A geologic and stratigraphic logging project report of Semiliki Basin-Albertine Graben-Ntoroko District Western Uganda from 15th/03/2021 to 24 th /03/2021

Abstract

This report documents my field excursion experience in the Albertine graben under the guidance of lecturers from the department of geology and petroleum studies-Makerere University. The area of study was Semliki basin located in the southern part of the Albertine graben in Ntoroko district western Uganda. It lies onshore south of Lake Albert and is bounded by an escarpment to the east and south, and River Semliki to the west. The field study lasted 10 days from 15th/03/2021 to 24th/03/2021. The aim of the study was to enable students apply the geological, geophysical and other relevant theoretical knowledge acquired during the course of the program to understand the geology, analyse the petroleum system and make conclusions based on field measurements and observations from the Semliki basin. A lot was learnt during the study and different materials and methods were used for data collection. Activities such as sedimentary logging, depositional system reconstruction, stream sediment studies, and structural measurements among others were carried out. The study was focused on the description and analysis of the depositional environments, facies and processes that took place in the Semliki area. The sediments (lithology) found in this area are of fluvial, lacustrine and deltaic origin. The sediments (clays, sandstones, siltstone, among others) are of Mid-Miocene to recent age, which are underlain by possible Jurassic to Early Tertiary age sediments resting unconformably over the basement rocks (granites and granite gneiss). Basin and facies analysis involved the interpretation of the depositional environments, basin architecture, paleo current flow directions and petroleum potential of the Semliki basin. The basin was affected by intensive tectonic activity,that led to the formation of vast types of structures. These include faults, joints, and veins among others in the basement rocks. Structures present in the soft sediments include bedding, cross-bedding, laminations, unconformities, mud diapirs, plunging, joints and faults among others. For the geophysics of the basin, Geophysical data ie potential field data of semliki basin and shallow, high resolution (150-200 Hz) seismic data with a vertical resolution of 3m from an intraslope salt withdrawal minibasin in the Gulf of Mexico were availed to students for interpretation and these were not obtained by students in the field. This data were later analysed by softwares such as Oasis montaj (Geosoft) and Teclog. In conclusion, the sediments in the Semliki basin are thick and the geophysical logs showed the presence of hydrocarbons due to the cross overs observed between the neutron porosity and density logs. Elements of the petroleum system such as reservoir rocks (sandstones), seal (clays), hydrocarbon-migration pathways among others were observed. The presence of oil seeps indicates that organic rich source rocks are present. Information from the field visit to sempaya hot spring indicate the presence of high enough geothermal gradients of over 67°C/km for the maturation and generation of hydrocarbons at shallow depths. Thus, the sediments in the Semliki basin represent a petroleum play for hydrocarbon accumulation for which we have most of the necessary elements of a working petroleum system. However, a suitable oil window is defined by the temperature range of 60-120°C but since the hot springs temperatures are higher than the oil window, then there are greater chances of cracking the oil to form thermogenic gases like methane.