川中北部地区下寒武统沧浪铺组一段碳酸盐岩微孔隙特征及其成因

Characteristics and genesis of carbonate micropores in the first member of Lower Cambrian Canglangpu Formation in north and central Sichuan Basin

周春宇 和源 李宗泽 周刚 杨岱林 孙奕婷 张亚 文华国 刘四兵

ZHOU Chunyu HE Yuan LI Zongze ZHOU Gang YANG Dailin SUN Yiting ZHANG Ya WEN Huaguo LIU Sibing

1. 成都理工大学 油气藏地质及开发工程国家重点实验室, 成都 610059; 2. 中国石油 西南油气田分公司 勘探开发研究院, 成都 610041

1. State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Chengdu University of Technology, Chengdu, Sichuan 610059, China; 2. Exploration and Development Research institute, Southwest Oil & Gas Field Company, PetroChina, Chengdu, Sichuan 610041, China

直径小于10 μm的微孔隙是目前深埋藏致密储层研究的热点。四川盆地下寒武统沧浪铺组具有良好的天然气勘探潜力，沧浪铺组一段(沧一段)白云岩中发育的大量微孔隙是天然气的主要储集空间，因此明确微孔隙的发育特征及成因机制是寻找有利勘探区的关键。通过铸体薄片鉴定、扫描电镜分析和岩心CT扫描，开展了川中北部地区寒武系沧一段白云岩微孔隙表征，并结合沉积环境与成岩作用探讨了微孔隙的成因机制。研究区沧一段鲕粒白云岩样品平均孔隙度为3.44%，最大孔隙度达9.44%，铸体薄片鉴定显示孔隙发育差，但扫描电镜分析、CT扫描等结果均显示存在大量微孔隙，表明研究区沧一段鲕粒白云岩储层孔隙度主要由微孔隙贡献。研究区沧一段储集空间以颗粒内广泛发育的晶间微孔隙为主，形态呈三角形、多边形，孔径分布主要集中在3~9 μm，呈点状或团状分布，宏观上呈现为鲕粒的轮廓，指示微孔隙主要分布在颗粒内部。研究区沧一段白云岩微孔隙的形成与沉积早期海平面变化导致的鲕粒滩被大气淡水选择性溶蚀密切相关，埋藏期的白云石化作用更是微孔隙形成的关键，而宏孔的消失则主要受压实、压溶作用及下伏地层油气充注的影响。

Micropores with diameters under 10 μm are a current research focus in deeply buried tight reservoirs. The Lower Cambrian Canglangpu Formation in the Sichuan Basin shows considerable potential for natural gas exploration. The abundant micropores in the dolomite of the first member of Canglangpu Formation (Cang 1) serve as the main storage space for natural gas. Therefore, clarifying the development characteristics and genesis mechanisms of micropores is key to identifying favorable exploration areas. Through cast thin-section identification, scanning electron microscopy (SEM) analysis, and core computed tomography (CT) scanning, the study characterized the micropores in the dolomite of the Cang 1 member in the north and central Sichuan Basin. Additionally, the genesis mechanisms were explored considering the sedimentary environments and diagenesis processes. The oolitic dolomite samples from the Cang 1 member in the study area had an average porosity of 3.44%, with a maximum porosity of 9.44%. Cast thin-section analysis indicated poor pore development. However, SEM and CT scanning images revealed the presence of abundant micropores, demonstrating that micropores were the primary contributors to porosity in the oolitic dolomite reservoirs of the Cang 1 member. The reservoir space in the study area is dominated by intercrystalline pores extensively developed in particles. These micropores exhibit triangular or polygonal shapes, with pore diameters mainly ranging from 3 to 9 μm, and are abundant, displaying a dot-like or clustered distribution. On a macroscopic scale, the micropores outline the contours of oolites, indicating that micropores are primarily distributed within particles. The development of micropores in the dolomite of the Cang 1 member was intimately linked to the selective dissolution of oolitic beaches by meteoric freshwater due to sea-level changes in early sedimentary stages. The dolomitization at the burial stage was crucial for micropore formation. The loss of macropores was mainly influenced by compaction, pressure solution, and hydrocarbon charging from the underlying formations.

国家自然科学基金面上项目(41972158)和四川省天然气成藏物质基础青年科技创新团队基金(2022JDTD0004)联合资助。

https://doi.org/10.11781/sysydz2025020311