Composite damage zones in the subsurface.

The cumulative displacement by multiple slip events along faults may generate composite damage zones (CDZ) of increasing width, and could modify the hydraulic and mechanical properties of the faults. The internal architecture and fracture distribution within CDZs at the subsurface are analysed here by using seismic attributes of variance, curvature and dip-azimuth of the 3-D seismic reflection data from tight sandstone reservoirs in northeast Sichuan, China. The analysed faults intersect the reservoir within a depth range of 2.4–3.0 km. The damage intensity mapping revealed multiple CDZs with thicknesses approaching 1 km along faults ranging 3–15 km in length, and up to 1000 m of cumulative slip. The identification of numerous fault cores and associate damage zones led us to define three classes of CDZs: banded shape, box shape and dome shape. The mechanical strength contrasts and distortion of fault cores suggest potential weakening and strengthening (healing) mechanisms for formation of CDZs that can be extended to faulting processes and earthquake simulations. [ABSTRACT FROM AUTHOR]