Reservoir properties of Turonian rudist-rich carbonates in Central Tunisia (the onshore of Sfax area)

The present study is based on core petrographical analyses of an oil-producing Turonian carbonate reservoir in the Sfax area (central-eastern onshore of Tunisia). Sedimentological studies show that these Early-Middle Turonian carbonates consist of shallow marine bioclastic limestones, sometimes partly dolomitised, and are rich in rudists which can be associated to other bivalves, gastropods, oncoliths, benthic foraminifers and calcisphers. Facies are organised into metre-scale shallowing upward cycles, each comprising three units corresponding, from base to top, to the following: bedded, slightly argillaceous wackestones containing floating rudist debris and sometimes calcisphers; alternation of massively bedded partly dolomitised packstones rich in entire joined rudists and finely bedded wackestones-packstones containing miliolids, oncoliths and gastropods; and micritic carbonates, partly argillaceous, nodular shaped and laminated. In terms of reservoir properties, the highest values of helium porosity (reaching 27%) and permeability (reaching 700 mD) were measured in rudist-rich carbonates. Pores, which are partly filled with oil, mainly correspond to the Radiolitid cells. In addition, some diagenetic features such as dolomitisation, dedolomitisation and dissolution have created additional pores and have clearly enhanced the reservoir potential of these rudist-rich carbonates.