Fluid flow and late diagenesis of fault-infill carbonates in the Aptian dolostones at Jabel Semmama, Kasserine area, western-central Tunisia

Abstract

Abstract In Western-Central Tunisia, Kasserine area, dolomitization of Jabel Semmama Aptian limestones of the Serj Formation (Fm) led to the dolostones of the Orbata Fm. These dolostones are cross-cut by several generations of faulting and fracturing. The current study focused on E-W trending faults and fractures system incorporating carbonate infills and freshwater flows which have stimulated continental diagenetic processes. The faulting, fracturing and rock-fluid interactions yielded nine fault-infill diagenetic and tectonic-sedimentary phases. The purpose of studying the tectonic stresses and the nine phases was to elucidate their late impact on the Aptian dolostone reservoir quality. This enquiry was based on lithological, tectonic, microfacies and oxygen/carbon (O/C) isotopes investigations. Analyses of stable O/C isotopes are performed on crushed and dedolomitized dolostones as well as on calcite cement laminae and spherulites (C1, C2). The crushed and dedolomitized dolostones δ18O and δ13C data (averages: −4.0‰V-PDB/+1.75‰V-PDB) remain close to the moderately depleted results of the Aptian replacive dolostone host rocks (averages: −3.6‰V-PDB/+2.0‰V-PDB). Inversely, calcite cement laminae and spherulites (C1, C2) yielded strikingly depleted oxygen values (δ18O averages: −7.77‰V-PDB/−8.0‰V-PDB) pointing out to freshwater controlled diagenesis. The discrepancy between the two records (−0.23‰V-PDB) is ascribed to cooling during the precipitation of calcitic cement spherulites. The divergence between the two depleted δ13C values of the two cements C1, C2 (averages: −6.19‰V-PDB and −4.0‰V-PDB) is attributed to climatic control (wet/dry alternations) that controlled the micritization phenomenon. The latter orchestered the release of Ca12CO3 being more mobile than Ca13CO3. This process is in agreement with the strong variation of fluid-flow supplies (vadose, phreatic) transcribed in the lithology as cement phases (lamellar, spherulites, gelatinous, vadose and granular). The study series record two major climatic changes, a wet period encompassing phases P1–P4, and a dry period extending on phases P5–P9. Two short cool and wet episodes accompany this period. The nine tectonic-sedimentary and diagenetic phases occurred into two periods through the telogenetic stage (uplift). They started at the Miocene-Pliocene during the Atlasic foldings along E-W faults and fractures (Phases P1–P6) and continued throughout the Quaternary period (Villafranchian) along a reactivation of the same direction (Phases P7–P9). We believe that the nine phases arose after the OAE's organic matter maturity and thus had nearly no impact on any early oil migration while the earlier diagenesis phases remain the most useful for the reservoir quality.