The Semliki Basin field study project report
Aleer, David Leek
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The Semliki basin is a rift basin and a half basin formed by tensional tectonics. The main objective of the study was field training on how to collect and interpret stratigraphic, petrographic, sedimentologic and structural data which are quite crucial in petroleum exploration. The major materials used during the fieldwork include GPS, maps used to locate ourselves in the field, geological compass used to measure structural attitudes, geological hammer used to collect samples among others highlighted in the main text, methods applied in the field included fieldwork and excursion, data collection using provided tools, data analysis and interpretation among others. Some of the data collected from the field was analyzed using Stereonet software for instance rose diagram was used to deduce the paleocurrent flow directions and type of river system responsible for the deposition of sediments. Geophysical data available at the department was analyzed using techlog and Geosoft softwares and sedimentary logs were obtained using Sedlog software. The sediments of the Semliki Basin are of the Middle Miocene to recent age, as seen from outcrops and well data, these most likely overly Jurrassic or Permo-Triassic to early tertiary sediments that unconformably lie on the basement rocks. The basement in this area comprises mainly granite gneiss and amphibolites rocks. The lithology of sediments include sands of various grain sizes, thicknesses and colors(mainly white and yellow), clay, conglomerates(both polymictic and monomictic) and siltstones. The sediments exhibit cyclicity suggesting changes in conditions in the depositional environment. The basin has some sections with coarsening-upwards and others with fining-upwards sequences. These are characteristic of deltaic and fluvial systems respectively. The blocky sediment sequences are indicative of the lacustrine paleoenvironment. The river systems in this area could have been braided and meandering systems. The presence of gypsum cement and iron(iii) minerals signifies semi-arid paleoenvironment. The general stratigraphy of The Semliki basin consists of Kisegi, Kasande, Kakara, Oluka, Nyaburogo, Nyakabingo and Nyabusosi formations in that order. Basin and facies analysis was done to interpret depositional environments, basin architecture, paleo flow directions and petroleum potential of the Semliki basin. The major structures in the study area include faults and joints encountered in the basement with a major NE-SW trend and A minor SE-NW trend. Structures present in the sediments include bedding planes, cross-bedding, laminations, unconformities, mud cracks, and minor folds among others. These have similar trends with the basement structures implying that they were affected similar tectonic forces. The geophysical data of the Semliki basin(both magnetic and gravity) show that the depocenter is located in the northern part of the basin and the seismic data shows presence of potential petroleum elements. Although the Semliki basin is relatively young(about 13-15ma), crustal thinning has led to high temperatures as evidenced by hot springs like Sempaya Hot springs. This provides a Time-Temperature-Index(TTI) sufficient to mature source to produce hydrocarbons. However, very high TIT would destroy the hydrocarbons. The kasande formation acts as source rocks and seals in the study area while the Kisegi formation acts as the Reservoir rocks. The reservoirs here are very compartmentalized by gypsum, thin clay layers which would production difficult and expensive. Normal faults act as potential traps, joints are potential migration pathways. The area also has a great geothermal potential.