Elastic Anisotropy Modeling of Organic-Rich Lower Gondwana Shale in Eastern India

Abstract

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.