Fracturing in carbonate sediments above a salt diapir (Jebel Madar, Oman)

Abstract

Salt-cored domal structures in the subsurface represent common reservoir settings. Understanding the generations and properties of the fracture network above such structures is therefore important. This outcrop-based study investigates fractures in the deformed carbonate layers exposed above a salt dome at Jebel Madar, Oman. Methods used to study the fractures and their infillings include fracture analysis, petrography and stable isotope geochemistry. Regionally occurring NNE/NE-SSW/SW and NW-SE trending fractures constitute important fracture sets across Jebel Madar. In addition, it was observed that the doming process led to the development of local fractures that overprint the regional trends. A study of the textures and composition of the fracture infilling material indicates that a dominantly advective mode of fluid transport occurred in the NNE/NE-SSW/SW trending veins, which developed due to repeated fracturing and re-cementation. These veins are associated with phases of doming and related pulses of high volumes of fluids during times when the maximum horizontal stress was oriented NE-SW. In contrast, a more diffusive mode of fluid transport is thought to have occurred in the NW-SE trending fractures, which developed during times when no doming was occurring. Although the diapir seems to be the main drive for fluid circulation and the precipitation of most diagenetic products occurred during periods of doming, vein infilling also occurred, to a lesser extent, during periods when doming was absent. The frequent occurrence of normal faults, extensional opening fractures and implosion breccia suggests that the layers above the diapir were highly extended during the salt emplacement. In addition, some of the extension was found to be accommodated by the development of shear movement (down-dip slip) on original mode I fractures. Their similar appearance on seismic sections to classic normal faults could lead to a misinterpretation of their fluid flow properties.