四川盆地五峰组—龙马溪组页岩脆性评价与“甜点层”预测

页岩气储层评价过程中,有机碳含量是表征富有机质页岩的主要指标,脆性指数则是反映压裂品质的重要参数,有机碳含量与脆性指数相结合可以更为精细地刻画"甜点层"的分布,从而为勘探开发工作提供有利依据。为此,依据岩矿资料,应用"石英+白云石+黄铁矿"三矿物脆性指数模型对四川盆地焦石坝、长宁和威远3个气田的上奥陶统五峰组—下志留统龙马溪组富有机质页岩进行了脆性评价。基于脆性指数(BI)与有机碳含量(TOC)之间的显著相关性,综合有机质丰度和可压裂性,建立了储层品质的分级标准:TOC3%且BI50%为优质储层(甜点层);2%TOC3%且40%BI50%为次优储层;1%TOC2%且30%BI40%为差储层;TOC1%且BI30%为非储层。依据该标准,长宁地区优质页岩气储层厚度为32 m,焦石坝地区优质储层厚度为22 m,威远地区优质储层厚度为13 m。探索脆性指数与测井响应之间的相关性,并建立了富有机质、高脆性页岩的常规测井响应图版,以自然伽马大于160 API、钍铀比小于3且密度小于2.58 g/cm~3作为优质储层的测井识别标准,进而预测了四川盆地五峰组—龙马溪组页岩"甜点层"的平面展布特征。

Evaluation of the Wufeng–Longmaxi shale brittleness and prediction of "sweet spot layers" in the Sichuan Basin

In the geological evaluation of shale gas reservoirs, organic matter content is a major indicator for characterizing organic matter- rich shale and brittleness indexes are the prominent parameters reflecting the quality of fracturing. And the combination of organic matter content and brittleness indexes can more finely depict the distribution of "sweet spot layers", thereby providing favorable foundation for the exploration and development of shale gas. In view of this, based on rock and mineral data and by applying the brittleness indexes model of three minerals of "quartz+dolomite+pyrite", the brittleness evaluation of organic matter-rich shale in the Upper Ordovician Wufeng Fm–Lower Silurian Longmaxi Fm in the three gas fields of Jiaoshiba, Changning and Weiyuan were conducted. Based on the significant correlation between brittleness indexes (BI) and total organic carbon (TOC), organic matter abundance and fracturing quality, the classification standard for reservoir quality was established: a high-quality reservoir (a sweet spot layer) (TOC＞3% andBI＞50%); a sub-high-quality reservoir (2%＜TOC＜3% and 40%＜BI＜50%); a poor reservoir (1%＜TOC＜2% and 30%＜BI＜40%); a non-reservoir (TOC＜1% andBI＜30%). According to this standard, the thickness of high-quality shale gas reservoir in the Changning, Jiaoshiba and Weiyuan areas is 32 m, 22 m and 13 m respectively. By exploring the correlation between brittleness indexes and logging responses, the conventional well logging response plate of organic matter-rich and high brittle shale was established. Taking GR＞160 API, Th/U＜3 and DEN＜2.58 g/cm3 as the logging standard for identifying high-quality reservoirs, the planar distribution features of the "sweet spot layers" for the Wufeng Fm–Longmaxi Fm shales in the Sichuan Basin were predicted.