Effect of basin evolutionary factors on reservoir quality of the Semliki basin, Western Uganda

Abstract

Sedimentation in the Semliki basin is controlled by tectonics and climatic changes. These factors together control the patterns of river drainage, that create variations in sediments composition, heterogeneities in the reservoir and unexpected variations in reservoir quality distribution. In spite of the good reservoir potential and the good petroleum generating, no commercial hydrocarbons have been discovered within the Semliki basin. This is most probably because the extent to which tectonics and climate have influenced reservoir quality distribution within the Semliki basin has not been determined and thus the cores focus for this research. Seismic and well data from the Semliki basin have made it possible to establish the effect of basin evolutionary factors on reservoir quality distribution within the basin. This has achieved through a structural evolution and reservoir quality distribution study of the basin using Petrel and Techlog software, punctuated with literature review. The study highlights a complex and multi-phase structural evolution of the Semliki basin consisting of three syn-rift episodes. The first episode (Mid-Miocene to Late Miocene) is characterized by intense fault activity with extension. The second episode (Late Miocene to Early Pleistocene) is characterized with transpression and increased rift shoulder uplift. In contrast, the third episode (Early Pliocene to present day) is characterized by transpression and continued reactivation of older faults. Extensional and transpressional (compression and strike-slip) tectonics associated with these episodes have had a decisive influence upon sediment source areas and topographic contrasts within the Semliki basin. The combined effect of several tectonic and climatic phases has produced three unique sandstone reservoirs, namely, Kisegi, Kakara, and Nyaburogo Formations. High effective porosity occurs both in the shallow and deeper parts of Kisegi reservoir. In Kakara and Nyaburogo reservoirs, effective porosity is mainly in the central and deeper portions respectively. Worth noting is that in all these three reservoirs, effective porosity does not match with permeability distribution in some areas. This is attributed to fining upward and coarsening upward sequences, typical of retrogradation and progradation of lake levels due to increased tectonic uplift and subsidence respectively. In addition, complex pore throat geometries in these reservoirs, discontinuous faults that are dynamic seals, shale interbeds, and undulating morphology result into random distribution of effective porosity and permeability.