中国海相页岩岩相精细划分及测井预测:以四川盆地南部威远地区龙马溪组为例

Fine lithofacies of China's marine shale and its logging prediction:A case study of the Lower Silurian Longmaxi marine shale in Weiyuan area,southern Sichuan Basin,China

开展海相页岩岩相精细划分及预测研究对页岩气勘探开发层位选择具有重要意义。本文基于页岩储层品质及压裂品质两个要素,提出以TOC含量、矿物组成为基础的"两步法"页岩岩相划分方案。在岩心观察描述的基础上,利用岩石薄片分析、X全岩衍射分析及TOC含量测量等资料,以川南威远地区龙马溪组页岩为例,开展了页岩岩相划分,分析了不同岩相的属性差异。结合测井资料,根据概率神经网络原理,建立了页岩岩相预测模型,开展了该地区单井岩相预测,揭示了威远地区龙马溪组页岩岩相组合及分布特征:(1)威远地区龙马溪组发育11种岩相类型,其中5种富有机质页岩岩相,6种贫有机质岩相;(2)富有机质硅质页岩相、富有机质碳酸盐质-硅质页岩相及富有机质硅质-碳酸盐质页岩相因具有高TOC含量和高脆性特征,保证了页岩良好的生气能力及可压裂性,为最有利岩相类型;(3)有利岩相组合主要分布在龙马溪组底部,平面上自威远地区北西向南东方向,组合厚度增大,组合类型变化。北西部W1井,以富有机质硅质页岩岩相与富有机质碳酸盐质-硅质页岩相组合为主,厚20m;往南东至W2井,以富有机质硅质页岩相、富有机质碳酸盐质-硅质页岩相、富有机质硅质-碳酸盐质页岩相夹薄层状富有机质黏土质页岩相为主,厚30m;至南东部W4井,为富有机质硅质页岩相、富有机质碳酸盐质-硅质页岩相、富有机质硅质-碳酸盐质页岩相夹厚层状富有机质黏土质页岩相组合,厚35m。

Fine lithofacies research and prediction of marine shale are of great importance for the successful shale gas exploration.Based on the key parameters of shale and fractures,a lithofacies classification scheme for marine shale is proposed by using TOC content and mineral composition.Combined with TOC,thin section,X-ray diffraction and other core testing data,differences of lithofacies for Longmaxi Formation in the Weiyuan area have been analyzed.Furthermore,based on probabilistic neural network（PNN）,shale lithofacies of Longmaxi Formation in the Weiyuan area have been predicted by using well log,and the lithofacies associations have been characterized in detail：（1）There are 11 lithofacies for Longmaxi Formation in the Weiyuan area,5kinds of organic rich shale facies,6kinds of organic lean facies.（2）Among these 11 lithofacies,the organic rich siliceous shale,organic rich carbonate-siliceous shale and organic rich siliceouscarbonate shale,which contain high TOC content and high brittleness,are favorable lithofacies for shale gas exploration.（3）The favorable lithofacies are mainly concentrated on the lowermost part of the Longmaxi Formation,in which thickness increased from northwest to southeast with different favorable lithofacies associations in different part.In the northwest,W1 well has thinner favorable lithofacies associations with thickness of 20 m,mainly composed of organic rich siliceous shale and organic rich carbonate-siliceous shale.W2 well,located in the central part,has different favorable lithofacies associations with thickness of 30 m,mainly consisting of organic rich siliceous shale,organic rich carbonate-siliceous shale,organic rich siliceouscarbonate shale and thin organic rich clay shale.In the southeast,the favorable lithofacies associations of W4 well are mainly composed of organic rich siliceous shale,organic rich carbonate-siliceous shale,organic richsiliceous-carbonate shale and thick organic rich clay shale,which has thickness of 35 m.