A modified rock physics model of overmature organic-rich shale: application to anisotropy parameter prediction from well logs

A well log penetrating the Lower Silurian shale in the south of Sichuan Basin shows that this organic-rich shale formation presents strong vertically transverse isotropy properties. The magnitude of the anisotropy is determined by clay content and the volume of clay-bound water, but kerogen content is found to have a negative correlation with seismic anisotropy. Core sample analysis reveals an extremely high level of maturity. This overmature shale includes a large volume of randomly distributed organic matter and developed pores of free water and gas; both phases reduce anisotropy. We propose a modified rock physics model based on the result of analysis, and this model can successfully describe the elastic behaviour of this overmature organic-rich shale. The intrinsic anisotropy is modelled from varied laminated clay minerals, and the clay-related pore space by using effective medium theories, while kerogen is considered as an isolated inclusion embedded with clay platelets. This model is applied to anisotropy parameter prediction from well log data. The clay-related pore geometry and a lamination parameter are inverted from C-33 and C-44 in a borehole. The end result is supported by agreement between prediction and borehole measurement of C-66 and Thomsen's anisotropy parameter, gamma. Besides, the anisotropic synthetic angle gather generated using the predicted logs shows a better seismic well tie than the isotropic synthetic angle gather. Therefore, this method can provide reliable anisotropy parameters for anisotropic seismic inversion and interpretation, and thus is useful for reservoir characterization.