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Integrated Petroleum Analysis for the Albertine Graben in East Africa

Abstract

The Albertine Rift in the Western branch of the East African Rift System (EARS) represents an Early-Miocene Northeast trending graben filled with ∼ 5km thick fluvio-lacustrine sediments deposited within a basin strongly controlled by Precambrian basement structures. Recent oil exploration within the Albertine Rift has shown a promising prospect with an estimated 3bn barrels, prompting further interest to better understand basin architecture and petroleum systems. Conventionally, hydrocarbon basin analysis relies heavily on seismic data and well log information. However, these two geophysical techniques are the most expensive and many developing countries cannot afford the cost. In this study, we integrated potential field geophysical methods (gravity and magnetics) and basin modelling approach to determine heat flow and source rock maturation within the Albertine Rift. We used two-dimensional (2D) power density spectrum methods to determine the base of the magnetized crust from aeromagnetic data that possibly represents the Curie point depth (CPD) and to estimate the crustal thickness from satellite gravity data. Based on back stripping analysis of multiple wells, we calculated tectonic subsidence which we used to validate crustal thickness estimates. We also performed one-dimensional basin modelling on selected wells to reconstruct burial history models to simulate thermal evolution and vitrinite reflectance (Ro). We used borehole data such as measured Ro and geochemical analysis (Tmax) to constrain our basin models. Here we report the heat flow and source rock maturation trends from data integration. Our results suggest 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°C/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°C/km). Modelled burial history indicates potential oil generation in the Kisegi formation, the major source rock, may have begun at ∼3Ma. Meanwhile, distribution of Ro values indicate minimum relationship with depth and very shallow (600m) maturation. We hypothesize this significant vertical heat anomaly that is responsible for shallow source rock maturation might be due to upward migration of hot fluids assisted by lithospheric scale boundary faults.