Rock-physics templates for clay-rich source rocks

Abstract

Shale source rocks are composed of various minerals, mainly smectite and illite, depending on the burial depth, and they can be described as transversely isotropic media. The “pore space” may contain kerogen, water, oil, and gas determined by the in situ conditions. A petroelastic description is based on the following: Smectite-illite transformation as a function of depth is described by a fifth-order kinetic reaction. Backus averaging to “mix” isotropic smectite and anisotropic illite is then used to obtain the elasticity constants of the mineral composing the frame. Porosity is obtained from density, and water is part of the mineral, whose elasticity constants are obtained from Gassmann equations. Oil and gas generated from kerogen are assumed to saturate the kerogen phase. The bulk modulus of the oil-gas mixture is calculated by a mesoscopic-loss model, and the stiffnesses of the kerogen/fluid mixture are obtained with the Kuster and Toksöz model, assuming that the fluid is included in a kerogen matrix. Two models are considered to obtain the seismic velocities of the shale, namely, Backus averaging and the Gassmann equation generalized to the anisotropic case with a solid pore infill. We built rock-physics templates (RPTs) containing only kerogen (immature) and kerogen plus hydrocarbons (mature). Pore-pressure effects were modeled and used as templates. We considered the Kimmeridge Shale at different depths. To model kerogen-oil and oil-gas conversions, we assumed a basin-evolution model with a constant sedimentation rate, geothermal gradient, and a first-order kinetic (Arrhenius) reaction. The detection of the hydrocarbons was investigated from RPTs, built with wave velocities, impedances, Lamé constants, density, Poisson ratio, Young modulus, and anisotropy parameters for varying kerogen content, fluid saturations, and pore pressure. Moreover, the amplitude variation with offset intercept and gradients were computed, corresponding to the seismic response of a source rock layer for varying kerogen content and fluid saturation.