上扬子地区下寒武统牛蹄塘组页岩储层脆性评价:以贵州岑巩区块为例

Brittleness evaluation of the Lower Cambrian Niutitang shale in the Upper Yangtze region:A case study in the Cengong block,Guizhou Province

页岩的脆性决定页岩储层的可压裂性,对页岩气的勘探开发影响重大。南方地区发育多套高脆性海相富有机质页岩,是我国页岩气勘探部署的重要战略地区。与已获得页岩气突破的龙马溪组页岩相比,下寒武统牛蹄塘组具有更大的沉积厚度与更广泛的分布面积,是我国页岩气勘探开发亟待突破的另一重点层系。为此,利用研究区牛蹄塘组岩心岩石力学实验、阵列声波测井、矿物组分和裂缝发育程度等资料,运用弹性参数法与脆性矿物含量法对牛蹄塘组页岩脆性特征与主控因素进行评价与分析。研究表明:牛蹄塘组页岩脆性与以石英为主的脆性矿物含量间并非呈单一正相关关系。页岩脆性与可压裂性受控于页岩的脆性矿物含量、TOC含量、热演化程度(成岩阶段)和天然裂缝发育程度。由于石英与TOC含量具有正相关性,当TOC含量小于6.5%时,牛蹄塘组页岩脆性与石英和TOC含量呈正相关关系;当TOC含量大于6.5%时,TOC对塑性贡献速率大于石英对脆性贡献速率,页岩脆性随石英与TOC含量的增加出现降低趋势。此外,岩心裂缝发育程度与页岩岩石力学脆性特征相关性较好,在过高TOC含量层段页岩脆性较低,岩心裂缝发育程度较差。对于中国南方海相页岩脆性评价,应结合不同地区、不同层位的实际地质条件进行多因素综合评价,以优选出最佳钻井与压裂层段。

The brittleness determines the fracability of shale gas reservoir,which has a significant impact on shale gas exploration and development.Recently,China has achieved a breakthrough of the Lower Silurian Longmaxi shale in the Sichuan Basin and its surrounding areas.Compared with the Longmaxi shale,the Lower Cambrian Niutitang shale has a great deposition thickness and wide distribution area,which is another significant stratum for China＇s shale gas.In this paper,the brittleness of Niutitang shale was evaluated and analyzed in the study area based on the data of core mechanical test,array sonic logging,mineral composition and fracture parameters.The results show that the relationship between brittleness and brittle mineral content is not a simple positive correlation.The brittleness and fracability of shale are determined by the content of brittle minerals and TOC（total organic carbon）,the degrees of thermal evolution（diagenesis stage）and fracture development.Because of the positive correlation between quartz and TOC contents,when the TOC content is less than 6.5%,the increase in the brittleness caused by the increase in the quartz content is greater than the increase in the ductility caused by the increase in the TOC content;when the TOC content is greater than6.5%,the ductility increases faster than the brittleness,which results in negative correlations of the brittleness with the TOC and quartz contents and low fracture density in the sections with high TOC content.This understanding will guide the exploration and development of shale gas and is very helpful for optimizing drilling and fracture stimulation intervals in southern China.