Constraining seismic rock-property logs in organic shale reservoirs

Abstract

One of the major challenges of unconventional shale reservoirs is to understand the effects of organic richness (total organic carbon, TOC), mineralogy, microcracks, pore shape, and effective stress on elastic properties. The generation of petrophysical parameters, such as TOC, and quantification of total and organic porosities through a physically consistent petrophysical model are described. Rock-matrix density, which is a key parameter in determining total porosity, is estimated as a function of the amount of TOC and its level of maturity. Then the petrophysical parameters are used as inputs for rock-physics modeling to constrain the beddingnormal compressional-wave velocity as a function of various parameters (e.g., TOC, porosity, mineralogy, pore shapes, and microcrack density) in combination with effective stress. Modeling results on three shale plays from North America show that compressional-wave velocity in these specific formations is controlled mainly by variations in TOC, mineralogy, and pore shape. Shear-wave velocity in organic shales also was refined as a function of compressional-wave velocity and amount of TOC.

Estimation of shear‐wave velocities in unconventional shale reservoirs

“…Following the work of Hu et al . (2015), Yenugu and Vernik (2015) and Vernik (2016) proposed an empirical model for shear‐wave velocity prediction that is based on correlations with total organic carbon and compressional‐wave velocity. Vernik et al .…”

Organic content and maturation effects on elastic properties of source rock shales in the Central North Sea

“…In recent years, research on characterization of source rock shales has become more frequent due to the value of exploiting unconventional shale reservoirs (e.g., Passey et al, 1990;Vernik and Landis, 1996;Passey et al, 2010;Vernik and Milovac, 2011;Alfred and Vernik, 2012;Zhu et al, 2012;Sayers, 2013a;Yenugu and Vernik, 2015;Zhao et al, 2018). However, the understanding and theories developed are in principle equally applicable for characterization of organic-rich shales as conventional hydrocarbon source rocks.…”

“…Multiple studies assess and predict the petrophysical and elastic properties of organicrich shales with focus on variations in kerogen content, fluid content, mineralogy, microstructure, texture and anisotropy from a rock physics modeling perspective, mainly aimed at unconventional reservoirs (e.g., Passey et al, 1990;Vernik and Landis, 1996;Sondergeld and Rai, 2011;Vernik and Milovac, 2011;Alfred and Vernik, 2012;Zhu et al, 2012; Guo et al, 2013;Sayers, 2013a;Yenugu and Vernik, 2015;Zhao et al, 2016).…”

“…Thermal maturation also directly affects the elastic properties of kerogen, since maturation involves formation of organic pores within the solid kerogen. The density of the remaining kerogen increases because the carbon fraction increases relative to hydrogen (Alfred and Vernik, 2012;Yenugu and Vernik, 2015;Dang et al, 2016;Ibrahim and Mukerji, 2017).…”