Most carbonates have a dual porosity and permeability (matrix and fracture). As fractures are preferential conduits for fluid flows, fracture networks strongly impact reservoir hydraulic properties. Two fracture patterns can affect reservoirs: random background fractures in the host rock and damage zone clustered fractures in fault zones. This study identifies the structural and diagenetic attributes of both fracture patterns and determined their respective impact on reservoir properties. The study focuses on the East part of La Fare Anticlinal (SE France). Lower Cretaceous, Urgonian facies carbonates underwent a polyphase tectonic history. Faults were set up as normal and were later reactivated as strike-slip. We made a 290m scan-line along the outcrop to characterize fracture network in and outside the fault zones. The diagenetic analysis on 45 thin sections in Polarized Light Microscopy, with SEM and cathodoluminescence evidenced 3 cementation phases and 2 micrite recrystallization phases. This study shows that fault zone structural properties and deformation are dependent of initial host rock background fractures network. Fault zone structure with damage zone fracture network encouraged the fluid to flow and the cementation of S2 phase. This fluid flow, absent in the host rock strongly modified the reservoir properties of the studied zone.
Abstract. Microporous carbonate rocks form important reservoirs with permeability
variability depending on sedimentary, structural, and diagenetic factors.
Carbonates are very sensitive to fluid–rock interactions that lead to
secondary diagenetic processes like cementation and dissolution capable of
modifying the reservoir properties. Focusing on fault-related diagenesis,
the aim of this study is to identify the impact of the fault zone on reservoir
quality. This contribution focuses on two fault zones east of La Fare
anticline (SE France) crosscutting Urgonian microporous carbonates. Overall, 122
collected samples along four transects orthogonal to fault strike were
analyzed. Porosity values have been measured on 92 dry plugs. Diagenetic
elements were determined through the observation of 92 thin sections using
polarized light microscopy, cathodoluminescence, carbonate staining, SEM, and
stable isotopic measurements (δ13C and δ18O).
Eight different calcite cementation stages and two micrite micro-fabrics
were identified. As a main result, this study highlights that the two fault
zones acted as drains canalizing low-temperature fluids at their onset and
induced calcite cementation, which strongly altered and modified the local
reservoir properties.
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