Integrated Geophysical Studies for Sub-basalt Imaging of the Magadi Basin in Relation to Hydrocarbon Prospectivity

Abstract

The study area, which falls within the southern parts of the Kenyan Rift System, is located in Kajiado County and is bounded by longitudes 35°30‘E to 36°30‘E and latitudes 1°30‘S to 2°30‘S. The area of interest is roughly 1296 square kilometers, accessible via the Nairobi-Magadi road. The main problem in the Magadi basin is the masking of target hydrocarbon-prone formations by a basaltic layer hindering reasonable subsurface imaging. The geological model of the Magadi basin is still obscure, and as a consequence, the hydrocarbon potential has yet to be realized. To unravel the problem, this study conducted geological and geophysical investigations with the aim of characterizing the basalts in the Magadi basin to develop an improved geophysical framework that images beneath the basalt layer. A geological survey was conducted to characterize and establish the surface distribution of basaltic rocks. For frequencies longer than 700 km, the strong correlation between gravimetric and topographical data with an admittance of about 50mgal km-1 necessitated a topographical study. Gravity Isostasy determined the overall potential of the basin. Aeromagnetic and gravity studies were conducted to establish the sub-surface distribution of the basalt layers. A non-conventional processing workflow was used to re-process the 2D seismic volumes to attenuate hybrid-type noise. This improved the seismic mapping of the sub-basalt structures. The improved seismic volumes were then integrated with gravity and aeromagnetic datasets during the 2D forward modeling process. Integrating seismic data into the 2-D modeling provided an informative depiction of the tectonics and underlying structures. The regional-residual separation of gravity and aeromagnetic anomalies, tilt derivatives, analytic signals, horizontal derivatives, vertical derivatives, and Euler solutions are used to identify pertinent signatures, lineaments, and prominent structures. Applying Butterworth low-pass filters to aeromagnetic data allowed for the isolation of magnetic layer responses that vary in depth. An unconstrained 3D susceptibility model reflects the compartments and surrounding geological formations. The depths and positions of the structures were determined using the Euler deconvolution and 2D integrated modeling. Volcanic flows in the Magadi basin exhibit a westward trend. Western movements of the volcano-tectonic flows have a greater impact on depocenters adjacent to the western boundary fault compared to depocenters adjacent to the eastern boundary fault. Internal compartmentalization occurred within the Rift, leading to the formation of depocenters characterized by sporadic and distinctive tectonic and sedimentary events. Rift segmentation resulted in adjustments to catchment areas, which subsequently induced abrupt fluctuations in sediment flow, culminating in the depocenters' isolation. The initial compartmentalized depocenters of Nguruman, Koora, and Forty-six are identified in this investigation. Sediment thickness ranges from 2000 to 4000 meters throughout the earliest Nguruman depocenter, which is around 2512 square kilometers in size and has an orientation of SW to NE. In the western and southern regions of Magadi, where compartmentalized depocenters are more prevalent, sedimentary formations are thicker than in the vicinity of the eastern boundary fault...