Characterization of proven Late Cretaceous Reservoir rocks in the Gulf of Gabes: Integrated case study

Abstract

Microstructural features control the petrophysical properties of rocks. Of these, pore size is particularly sensitive when non-wetting fluids saturate the reservoir. The pore networks and physical properties (helium, water and mercury saturation porosity, bulk density, nitrogen permeability, P-wave velocity and thermal conductivity) of different rock types from a productive Upper Cretaceous (Coniacian) reservoir in the tunisian offshore are measured on hydrocarbon-washed samples. The facies sampled of Douleb Member are wackestone, packstone and grainstone textures as well as dolostone, dolomitized packstone and anhydrite textures. Based mainly on the results obtained by mercury injection, the porous facies of the Coniacian Douleb Member are characterized by a complex pore system with a large morphological and pore size variability in the rock. Porosity values range from 0.3 to 23.6%, bulk densities vary from 2.05 to 2.92 g.cm−3. The permeability is variable from 3760 mD to values below 0.01 mD (measurement limit of the device). P-wave propagation velocity values range from 2060 to 6084 m.s−1 and thermal conductivity varies from 1.43 to 3.77 W.m−1.K−1. The oil-impregnated facies with the best petrophysical characteristics are mainly the rudist-rich limestones and dolomites of the first unit (U1) of the Douleb Member. The well-sorted grainstones with small rudist debris and peloids have the best reservoir qualities. Porosity is the first order characteristic that controls petrophysical properties. The variability of permeability values around this first-order relationship is attributed to variations in the size of the pore access thresholds and connectivity. The variability in velocities is due to the shape of the voids, while the variability in thermal conductivity measurements is due to the nature of the contacts between the grains.