The determination of fracture density and orientation is important in plays in which reservoir permeability is associated with micro-fractures. In these reservoirs, effective porosity is dependent upon the density of open fractures and permeability has a directivity component associated to the fractures’ orientation. Accurate determination of fracture density and orientation using surface seismic helps, then, in determining locations of good reservoir storability, as well as directional drilling orientation for optimum permeability. The anisotropy of P- and S-wave velocities is commonly associated with the magnitude and orientation of stress fields or open fractures and hence, is the medium through which factures can be characterized using surface seismic data.
A case study in the Marcellus Shale (NE United States) is presented in which PP and PS wide azimuth seismic data, acquired and processed to preserve the amplitude and velocity information of the source-receiver azimuth, are used to compute anisotropic attributes from which a qualitative analysis of fracture density and orientation is done.
The Marcellus Shale is known to have two sets of fractures (joints) associated to different tectonic events. Particular to this play is that one set of joints is perpendicular to the co-located macro-fractures. In this case, although curvature or other geometric attributes reveal the strike of macro-fractures, this does not correspond to the joint sets, which are at sub-seismic resolution. Anisotropy analysis through elliptical inversion of P-wave velocities identifies both sets of micro-fractures and verifies their expected position in relationship to the oroclinal belt and the macro-fractures. Analysis of PS migrated stacks supports these observations.