Lithofacies Analysis of Lower Acacus Reservoir and Its Impact on Reservoir Quality Utilizing Borehole Images, Ghadamis Basin—Northwest Libya

Abstract

AbstractThe Silurian Lower Acacus reservoir is known as one of the prolific hydrocarbon intervals within Ghadamis Basin. It is characterized by an alternation of siliciclastic succession represented by sandstone, siltstone and shale. These were deposited in a complex marginal depositional setting. Previous regional basin scale studies indicated a transitional pattern from fluvial channel sandstone in the south, coastal deltaic sandstone and siltstone, to offshore marine siltstone in the north. However, the study area (NC4) is located within the coastal-deltaic dominated facies. There was an attempt to understand the variations in the exploration license NC4 for further field development. From the exploratory wells drilled according to the license, it is observed that the vertical association of these rocks highly influences the reservoir quality as well as its lateral continuity. Therefore, understanding both the vertical and lateral reservoir geometries is crucial for further exploration and field development plans of the Lower Acacus play. The reservoir succession shows variations in the depositional pattern sequences from coarsening to fining upwards as per the GR log signature, paleo-flow directions and the possible depositional setting. To achieve this objective, an integration between the core, wireline logs and borehole images (full borehole MicroImager tool—FMI) data was implemented. The integration of core image and FMI analysis assisted in the recognition of four main lithofacies within the Lower Acacus reservoir, namely heterolithic bedding, cross-bedded sandstone, shaly sand and laminated shales. The association of these facies reflects a clear tidal effect on the deltaic deposition as supported by paleocurrent direction analysis. The paleocurrent measurements from the cross-bedded facies show a dominant bimodal trend toward NW and SE, with modest polymodal and few unimodal models.KeywordsLower AcacusGhadamis BasinLithofaciesSedimentary structuresReservoir qualityBorehole imageLibya