Hercynian Deformation and the Creation of Fractured Reservoirs along Wrench Faults of the Central Sahara

By

 John C. Lorenz¹

(1) Sandia National Laboratories, Albuquerque, NM

 Several continental-scale, north-south trending faults extend northward from the Hoggar massif into the central Saharan platform. These faults originally formed during late Precambrian time but were re-activated in late Paleozoic time to create reservoirs in the overlying, early Paleozoic sandstones. Surface expressions of the faults consist of broad anticlines, abrupt folds, vertical offsets with abrupt changes in the sense of motion along strike, and east-west directed thrusts. These characteristics are suggestive of east-west compression but are also compatible with transpression along the faults, a more regionally plausible interpretation. Right-lateral, kilometer-scale, wrench offsets accommodated northward and northeastward translation of the Mauritanian craton during the Hercynian orogeny, ultimately related to collision between Africa and the Americas and recorded by thrusting in the Mauritanides chain along the west coast of northwest Africa. Simultaneous northeast-southwest shortening across the Ougarta arch, where northeastward motion of the craton was directly accommodated by folding and thrusting, supports this model. Strata overlying the sinuous and multi-stranded faults were folded and faulted at fault bends, offsets, and asperites during wrench faulting. These structures formed as a result of local strain partitioning in the shallow strata, and are a common feature of other continental-scale transpressive fault zones as well as of published sand-box models. Deformation and folding of the strata along faults created traps for hydrocarbons and fracture-related permeability within the local well-cemented Paleozoic sandstones. More systematic, regional fractures are present throughout the minimally deformed strata found between faults.