四川盆地东部地区中二叠统茅口组白云岩储层特征及其主控因素

四川盆地东部(以下简称川东地区)中二叠统茅口组"似层状"白云岩储层天然气勘探获得了重大发现,但对该层位白云石化的成因和分布一直未能达成共识,制约了该区的天然气勘探部署。为了明确该区茅口组下一步的天然气勘探方向,基于岩心和露头样品的岩相学及地球化学特征分析,研究了茅口组白云岩储层的特征、成因及主控因素,建立了白云岩发育模式。研究结果表明:①川东地区茅口组白云岩纵向上主要发育于茅三段中下部,多呈层状、似层状产出,岩性主要为细—中晶云岩、硅质云岩、生屑云岩等,储层物性较好,主要发育孔隙、热液溶蚀缝洞系统两类储集空间;②该区茅口组基质云岩与鞍状云岩的微量元素及其含量、锶同位素比值及包裹体均一温度均存在着差异,表明两类白云岩的形成流体或期次相差较大,早期热水沉积形成基质云岩,晚期受热液溶蚀改造形成缝洞充填的鞍状云岩;③该区茅口组白云岩发育分布受生屑滩、热水沉积和基底断裂的联合控制,其中生屑滩是白云岩储层发育的基础,热水沉积形成了早期层状基质云岩,基底断裂为后期富镁热液流体提供了运移通道、改善了储集性能。该研究成果有助于深化对川东地区茅口组白云岩储层的认识。

Characteristics and main controlling factors of the Middle Permian Maokou dolomite reservoirs in the eastern Sichuan Basin

Significant discoveries have been made in the gas exploration of layered-like dolomite reservoir of the Middle Permian Maokou Fm in eastern Sichuan Basin. However, no consensus has been reached on the genesis and distribution of dolomitization in this formation, which restricts the exploration and deployment of natural gas in this area. For clarifying the direction of natural gas exploration in the Maokou Fm in this area, based on the petrographic and geochemical characteristics of core and outcrop samples, the dolomite reservoir characteristics, genesis and main controlling factors in the Maokou Fm were studied, and the dolomite development pattern was established. The results show that: (1) the Maokou Fm dolomite is mainly developed in the middle and lower part of the third member of the Maokou Fm, generally in a layered or stratoid style. The lithology is mainly fine–medium crystal dolomite, siliceous dolomite, and bioclastic dolomite etc. The reservoir physical properties are better, and there are mainly two types of reservoir space developed: pore and hydrothermal dissolution fracture–cave; (2) different features are shown in trace elements and their contents, strontium isotope ratios and inclusion homogenization temperatures of between dolomite and saddle dolomite in the matrix of the Maokou Fm, which indicates that the formation fluids or periods of the above two types of dolomite differ greatly. The early hydrothermal sedimentation formed the matrix dolomite, and the late hydrothermal dissolution reformation formed the saddle-like dolomite filled with fractures and caves; (3) the development and distribution of the dolomite in the Maokou Fm are jointly controlled by the bioclastic beach, hydrothermal sedimentation and basement faults. Among them, the bioclastic beach is the base of developing dolomite reservoir, hydrothermal sedimentation formed early layered matrix dolomite, and basement faults provided migration channels for later Mg-rich hydrothermal fluids and improved reservoir performance. The research results are of great significance to deepening the understanding of the genesis and distribution of the Maokou dolomite reservoir in the eastern Sichuan Basin.