Layering and reservoir modeling of Silurian uppermost Tannezuft and Acacus reservoirs in the Tunisian Ghadames Basin: Inferred static model and prolific layers

Abstract

Upper Silurian sediments of the uppermost Tannezuft Formation and the overlying Acacus A Unit in the Ghadames Basin of Southern Tunisia are considered as an important oil and gas target. For that, four wells, spanning these reservoirs, are evaluated herein. The study uses well logging tools via the Interactive Petrophysics (IP) software to determine hydrocarbon bearing layers. A static model is also inferred.The studied interval shows twenty-four sand bodies associated to the four petrophysical packages (T0 to T3) of the Uppermost Tannezuft and the nine petrophysical packages of the Acacus A (A1 to A9). Sand bodies show variable thicknesses (from 1.5 to 38m) and are laterally correlated following a North-West (NW) to South-East (SE) trend. They are composed of stacked thin bedded alternating sands and shales. The petrophysical packages A5 (A5-1 and A5a-2) and A9 (A9a and A9b) of the Acacus A are identified as the most interesting layers.The computed petrophysical parameters of the sand bodies reveal a clay volume ranging between 8% and 29% with an average of 22%. The effective porosity ranges from 10% to 29%. Relatively high-water saturation is depicted in most sand bodies reaching up to 63%. In addition, and due to the complex matrix composition, relatively low permeabilities characterize reservoirs zones (an average of 20mD). The integration of all data using the IP software reveals significant accumulations of gas condensates. The volumetric assessment via static modeling using the Petrel software indicates that the A9b unit is, predominantly, the most prolific reservoir.