ESTIMATING HEAT FLOW AND GEOTHERMAL GRADIENT FROM MAGNETIC AND GRAVITY DATA. AN EXAMPLE FROM THE ALBERTINE GRABEN EAST AFRICA

Abstract:

Conventionally, hydrocarbon basin analysis relies heavily on seismic data and well log information to understand oil and gas reservoirs. However, the two geophysical techniques are the most expensive and many developing countries cannot afford the cost. Potential field geophysical methods (gravity and magnetics) avail us the opportunity of determining the crustal thickness and temperature gradient, the two most important inputs in the determination of heat flow critical for predicting hydrocarbon generation in potential hydrocarbon reservoirs. In this study, we investigated the thermal and crustal structure beneath the Albertine rift which is part of the Western branch of the East African Rift System (EARS). We analyzed aeromagnetic data using two-dimensional (2D) power density spectrum methods to determine the base of the magnetized crust that possibly represents the Curie point depth (CPD). In-situ temperature measurements from boreholes were correlated with heat flow estimates derived from the spectral analysis of the magnetic data. The crustal thickness beneath the rift was determined from satellite gravity data; this was used to establish the relationship CPD, temperature gradient and the observed heat flow. Our results suggest that shallower CPD’s (19-22± 1 km), high heat flow (~67-79±0.2 mWm^-2), thin crust (26-33 ±2 km) and higher temperature gradients (27-31^oC/km) occur beneath the Lakes George-Edward, Albertine and the Rhino rift basins. The surrounding basement were found to have deeper CPD (~24-26±1 km), lower heat flow (~57-63 ±mWm-2), thicker crust (33-39±1 km) and temperature gradients (23-27^oC/km). The relatively shallow CPD’s, elevated heat flow and thinned crust are consistent with previous seismic studies. Additionally, we modeled the burial history, thermal maturity and potential timing of petroleum generation from multiple wells and seismic section. Integration of results generated from the gravity and magnetics to those of basin subsidence and temperature gradient curve generated from basin modelling based on the stratigraphic data has significantly improved our understanding of the petroleum system of the Albertine Graben and generally frontier basins as potential oil and gas reservoirs.