From fractures to flow: A field-based quantitative analysis of an outcropping carbonate reservoir

Fractures can play an important role in the fluid storage–migration properties of fault damage zones. In this present contribution, we document the role exerted by fractures on fluid flow in carbonate damage zones of hydrocarbon-bearing, km-long, oblique-slip normal faults with 10's of m-throw. The carbonate fault damage zones were analysed by mean of scan line surveys conducted in both tar-free and tar-rich outcrops. In this way, the relationships among the individual fracture characteristics (length, spacing, aperture, orientation, connectivity and distance from slip surfaces pertaining to small faults of the fault damage zones) and hydrocarbons have been established. Data obtained by scan line surveys were also used to compute the amount of fracture porosity, the degree of fracture connectivity and, based upon simple assumptions, the orientation of the local σhmax at times of faulting. Additionally, scan line surveys were also carried out along outcrops exposing unfaulted carbonate host rocks. The results of our computation are consistent with a carbonate host rock made up of a quite isotropic fracture array comprised of isolated and coupled fractures, in which individual fracture sets have negative exponential spacing distributions. In terms of fluid flow, the fracture array of the carbonate host rock enhances the fluid storage. Conversely, the fracture array of the fault damage zones is characterized by a pronounced anisotropy due to interconnected fractures, which enhance the fluid migration. Fractures in the fault damage zones include those inherited from background deformation and others related to the faulting processes. The latter fracture sets are characterized by power law spacing distributions. In conclusion, counter-intuitively, both fracture length and fracture spacing do not have any correlation with hydrocarbons in the fault damage zones. On the contrary, fracture anisotropy, fracture spread and fracture orientation are positively correlated with hydrocarbons.