Estimation of Anisotropy Parameters for a Clay-rich Shale Formation Based on a Rock Physics Model

Shale represents strongly intrinsic vertical transverse isotropy (VTI) due to its unique microstructure. One of the major reasons causes the seismic anisotropy of shale is the platy shape and preferred orientation of clay particles and kerogen. Effectively modelling the seismic anisotropy is important in the seismic processing and inversion, as well as the hydraulic fracture monitoring. In order to construct a realistic rock physics model for clay-rich shales, it is important to simulate the intrinsic VTI anisotropy caused by the present of various clay minerals.

In this paper, we propose a practical framework for the estimation of effective elastic stiffness of clay-rich shales. The intrinsic VTI anisotropy is modelled by the construction of background basic units using variously aliened clay minerals as well as kerogen, which is more general than the existing models. Other minerals and pores are included in the anisotropic background by means of the anisotropic differential effective medium method. We apply the rock physics model to predict the elastic properties of a shale formation in Western Sichuan basin, and the results are demonstrated with the corresponding logs. The predicted vertical P- and S-wave velocities show good agreement with the real logs; the estimated anisotropy parameters are used to calibrate the AVA (amplitude versus angle) response and to correct the P-wave velocity log in the horizontal well.