A model for seismic velocity and attenuation in petroleum source rocks

Petroleum source rock is modeled as a viscoelastic transversely isotropic medium composed of illite/smectite and organic matter. The wave velocities and attenuation of petroleum source rocks are obtained as a function of excess pore pressure, initial kerogen content, and water saturation. The model generalizes a previous approach based on a pure elastic formulation of Backus averaging and introduces the pressure effect and the presence of fluids (oil and water). The model allows the simulation of different maturation levels induced by pore-pressure changes caused by the conversion of kerogen to oil. The higher the oil saturation, the higher the maturation level. Assuming that the source rock has a very low permeability, the excess pore pressure can be calculated:as a function of the conversion factor. Then the bulk modulus and density of the kerogen/oil mixture are obtained with the Kuster and Toksoz model, assuming that oil is the inclusion in a kerogen matrix. Finally, Backus averaging of this mixture with the illite/smectite layers gives the complex stiffnesses of the transversely isotropic and anelastic medium. Computed P- and S-velocities and quality factors parallel to bedding are higher than those normal to bedding, with attenuation anisotropy higher than stiffness anisotropy. In particular, for the North Sea Kimmeridge Shale and at maximum anisotropy, P and S parallel velocities are approximately 0.7 km/s higher than the corresponding P and S normal velocities. The maximum attenuation and stiffness anisotropies are obtained for 18% and 30% volumetric kerogen content, respectively. Both velocities and quality factors decrease with increasing kerogen content at a given pore pressure. The decrease in wave velocity is 2 km/s for P-waves and 1 km/s for S-waves when kerogen increases from zero to 100%. Moreover, anisotropy increases and velocities decrease with increasing pore pressure, i.e., with higher kerogen-to-oil conversion. Finally, the presence of water affects the normal-bedding velocity, i.e., higher water saturation implies lower velocities.