川西南自贡地区二叠系茅口组 储层裂缝特征及期次演化

Characteristics and formation period of fractures in the reservoirs of Permian Maokou Formation,Zigong area,Southwest Sichuan Basin

四川盆地自贡地区二叠系茅口组海相碳酸盐岩储层裂缝普遍发育,对储层的储集性能、渗流规律和油气富集具有重要影响。综合利用岩心、薄片、成像测井及分析测试等资料,明确裂缝成因类型及发育特征,分析裂缝形成时间,确定裂缝形成期次。结果表明:自贡地区茅口组海相碳酸盐岩储层发育构造剪切裂缝、构造张性裂缝、水平层理缝和成岩缝合线;构造剪切裂缝为茅口组储层主要裂缝类型,以NEE和NNE走向为主,裂缝倾角介于20°~80°,延伸长度小于60 cm,充填程度较低,有效性较好;结合分析测试资料确定自贡地区茅口组储层裂缝形成于3期构造运动:第1期为海西晚期-印支早期,在华南板块顺时针运动派生的SW向应力作用下,发育少量剪切裂缝,裂缝多被矿物充填,属于裂缝次要发育时期;第2期形成于燕山晚期-喜山早期,在江南雪峰隆起产生的NW向应力作用下发育大量构造裂缝,是研究区裂缝主要形成时期;第3期为喜山晚期,裂缝形成于印度洋板块碰撞产生的NEE向挤压应力下,且裂缝多无充填,有效性较好。通过上述裂缝相关研究,为研究区有利勘探区带确立提供了依据。

[Objective]Fractures commonly occur in the marine carbonate reservoirs of Permian Maokou Formation in Zigong area of Sichuan Basin and have important impacts on reservoir properties,seepage patterns and hydrocarbon enrichment.[Methods]Cores,thin sections,image logs and experimental test data were used to clarify the type of fracture genesis and development characteristics,analyse the time of fracture formation,and determine the period of fracture formation.[Results]The results show that the marine carbonate reservoirs of Maokou Formation in Zigong area are divided into two types,namely,tectonic fractures and diagenetic fractures,among which the tectonic fractures include tectonic shear fractures and tectonic tensile fractures,and the diagenetic fractures include horizontal bedding fractures and diagenetic sutures.Tectonic shear fractures dominate Maokou Formation reservoir and occur mainly in the NEE and NNE directions.The fracture inclination angle ranges from 20°to 80°,and the extension length is less than 60 cm.The fracture degree of filling is low,and the validity is good.Combined with the analysis and test data,the reservoir fractures of Maokou Formation in Zigong area were determined to have formed by 3 stages of structural movement.The first stage included the late Hercynian and early Indosinian periods,approximately 240-220 Ma.Under the SW stress derived from the clockwise movement of the South China Plate,a small number of shear fractures developed,and the fractures were mostly filled with minerals,representing the secondary development period of fractures.The second stage occurred in the late Yanshan-early Himalayan period,approximately 78-69 Ma,and a large number of tectonic fractures developed under the NW-trending stress generated by the Xuefeng uplift in Jiangnan,which was the main period of fracture formation in the study area.The third period was the late Himalayan period,approximately 13-0 Ma.Fractures were formed under the NEE compressive stress generated by the collision of Indian Ocean Plate,and most of the fractures were unfilled,indicating good effectiveness.[Conclusion]The above fracture-related research provides the basis for establishing favourable exploration zones in the study area.