AVO effects of a hydrocarbon source-rock layer

The organic content of petroleum source-rock layers can be determined from seismic data with a suitable AVO technique. Since the source rock has a layered structure, the best forward model is a transversely isotropic layer between two isotropic half-spaces, parameterized by the layer thickness and the amount of organic material. This limited number of parameters is essential to obtain a robust inversion method. Here, I compute the PP- and PS-reflection coefficients of the source bed as a function of layer thickness and organic content, including attenuation mechanisms related to different stages of maturation. The source rock is modeled as a viscoelastic medium composed of illite/smectite, solid organic matter (kerogen), and fluids (oil and water). The properties of the kerogen/oil/water mixture are obtained with the Kuster and Toksoz model, assuming that oil and water are the inclusion in a kerogen matrix. Then, Backus averaging gives the complex stiffnesses of the layer. To derive the reflection coefficients, I relate the particle velocities and stresses on the top and bottom interfaces between the layer and the substrates. The results indicate that there is ambiguity regarding the layer thickness, since two or more values can have similar reflection coefficients. For instance, for moderate offsets and 25% kerogen content, the reflection-coefficient curves oscillate with a period of 70 m for PP-waves and 40 m for PS-waves. On the other hand, the reflection coefficients have a minimum at approximately 25% kerogen content and increase monotonically above this value. Therefore, the inversion may provide reliable values above approximately 10% kerogen content.