Characterization of Reservoir Properties for the Kisegi Formation in Semliki Basin, Albertine Graben, Western Uganda using Rock Physics Diagnostics

Abstract

The quality of a reservoir rock is a fundamental input in evaluating its productivity during exploration and appraisal. Therefore, attempts to characterize the formation using rock physics diagonostic models make it possible to assess diagenetic heterogeneities at micro scale.Intracontinental rift basins experiencedifferent episodes and kinds of tectonism, followed bymultiple diagenetic transformations at micro level during burial resulting into reservoirs with complex lithologies.Micro structures produce unique poro-elastic properties and rock physics diagnostic modelsmake it possible to determine thesepropertiesTheKisegi Formation is situated intheSemliki basin of the Albertine graben, which is part of the western branchof the East African Rift System. Kisegi is a fluvial-deltaic feldsparthic sandstone and is the main reservoir of the Semliki basin. Rock physics modeling using well log data and thin section analysis using Scanning Electron Microscope (SEM) unraveled the cementation and sorting nature of this reservoir. The results indicateda reservoir with both contact and pore-fillingcement. Quartz and calcite was observed holding grains together while mainly authigenic clay was filling the pores. Patchy calcite cementation was observed sealing some pores that already contain clay matrix. The constant cement model provided the best fit for the data from the three wells. Total cementation varies inversely with porosity and this explained large porosity ranges of 5-35%for Turaco prospects. The model also revealed high sand bulk modulus of 55 GPa attributed toCalcite cement, and lower shear modulus of 18 GPa attribute to feldsparsand intensive fractures. The patchy cementation also can produce relatively higher and localized P-wave velocities resulting into large impedance changes, whichcan be misinterpreted as a hydrocarbon indicator. This probably explains the reasonwhy exploration is still unsuccessful in Turaco and Kanywataba prospects with good indicators of oil presence such as nearby Kibuku oil seep. Therefore, with good data quality, and pre-stack seismic data, the results could be extrapolated in the entire basin. This helps to identify other prospects in this frontier area. It can be argued that determining micro structural changes is vital and maybe one of the keys to exploration success in intracontinental basins