Elastic Anisotropy Modeling of Organic-Rich Lower Gondwana Shale in Eastern India
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Pure and Applied Geophysics
Elastic Anisotropy Modeling of Organic-Rich Lower Gondwana Shale in Eastern India RANJANA GHOSH,1
PIYUSH SARKAR,2,3 and KUMAR HEMANT SINGH2
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. Keywords: Gondwana basin, shale gas, rock physics modeling, SCA-DEM theory, ODF, anisotropy.
1. Introduction Sedimentary basins are mostly (about 60–70%) covered by the fine-grained fissile shales and non-
1 CSIR-National Geophysical Research Institute, Hyderabad, India. E-mail: [email protected] 2 Indian Institute of Technology, Mumbai, India. 3 Indian Institute of Technology (Indian School of Mines), Dhanbad, India.
fissile mudstones (Broichhausen et al. 2005). These clay-rich sediments are significant because they act as seals, source rocks, and also unconventional reservoirs. Shale of low permeability due to fine-grained texture plays a vital role in regulating fluid flow (Sayers 2005). Clay-rich sediments are characterized by intrinsic anisotropy due to alignment of clay platelets as evident from the seismic (Alkhalifah and Rampton 2001; Banik 1984) and ultrasonic measurements (Wang 2002; Johnston and Christensen 1995; Hornby et al. 1994; Vernik and Nur 1992; Jones and Wang 1981). Such anisotropy needs to be appropriately taken into account to avoid erroneous interpretation of amplitude variation with offset analyses, seis
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