The western Bechar basin contains a 1500-2000-m-thick Mississippian succession of carbonates consisting of superposed shoaling-upward sequences, whose porosity or diagenesis have never been investigated to establish the reservoir potential of these carbonates. In this paper, we present a diagenetic and porosity evolution of the Visean part of the Mississippian carbonate platform that may lead to the development of new exploration fairways in the region. A diagenetic model is proposed in which ooid and crinoid grainstones may have a reservoir potential if hydrocarbon migration occurred before the end of the Serpukhovian, prior to complete occlusion of porosity.
The two facies investigated are Waulsortian-type buildups and overlying ooid and crinoid grainstones that form regularly alternating packages in the thick upper Visean succession. The diagenetic sequence contains five calcite cement phases (C1-C5) that were investigated for isotope geochemistry and fluid inclusion microthermometry. The isotopic signatures of calcite cements range from marine for C1 to deeper burial environments for the 18O-depleted C3 and C4. Fluid inclusion data indicate that C3 and C4 (60-80% of the total cement) precipitated from hot, saline basinal fluids, whereas a meteoric-phreatic origin is suggested for C5. Temperatures of cementation, inferred from stable oxygen isotopes and homogenization temperatures of fluid inclusions, indicate that most pores within the buildups (shelter, stromatactoid, moldic, breccia, and vuggy) were occluded after only a few hundred meters of burial. In contrast, pores in the ooid and crinoid grainstones (mainly interparticle) stayed open after thousands of meters (900-2500 m) of burial. We hypothesize that these grainstones may have served as a regional conduit for hydrocarbon migration from the basin to the carbonate platform.
Basin subsidence rate ranged from 220 m/m.y. during the Visean to 40 m/m.y. during the Serpukhovian-Bashkirian, which suggests that 20 m.y. was needed to fill most porosity. As a result, the western Bechar limestones can be considered a good target for petroleum exploration if the hydrocarbon migration occurred before the end of the Serpukhovian. This conclusion has important implications because the Mississippian limestones lie directly above potential Upper Devonian source rock in the Bechar basin.