Effect of normal fault activity on carbonate reservoir diagenetic evolution (Urgonian facies, SE France)

Abstract

In carbonates rocks, which are sensitive to fluid-rock interactions, diagenetic processes can strongly modify the structure and volume of pore pattern. In highly reactive carbonate rocks, fault zone related fluid flow can increase diagenetic alterations (dissolution, replacement, cementation deposits). To define the diagenetic evolution of faulted rock as an analog to nearby reservoirs, we studied the diagenetic properties of Urgonian (upper Barremian) microporous limestones affected by a polyphase normal fault located on the northern flank of Nerthe anticline (SE France). Eight different cements have been identified under cathodoluminescence (C0 to C7). Their distributions and geochemical signatures (18O and 13C stable isotopes, Δ47 thermometry) allowed to determine the paragenetic sequence, as well as the origin (marine, meteoric, burial brines) and temperature of the fluids that generated C1 and C7 cements. U–Pb dating allowed us to relate the C7 cement phases with the Oligo-Aquitanian rifting stages. However, the results show that most of the diagenetic alterations occurred prior to the Cenomanian. Subsequently, localized but intense alteration related to the fault is limited to one stratigraphic unit, where effect extend as much as 700 m from the fault surface.