广西东兰-凤山地区石炭纪-二叠纪碳酸盐岩洞穴发育特点及其控制因素

碳酸盐岩次生溶蚀孔洞是优质的储集空间,为油气的运聚提供条件。为研究溶洞发育的控制因素,选取广西东兰-凤山地区石炭纪-二叠纪碳酸盐岩溶蚀特征进行分析。镜下显示,研究区岩石类型为亮晶生屑灰岩、颗粒灰泥灰岩、含颗粒灰泥灰岩、晶粒灰岩和灰泥颗粒灰岩等,孔隙类型主要为粒间溶蚀孔、粒内溶蚀孔、晶间孔和微裂缝。研究发现,影响洞穴发育的主要因素有构造、岩组类型、流体性质、气候及岩溶古地貌。构造控制洞穴发育规模,在构造破碎带及其附近容易发育大规模洞穴;目的层大套灰岩是溶蚀的物质基础,力学强度较大的亮晶生屑灰岩易于形成大型洞穴,当岩石含较多灰泥或发生重结晶时,溶蚀难度增加;酸性流体、温暖潮湿气候和岩溶斜坡地貌,为洞穴发育的有利条件。其中,构造对洞穴发育的控制作用最大。

Development Features and Controlling Factors of Carboniferous-Permian Carbonate Caves in Donglan-Fengshan Area,Guangxi

Secondary porosity generated by dissolution of carbonate rocks are high-quality reservoirs for oil and gas, and provide favorable conditions for hydrocarbon accumulation and migration. To study the controlling factors of karst development, we analyzed the Carboniferous-Permian carbonate dissolution characteristics in Donglan-Fengshan area (Guangxi Province). Under the optical microscope, rock types identified include crystalline limestone and granular muddy limestone. The pore types are mainly intergranular dissolved pore, intragranular dissolved pore, intercrystalline pore and micro-fractures. The study shows that the major controlling factors on the cave development in the study area are tectonics, rock-fabrics, fluids properties, climate and paleogeomorphology. Among these factors, tectonics controlled the size of the caves. Larger caves are better developed along/around major structural zones. Moreover, large beds of limestone contribute the basic material for dissolution. Skeletal granular limestone with high mechanical strength can readily develop large caves. Difficulty of dissolution increases when the rocks are more muddy or recrystallized. Acidic fluids, humid climate and karst slope are favorable conditions for karst development in the study area. All in all, tectonics is the prime controlling factor in karstic cave development.