The Semliki Basin, Uganda : its sedimentation history and stratigraphy in relation to petroleum accumulation
Master Thesis
2005
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University of Cape Town
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Abstract
The Semliki Basin is covered by sediments that represent the Middle Miocene to Recent, which are described from outcrop and well data, underlain by possible Jurassic or Permo-Triassic to Early Tertiary sediments, which rest unconformably on Basement, described from seismic data. Thin-section analysis of selected samples, collected from the field, has shown that sandstones from the Semliki Basin are predominantly composed of quartz, potassium feldspars and plagioclase feldspars with subordinate clay minerals. Accessory minerals, such as micas (biotite and muscovite), heavy minerals, garnet and epidote, are present in minor amounts. This mineralogy indicates that the sediments have a granitic and gneissose origin, related to continental-block provenances. The X-ray diffraction scans of bulk samples reveal that the mudrocks/claystones are dominated by clay minerals with subordinate quartz, feldspars and calcite. The clay minerals include illite, illite-smectite, kaolinite, montmorillonite, illite-monnnorillonite, and mica with mixed layer illite-smectite and illite layers dominating. The clay minerals in the sediments were interpreted to be as a result of weathering of feldspars and volcaniclastic sediments. Authigenic minerals such as anatase and jarosite and secondary precipitates such as calcite and gypsum have also been interpreted as oxidation products of sulphides in the sediments. The study has allowed a better understanding of the stratigraphic relationship of the different rock units that are exposed on outcrop, those encountered in the wells, plus a section interpreted from seismic data. In general, the depositional environment of the sediments in the Semliki Basin is fluvial-lacustrine/deltaic showing significant variations in gamma-ray character, which reflect the water-level changes and river interactions through the depositional period and the influence of rifting tectonics on sediment deposition through time and space. The sediments in the Semliki Basin represent a petroleum play for hydrocarbon accumulations, in which the necessary elements of a valid petroleum system were identified. These include excellent or good potential for reservoirs and top seals as well as circumstantial evidence of regionally mature source rocks, possible seals, traps and hydrocarbon-migration pathways.
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Includes bibliographical references (leaves 140-147).
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Kiconco, L. 2005. The Semliki Basin, Uganda : its sedimentation history and stratigraphy in relation to petroleum accumulation. University of Cape Town.