Petrography, Sedimentology and Reservoir Geology of Aradeiba and Bentiu Formations, Block 5A (Tharjath Oil Field) South Sudan

Abstract

A research on five wells data was achieved with the objectives of carrying out a detailed and comprehensive study to investigate and describe textures, mineralogy, digenesis, facies, porosity and permeability characteristics and controls on the Bentui and Aradeiba reservoirs. The aim was also to essentially construct a conceptual depositional model summarizing the probable geometries and relationships of the major sand-body types and properties in order to fully understand and asses the reservoir sands characteristics, controls and distribution. Methods of petrographic analysis, XRD, and SEM on cores and ditch samples aided by Petrophysical wire line logs and Cyclolog® technique were used for determination of facies, porosity, permeability, sand to shale ratios, correlations and spatial sands distribution. Interpretation and integration of the study results revealed that the Aradeiba Formation consist mainly of interbedded sequences of shale and sandstone but mostly argillaceous. In the lower part, the interbeded sands thicknesses range from 2.5 to 18 m having good reservoir properties with porosity range from 7.9% to 24.9 % and variable permeability from 0.08 mD to 5.3D. The identified lithofacies and cycles patterns indicate that Aradeiba has been deposited within a high sinuosity/meandering fluvial channel and locally prograding deltaic deposits system, with less sands lateral continuity. The upper part is considered essentially as non- reservoir due to abundant mud materials and acts as top/lateral seal within Tharjath field. The underlying Bentiu reservoir sandstones represents thick stacked poorly consolidated sands with 2.3 to 21 m in thickness having porosity ranges from 12.9% to 31.6% and permeability varying between 0.54 mD to 7.9D. The deposional cycles and facies recognized indicate the dominance of low sinuosity/braided fluvial channel environment characterized by good reservoir sands quality and lateral continuity. This reservoir characterization is very essential for future planning of wells drilling, optimum well completion, field development and reserves calculation economics.