川南长宁地区五峰组—龙马溪组页岩脆性特征

脆性是页岩气储层地质评价的重要内容。以露头和钻井资料为基础，提出基于“石英、白云石和黄铁矿”3种矿物的脆性指数计算方法，并结合弹性参数脆性指数法对川南长宁地区五峰组—龙马溪组页岩脆性特征进行了定量评价；在此基础上着重分析了脆性指数与矿物组成和有机碳含量的相关性；应用“矿物三端元+沉积微相”法，在有利岩相研究的基础上，综合页岩脆性特征对研究区“甜点层”进行了进一步的优选。结果表明：研究区五峰组—龙马溪组底部厚度约30~40m的页岩段脆性最好，矿物脆性指数达50%~75%，杨氏模量平均为37.84GPa，泊松比平均为0.19，弹性参数脆性指数平均为61%，且自下而上脆性变差；脆性指数与矿物组成和有机碳含量的相关性显著，反映了沉积环境对页岩脆性特征的重要影响；长宁地区的“甜点层”为深水陆棚发育的硅质页岩相和钙质硅质混合页岩相，平均脆性指数大于50%，平均有机碳含量大于3%，分布于五峰组至鲁丹阶中下部，累计厚度约为42m。

Brittleness characteristics of Wufeng-Longmaxi shale in Changning region,Southern Sichuan,China

Brittleness is significant for geological evaluation of shale-gas reservoirs.Based on outcrop and drilling data,this paper first proposed a new brittleness index,which use quartz,dolomite and pyrite as brittle components,and evaluated the brittleness of Wufeng-Longmaxi shale in Changning region,Southern Sichuan combined with the elastic parameters-based index.And then we analyzed the relationship between brittleness and mineral composition,TOC.Applying “ternary diagram combined with depositional microfacies” method to classify the lithofacies types of Wufeng-Longmaxi shale,we further selected the “sweet spot” layer on the basis of favorable lithofacies according to previous studies.The results showed that:(1)Thelower part of Wufeng-Longmaxi shale,with appropriate thickness of 30-40m,has the best brittleness characteristics:mineral-based brittleness index of 50%-75%,Young’s Modulus of 37.84GPa,Poisson’s Ratio of 0.19,and average elastic parameters-based brittleness index of 61%.And the brittleness index decreases upward.(2)Brittleness index correlates well with mineral composition and TOC,reflecting the control of depositional environment on shale brittleness.(3)Siliceous and calcareous-siliceous shale deposited in deep-water shelf,which were developed in Wufeng to the middle-lower part of Rhuddanian with average brittleness index>50%,average TOC>3% and total thickness of appropriately 42m,are “sweet spot” layer in Changning region.