Analysis of cores from the Lower Gondwana basin in eastern India confirms the presence of abundant organic matter, indicating a high shale gas prospective zone. Shale is intrinsically (transversely) anisotropic due to the preferential orientation of anisotropic clay platelets depending on the geological processes of deposition. Such anisotropy needs to be incorporated in modeling for an accurate explanation of observed seismic anisotropy in shale. Combined anisotropic formulations of self-consistent approximation (SCA) and differential effective medium (DEM) theory is the most suitable among existing theories to deal with the complex microstructure of shale. We start by predicting the bulk and rigidity moduli of clay mineral aggregates as 21 GPa and 10 GPa from our model, which are difficult to measure in the laboratory due to the small crystal size of clay. A concept of host medium (HM) is presented here, which is constituted of clay mineral aggregates, kerogen, and unconnected pores. Other minerals are added in the biconnected composite constituted of HM and connected pores filled with water. An orientation distribution function (ODF) of clay particles is determined using the combined SCA-DEM theory from the observed ultrasonic velocity measurements. Our model shows strong intrinsic anisotropy at the shallow depth that decreases with depth because of the changes in the microstructure of the shale. The P-velocity predicted from our model, widely used Biot–Gassmann theory (BGT) and Biot–Gassmann theory modified by Lee (BGTL) match well with the measured data where P-wave anisotropy is insignificant. We also predict from our model the volume of kerogen and total organic carbon as 26–43% and 6–8%, respectively.

中文翻译：

---

印度东部冈瓦纳下层富含有机质页岩的弹性各向异性建模

对印度东部下冈瓦纳盆地岩心的分析证实存在丰富的有机质，表明页岩气远景带。页岩本质上是（横向）各向异性的，这是由于各向异性粘土片的优先取向取决于沉积的地质过程。为了准确解释在页岩中观测到的地震各向异性，需要将这种各向异性纳入建模中。自洽近似 (SCA) 和微分有效介质 (DEM) 理论的各向异性组合公式是现有理论中最适合处理页岩复杂微观结构的方法。我们首先根据我们的模型预测粘土矿物骨料的体积和刚度模量分别为 21 GPa 和 10 GPa，由于粘土的晶体尺寸小，在实验室中难以测量。这里提出了宿主介质 (HM) 的概念，它由粘土矿物聚集体、干酪根和不连通的孔隙构成。在由HM和充满水的连通孔构成的双连通复合物中添加了其他矿物质。粘土颗粒的取向分布函数 (ODF) 使用组合 SCA-DEM 理论从观察到的超声速度测量中确定。由于页岩微观结构的变化，我们的模型在浅层显示出强烈的内在各向异性，随着深度的增加而减小。从我们的模型、广泛使用的 Biot-Gassmann 理论 (BGT) 和 Lee 修正的 Biot-Gassmann 理论 (BGTL) 预测的 P 速度与 P 波各向异性无关紧要的测量数据非常匹配。