mud-powder dolomite outside the geopetal structure,but are obviously different from the fine-mesocrystalline dolomite formed in the deep burial period.The evolution of geopetal structural is an important component of gypsum dolomite as a favorable reservoir.Governed by the principle of chemical and ionic equilibrium,dissolution and filling alternate and repeat throughout the different diagenetic stages,demonstrating that chemical instability is the intrinsic cause of gypsum dissolution,the key to the formation of gypsum dolomite reservoir.