--> --> Coniacian Douleb Carbonates Member at Jebel Khsham El Artsouma, Central Eastern Tunisia; Reservoir Characterization and Subsurface Analogue

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Coniacian Douleb Carbonates Member at Jebel Khsham El Artsouma, Central Eastern Tunisia; Reservoir Characterization and Subsurface Analogue

Abstract

The Coniacian of central Tunisia is known as Douleb limestone dominant Member within the Aleg formation. It tested and produced oil in eastern onshore and offshore Sfax area. Due to its heterogeneity and despite previous results, this reservoir is still representing a challenge for explorationists. That's why collection and studies of outcrop data are necessary for better understanding subsurface parameters. Sedimentological study and petrophysical analysis of two sections (Oued Mahloul OM and Oued Mahloul East OME) outcropping at Jebel Khsham El Artsouma, provide more knowledge about Coniacian Douleb reservoir. This Member is composed of three distinctive units; (U1) lower alternations of bioturbated bioclastic/ lumachellic carbonate and light green marls, (U2) middle oolithic dominant beige carbonate and (U3) upper alternation of light grey carbonate and bioturbated marls. Limestone beds of these units are partially dolomitized at OM section and highly Dolomitized at OME. The main facies is oolithic dominant deposited in a shoal complex environment within a gentle slope Ramp. Bioclasts increase in fore-shoal setting however in back-shoal mud-coated grains and peloids are commons. Oolithic sand bodies are oriented from North West to South East separating a restricted domain landward to the South West and a storm influenced open platform toward the North East. The Douleb limestone is made of shallowing upward sequences bounded topwards by aerial/ subaerial surfaces outlining exposures/ sub-emersion of shallow Ramp during regressive phases. In fact, initial petrophysical parameters of rocks were highly enhanced by early meteoric and burial diagenesis. Dolomotization and Dissolution are spectacular processes giving a significant secondary porosity and permeability. Porosity versus Permeability cross plot showed that the OME section dolostones is a potential reservoir. The main pore types are; inter-crystalline, molds of leached grains and vugs. The three units of the Douleb Member previously described in outcrop were encountered in drilled wells. Every unit was characterized by a distinctive diagraphic signature or log type. Their limits are marked by observable shifts in Gamma ray and Acoustic readings indicating a lithofacies contrast. In addition, sedimentological investigations showed a big similarity in reservoir parameters with studied sections. Indeed, the Coniacian of Khsham El Artsouma outcrop could be a good candidate for subsurface analogue.