Gravity and magnetic data analysis to detect the origin of carbon dioxide in Semliki Basin, Albertine Graben

Abstract

Semliki basin is one of the most prospecting basins for Petroleum exploration in Uganda. An exploratory well (Turaco-3) drilled to the depth of 2960 m by Heritage Oil and Gas Limited in the Semliki Basin encountered a hydrocarbon-bearing formation, which was found to be heavily contaminated by carbon dioxide (CO2), however, the origin of CO2 is unclear. In order to investigate the origin of CO2, gravity and magnetic data was used in this study to: (a) define the geometry/shape of the gravity and magnetic source bodies by delineating their edges. (b) estimate the depths to the gravity and/or magnetic source bodies in the study area and (c) to integrate the model results with the qualitative analysis of the potential field signals from the subsurface sources in order to establish the origin of the CO2. Regional- residual separation to enhance the local changes in the potential field was done by applying Band pass filter, analytical signal, tilt derivative and horizontal gradient. Euler Deconvolution was then used to define the basement faults and fractures. The results provided new insights into the gravity and magnetic anomalies of the southern part of Lake Albert and the Semliki basin. Some prominent magnetic anomaly trends were observed, namely NE-SW, E-W and NW-SE. The positive correlation between these anomalies and fault trends is an indication that the observed magnetic anomalies and their associated subsurface sources are fault controlled. The faults consequently would play a significant role in the movement of subsurface fluids across the region. The presence of very high amplitude magnetic anomalies and the values of their respective estimated magnetic susceptibilities are considered to support the possibility of magmatic intrusions existing within the Semliki basin, these could be the source of CO2 in the Basin, and migrates through faults. The Rwebisengo anomaly in particular has very high significant association with subsurface structuring and the high amplitudes associated with these anomalies are a good indication of their correlation with magmatic sources in the subsurface. The models further indicate that the sources could be within 3 km depth and shallower.