Source-rock seismic-velocity models: Gassmann versus Backus

Abstract

Source rocks are described by a porous transversely isotropic medium composed of illite and organic matter (kerogen, oil, and gas). The bulk modulus of the oil/gas mixture is calculated by using a model of patchy saturation. Then, the moduli of the kerogen/fluid mixture are obtained with the Kuster and Toksöz model, assuming that oil is the inclusion in a kerogen matrix. To obtain the seismic velocities of the shale, we used Backus averaging and Gassmann equations generalized to the anisotropic case with a solid-pore infill. In the latter case, the dry-rock elastic constants are calculated with a generalization of Krief equations to the anisotropic case. We considered 11 samples of the Bakken-shale data set, with a kerogen pore infill. The Backus model provides lower and upper bounds of the velocities, whereas the Krief/Gassmann model provides a good match to the data. Alternatively, we obtain the dry-rock elastic moduli by using the inverse Gassmann equation, instead of using Krief equations. Four cases out of 11 yielded physically unstable results. We also considered samples of the North Sea Kimmeridge shale. In this case, Backus performed as well as the Krief/Gassmann model. If there is gas and oil in the shale, we found that the wave velocities are relatively constant when the amount of kerogen is kept constant. Varying kerogen content implies significant velocity changes versus fluid (oil) saturation.

Effect of kerogen on rock physics of immature organic-rich chalks

“…Sayers (2013) concluded that the kerogen in the Bakken reduces elastic stiffnesses when forms an interconnected network instead of discrete inclusions. Carcione et al (2011) indicated that if kerogen is treated as part of the solid skeleton, it is optional to model the effective moduli using Hashin-Shtrikman bounds (Hashin and Shtrikman, 1963). Although Gassmann's theory is frequently used for predicting elastic moduli, it is noteworthy that organic-rich rocks do not obey some basic assumptions of that theory (Gassmann, 1951;Mavko et al, 2009).…”

“…The Bakken shale is a case in point of rock physics of transversely isotropic source rocks (e.g. Carcione et al, 2011;Sayers, 2013;Vernik and Nur, 1992). Shale fabric is conceptualized as lenticular grains of illite rather than continuous sheets (Vernik and Landis, 1996;Vernik and Milovac, 2011).…”

Fracturing of organic-rich shale during heating

“…The authors reported poroelastic constants of 0.3-0.6 and 0.1-0.8 for Haynesville and Bossier shales, respectively; the low values that are specific for stiff, low porosity rocks. Carcione et al (2011) theoretically studied effects of kerogen fraction and partial saturation on elastic properties of organicrich shales at different frequencies. Sayers (2013) used the database provided by Vernik and Liu (1997) to test three different effective medium theories.…”

A rock physics model for the characterization of organic-rich shale from elastic properties

“…The shale is composed of quartz, clay, carbonate, the mixture of kerogen and hydrocarbons, and fluid-saturated pores in the shale rock matrix. Elastic properties of the constituents and oil, gas, and water are given in Table 1 (Carcione et al 2011;Mavko et al 2009). …”

“…Recently, the impact of kerogen on elastic properties and seismic responses of shales has received more interest. Carcione et al (2011) and Vernik and Milovac (2011) researched and compared rock physics models for organic-rich shales. Zou et al (2011) analyzed the capacity of reservoirs in terms of nano-porosity in kerogen in shales.…”