川中合川地区二叠系茅二段白云岩储层特征与成因分析

Characteristics and Genesis Analysis of the Dolomite Reservoir in the Member 2 of the Permian Maokou Formation in Hechuan Area,central Sichuan Basin

与川西南地区茅口组二段白云岩储层不同,合川地区茅二段发育两类储层:灰岩储层、白云岩储层。结合区域地质背景,利用钻井取芯样品,通过岩芯薄片、阴极发光、碳氧同位素、常量元素、微量元素、盐水包裹体等定量分析,对合川地区茅二段储层岩石学特征、地球化学特征及其主控因素进行了研究。认为研究区茅二段储层岩性以粒屑灰岩及白云岩为主;粒屑灰岩具明显相控特征,残余粒间孔发育;白云岩储层以残余砂屑中—细晶白云岩和中粗晶白云岩为主,形成于埋藏白云石化作用叠加后期热液白云石化作用的对粒屑灰岩和泥晶灰岩的交代改造。储集空间以残余粒间孔、晶间孔及沿裂缝带发育的溶蚀孔洞缝为主。研究区内茅口组形成储层的控制因素有:沉积相带、白云石化作用、构造作用及海平面升降变化等,沉积相带是茅二段灰岩储层的主控因素,同时也是白云岩储层形成的先天基础,埋藏白云石化作用增强初始储集空间的保存及改善,构造断裂带是改善储层物性及油气运聚的必要条件。

Different from the dolomite reservoirs of the Maokou Formation in the southwestern Sichuan region,there are two types of reservoirs developed in the Maokou Formation in the Hechuan region:limestone reservoirs and dolomite reservoirs.Combined with the regional geological background,the Petrology characteristics,geochemical characteristics and main controlling factors of Mao-2 reservoir in Hechuan area were studied by using drilling core samples and quantitative analysis of core slices,cathodoluminescence,carbon and oxygen isotopes,major elements,trace elements,and brine inclusions.The results show that the reservoir lithology of the Mao-2 section in the research area is mainly composed of granular limestone and dolomite.Granular limestone has obvious facies control characteristics,and residual intergranular pores are developed.The dolomite reservoir is mainly composed of residual fine-grained dolomite and coarse-grained dolomite,which were formed during the replacement and transformation of granular limestone and muddy limestone during the later stage of buried dolomitization and hydrothermal dolomitization.The storage space is mainly composed of residual intergranular pores,intergranular pores,and dissolution pores and fractures developed along the fracture zone.The control factors of Maokou Formation reservoir formation in the study area include sedimentary facies belt,dolomitization,tectonism and sea-level change,etc.The sedimentary facies belt is the main control factor of Mao-2 limestone reservoir,and is also the innate foundation of dolomite reservoir formation.Buried dolomitization enhances the preservation and improvement of initial reservoir space,and structural Fracture zone is a necessary condition for improving reservoir physical properties and oil and gas migration and accumulation.