四川盆地中二叠统热水白云岩成因及其分布

四川盆地油气勘探过程中,在钻揭中二叠统时常发现厚度不等的白云岩储层,但过去对该套储层的特征、成因机制、分布规律等认识不一致,制约了其勘探进程。为此,在系统分析川东地区该套白云岩储层的岩性特征和储集空间类型,以及基底断裂对中二叠统沉积影响的基础上,初步落实了该区中二叠统茅口组茅二a亚段白云岩储层的平面分布范围;进而探讨了白云岩储层的成因机制,并提出了热次盆、热次盆沉积微相的概念。结论认为:1茅二a亚段层状白云岩储层厚度可达30m,以细—中晶、硅质白云岩为主,其溶蚀孔洞、裂缝发育,储集性能好;2地球化学特征分析结果表明,茅二a亚段白云岩为热水成因沉积(非正常海水水成沉积),基底断裂控制了该套白云岩储层的平面展布;3过去认为该区W67、W83、W93等3口井因钻遇特大裂缝系统从而造就了石灰岩储层高产,而此次研究则证实,这3口井均属同一压力系统,高产原因在于其储层均为茅二a亚段白云岩储层;4热次盆沉积微相为川东地区茅口组最有利的储集相带,勘探工作应围绕这一沉积微相的分布范围展开。

Origin and distribution of hydrothermal dolomites of the Middle Permian in the Sichuan Basin

Dolomite reservoirs with various thicknesses are often encountered during drilling through the Permian strata in the Sichuan Basin. However, no consensus has ever been reached on their characteristics, genetic mechanisms and distribution rules, which impedes the exploration process. The dolomite reservoirs in the P₂m₂a (the interval “a” of the 2^nd member of the Middle Permian Maokou Fm) were mapped based on the systematic analysis of the lithologic features and reservoir space types of the dolomite reservoirs as well as the impacts of basement faults on the deposition of the Permian strata in eastern Sichuan Basin. Moreover, the genetic mechanisms of the dolomite reservoirs were analyzed and the concepts of a hydrothermal sub basin and sedimentary microfacies within a hydrothermal sub basin were presented. The following conclusions were obtained. (1) The layered dolomite reservoir in P₂m₂a with the thickness of up to 30 m is dominated by fine intermediate crystalline siliceous dolomites with well developed dissolution pores and cavities as well as fractures, and thus has good poroperm characteristics. (2) Geochemical analysis reveals that the dominated dolomites there are of hydrothermal origin instead of aqueous deposits, and the basement faults control its plane distribution. (3) The high flow rates of the limestone reservoirs in W67, W83 and W89 wells were previously attributed to the penetration of super large fracture systems, while this study disclosed that these three wells belong to the same pressure system and their high flow rates would be attributed to the dolomite reservoirs inP₂m₂a. (4) The sedimentary microfacies of a hydro thermal sub basin are the most favorable reservoir facies belts for the gas exploration, on which the exploration focus should be in this study area.