A model for seismic velocity and attenuation in petroleum source rocks

Petroleum source rock is modeled as a viscoelastic transversely isotropic m edium composed of illite/smectite and organic matter. The wave velocities a nd attenuation of petroleum source rocks are obtained as a function of exce ss pore pressure, initial kerogen content, and water saturation. The model generalizes a previous approach based on a pure elastic formulation of Back us averaging and introduces the pressure effect and the presence of fluids (oil and water). The model allows the simulation of different maturation le vels induced by pore-pressure changes caused by the conversion of kerogen t o oil. The higher the oil saturation, the higher the maturation level. Assu ming that the source rock has a very low permeability, the excess pore pres sure can be calculated:as a function of the conversion factor. Then the bul k modulus and density of the kerogen/oil mixture are obtained with the Kust er 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 hi gher 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 k m/s higher than the corresponding P and S normal velocities. The maximum at tenuation and stiffness anisotropies are obtained for 18% and 30% volumetri c kerogen content, respectively. Both velocities and quality factors decrea se with increasing kerogen content at a given pore pressure. The decrease i n wave velocity is 2 km/s for P-waves and 1 km/s for S-waves when kerogen i ncreases from zero to 100%. Moreover, anisotropy increases and velocities d ecrease with increasing pore pressure, i.e., with higher kerogen-to-oil con version. Finally, the presence of water affects the normal-bedding velocity , i.e., higher water saturation implies lower velocities.