Structure and Stratigraphy of the Bogal Inversion Zone, Anza Basin, Kenya:implications for the Petroleum System University of Nairobi

Abstract

The Anza basin is part of the larger Central African Rift System (CARS) that traverses the African continent from the west to the east. Inversion tectonics occurred in the various sub-basins of the CARS, including the Anza rift during the Late Cretaceous to Early Tertiary. Inversion affects the petroleum system in various ways. It can affect the petroleum system positively by creating new structural and stratigraphic traps or negatively through the re-migration of hydrocarbons, erosion of reservoirs and seals and the redistribution of source rocks and reservoirs. The aim of the study is to apply an integrated structural-stratigraphic and geochemical concept in the evaluation of the hydrocarbon potential of the Bogal play and to establish the consequence of inversion tectonics on the petroleum system of this area. The interpretation of 2D seismic and borehole data from the Bogal-1 well was applied in the analysis of the stratigraphy, structural geometry and the petroleum system elements within the Bogal play. Time-Temperature Indexes (TTI) analysis was used to determine the maturity of the Cretaceous source rocks, and to estimate the timing of hydrocarbon expulsion. The stratigraphy of Bogal-1 well is composed of thick Cretaceous and thinner layers of Tertiary and Quaternary sediments. An angular unconformity separates the Neogene and the Paleogene sediments, forming a potential stratigraphic trap for hydrocarbons. The Upper Cretaceous sediments contain reservoir-quality sandstones and thick mudstone layers that could possibly form the sealing rocks of the Bogal play. The thick Lower Cretaceous sediments contain mudstones that form the potential source rocks of this area. A 100m thick diabase sill penetrates the Lower Cretaceous sediments. It is believed to be of Tertiary origin. Seismic data reveals that the Bogal structural zone is composed of inversion anticlines with wavelengths of approximately 16 Km. The inversion phase occurred after initial extensional forces were replaced by compressional stress. The presence of positive, negative and hybrid flower structures form viable structural traps for hydrocarbons. Fault restoration of the Cretaceous sediments estimates a shortening of approximately 1,714.6m. TTI analysis of the Cretaceous sediments gives a value of 6.04 for the Upper Cretaceous sediments, implying that they are located at the onset of the early oil window, and are therefore immature. The Lower Cretaceous sediments obtain a TTI value of 457.2, suggesting maturation within the wet gas zone. The maturation and expulsion might have occurred in the Oligocene, approximately 27 Ma, probably at the same time as the inversion. The timing of inversion and hydrocarbon expulsion is therefore considered sufficient for petroleum entrapment. More wells should be drilled in this area to examine the trap potential of the lateral angular unconformity that extends throughout this area.