A report on the geologic and stratigraphic logging project of Semliki Basin-Albertine Graben in Ntoroko district Western Uganda.

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 8 days from 03/01/2022 to 10/01/2022. The aim of the study was to enable us apply the geological, geophysical and other relevant theoretical knowledge acquired during the course of the program to understand the geology, analyze 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 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 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, and plunging folds among others.

As far as the geophysics of the basin is concerned, geophysical data (gravity, magnetic and seismic) from Turaco well and Gulf of Mexico wells was analysed and logs interpreted. 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. Hot springs with in the Semliki area indicate the presence of high enough thermal gradients for oil maturation.