Master Thesis Subsurface Reservoir Evaluation and Image Log Analysis of the Shaly-Sand Lower Acacus

Abstract

The Lower Acacus reservoir in the northeastern part of Ghadames Basin (Block-114) forms a tidally influenced deltaic system. Structural and stratigraphic cross-sections that were constructed using Petrel (Schlumberger Trademark) have shown a general thickening of the reservoir formation towards the north-northeast. Petrophysically, this reservoir is divided into three main units based on log characters; these are (Unit A, B, and C) with Unit A being at the bottom followed by B & C. The petrophysical analysis using IP (LIoyd’s Register Trademark) indicates that Lower Acacus reservoir is hydrocarbon bearing in the study area with very good reservoir parameters. Average volume of shale is around 25 percent, porosity ranges from 15 to 20 percent, and water saturation is around 35 percent in the oil wells. Hydrocarbon distribution and reservoir intervals are both mainly controlled by depositional facies and to a lesser extent by structural influence. Consequently, FMI and core images were analyzed using Techlog (Schlumberger Trademark) to define the main sedimentary structures in the available wells. These sedimentary structures were grouped into four main facies in order to delineate the vertical and lateral variation among the wells in the study area. The identified facies are heterolithic bedding, crossbedding sandstone, shaly-sand bedding, and laminated shale facies. The bioturbation activity on these facies might have a direct influence on the reservoir performance by destroying the rock primary fabric. However, the association of these facies reflect the tidal effect on the delta as supported by paleocurrent direction analysis. Additionally, the paleocurrent measurements from the cross-bedded facies show a dominant bimodal trend towards NW-SE, with modest polymodal and unimodal models.