Summary
The goal of this study is to more accurately establish the cause-and-effect relationships between organic geochemistry, rock physics, and shale microstructures as the organic-rich Barnett shale matures. We build upon previous work by performing laboratory pyrolysis of core plugs at both temperature (360-425 °C) and hydrostatic confining pressures (22-25 MPa). By iteratively characterizing the velocity, porosity, permeability, and microstructure of an initially thermally immature, outcrop Barnett shale sample, we are able to determine the effect of the thermal decomposition of organic matter on the rock physics signatures of the intact composite rock.
Introduction
The effect of the thermal maturation of organic matter on the elastic anisotropy of shales remains poorly understood. This lack of understanding contributes to a central problem in rock physics: how to interpret the often-scattered elastic anisotropy trends observed in shale. Given that shales account for 75% of rocks in sedimentary basins (Jones and Wang, 1981), it is pivotal that the elastic anisotropy of shale and its evolution with thermal maturity be more fundamentally understood. Additionally, in order to provide more accurate constraints to rock physical models, it is necessary that the elastic anisotropy of shales and processes governing its evolution be more fully understood.
Previous work has identified multiple sources of anisotropy in shale. Primary anisotropy in shale results from a preferred orientation of the mineral frame (Kaarsberg, 1959; Jones & Wang, 1981; Vernik and Nur, 1992; Hornby, 1995; Johnston and Christensen, 1995; Sayers, 1999 and 2005; Wang, 2002; Valcke et al., 2006; Wenk et al., 2007; Lonardelli et al., 2007; Kanitpanyacharoen et al., 2011; Allan et al., 2014b). This primary anisotropy is overprinted by microcracking arising from the generation and expulsion of hydrocarbons with increasing thermal maturity (Vernik, 1993; Johnston and Christensen, 1995; Dewhurst and Siggins, 2006; Allan et al., 2014a, 2014b). Finally, the mobilization of crack-filling, increasingly aromatized pyrobitumen is hypothesized to contribute to shale anisotropy as the organic matter matures (Tissot et al., 1974; Taylor et al., 1998; Teerman et al., 1987; Peters et al., 2004; Prasad et al., 2011).
