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Superdeep fault-karst carbonate reservoirs discovered over 7 km deep are controlled by strike-slip fault zones and karst collapses in Tarim Basin, Northwest China. The resulting fracture-cave system provides favorable migration channels and reservoir spaces for hydrocarbon, while the characterization of the internal fault structures remains enigmatic. Based on seismic imaging data, we conducted an integrated study on fault damage zones by seismic curvature attributes, velocity anisotropies, and seismic attenuations. The results show that three typical fault-zone patterns can be identified in the study area, including paratactic multiple fault cores, interactive fault cores and one primary-several subsidiary fault cores. These typical patterns can be clearly characterized via curvature attributes. The elastic characteristics of fault damage zones are significantly affected by seismic frequencies, which are manifested from velocity anisotropies and seismic attenuations. The maximum seismic attenuation occurs along with the orientation of fault cores. There is a strong anisotropic characteristic of P-wave phase velocity with incident angle of three fault-zone models. It appears that seismic attributes associated with geological steering are an effective tool for the subsurface characterization of fault damage zones. [ABSTRACT FROM AUTHOR]

Natural fractures are usually multiscale in size, orientations and distribution, resulting in complex anisotropic characteristics. Analytical methods for quantifying the associated effective elastic properties are based on some assumptions, such as dilute fracture concentration and regular-shaped fractures, which do not occur in actual reservoirs. Here, we conduct anisotropic finite-element modeling of effective elastic properties of complex fractured rocks using the least-square fitting method. The algorithm is developed for a 3D case and validated by classical effective medium theories for models with different fracture densities. The results of the 3D numerical method agree well with the theoretical predictions at low fracture density. The model also considers the interactions among fractures to calculate equivalent stiffness tensors at high fracture density. Three 2D fracture models are simulated to demonstrate the basic behavior of stress interactions and their effect on the overall elasticity under different fracture densities. We applied the developed model to 3D natural fractures built from a real outcrop, and we found that the fracturing pattern significantly affects the effective anisotropy properties. The resultant P-wave phase velocities as functions of the incidence angle and frequency are anisotropic. This study provides a great potential to calculate equivalent stiffness tensors and anisotropic properties of 3D multiscale natural fractures. [ABSTRACT FROM AUTHOR]