Fieldwork report on stratigraphy and oil deposition in Semliki Basin.

Abstract

Semliki Basin is found in Ntoroko District and is about 30km from Fortportal town. The Oil seeps in the area especially Kibuku oil seep provided evidence of existence of petroleum system. Semliki basin is made up of several formations of which Kisegi and Kasande was studied in detail. The study assignment was based in the Kisegi formation with large exposure of sands that had geological facies and structures of interest. A full description of the color of the rock unit, Grain size and Texture was done. Dip and Strike measurements of different structures were taken using Brunton campuses and grain size card was used to measure the size of different sand grain sizes. Coarse to medium unconsolidated yellow and white sands intercalated with some clay material with structures such as angular unconformity were present. The cyclic nature of the lithology is very strong with some sections indicating finning upwards and some suggesting coarsening upwards, this meant there was a different depositional environment and period of deposition. The sand/clay associations reflect that there was an interaction of sandy fluvial deltaic and muddy lacustrine environment systems. Structures such as folds had an orientation of NE-SW and plunge was 16SW, in the fault encountered there was Normal faults observed in silty clays with fault plane in NE-SW and dipping in NW several structures were found to exist in the sediments such as, load casts, mud diapirs, concretions. Sempaya Hot springs was visited largely because of the role they are believed to play in providing high geothermal gradient which is sufficient for the source rock maturation. We concluded that Semliki has a good petroleum system where faults play an important role in hydrocarbon migration; they act as conduits where formation fluids may escape to other places with evidenced of Kibuku oil seeps which lie in the Semliki fault plains.