Chemostratigraphy of the Bireno member (Turonian, upper cretaceous) in the Sfax area, Tunisia: characterization of a pertroleum reservoir

Geochemical analysis of 254 core and cuttings samples from three petroleum exploration and development wells (Guebiba 3 and 4, El Jem 1) in the Sfax area of eastern Tunisia, has successfully characterised the Cenomanian Fahdene Formation, and the Turonian Annaba, Bireno and Douleb members of the Aleg Formation. The cored mid-Turonian Bireno Member in the Guebiba 4 well is subdivided into two lithostratigraphic units: (i) Unit 1, attributed to a shallow subtidal environment; (ii) Unit 2, believed to represent an intertidal to supratidal sabkha environment. Vertical geochemical profiles established for each well for 11 major elements (Si, Ti, Al, Fe, Mn, Mg, Ca, Na, K, P, S) and 7 trace elements (Ba, Cr, Sr, Th, U, Y, Zr), have allowed the recognition of marker geochemical shifts that reflect member boundaries and the two Bireno Member sub-units. Chemostratigraphic correlation of the different members has been established between the three wells, and the lithological units recognised in the Bireno Member core of Guebiba 4 have been successfully correlated to the other wells using geochemical criteria. Porosity seems to be strongly correlated with Ca and Mg, reflecting the importance of peri-reefal limestones and sabkha dolostones as important reservoir horizons, while clay minerals (represented by Al and Ti curves) provide the main source of gamma ray response, and clay-rich beds typically represent relatively tight intervals. Chemostratigraphic studies of shallow-marine sediments such as the Bireno Member, offer considerable potential for the characterisation, subdivision and correlation of strata in petroleum basin.