William S. Okoth

The purpose of this research work was to evaluate hydrocarbon potential of offshore Kenya, covering an area of approximately 39,500 square kilometres. A total of 3,673 line kilometres of multichannel seismic reflection shot by Total Oil Exploration Company were interpreted. Three horizons designated A, B and C were mapped and horizons A and B correlated with Simba-1 well 90 kilometres offshore. Additional 910 line kilometres of multichannel seismic reflection conducted by Seagap were used in the northern part of the survey area to confirm the B horizon. Gravity data was compiled to interpret structural highs.

On the southern part of the survey area, a north-south ridge complex beneath the continental slope continues the trend of the Lamu embayment anticline onshore to the north. West of this is a major Mesozoic basin with isolated diapiric and basement highs that is flanked on the west by a complex series of Tertiary faults. Similar Tertiary faults, accompanied by abundant plug-like diapirism characterise the northern part of the survey area.

Published data on onshore coastal stratigraphy were synthesized to illustrate reservoir and cap rock potential. Rock cuttings from Simba-1 well were analysed to indicate source rock material.

There are potential reservoir sandstones and limestones from Lower Miocene to paleocene in the coastal onshore wells, however, their permeabilities and porosities are low and decrease with depth. Lower Cretaceous to Jurassic beds have in general low to fair permeability. Cap rock are poorly developed in Tertiary of the onshore coastal wells where only occasional thin shales are found. Offshore, both velocity analysis and Simba-1 well show that the coastal limestone and sandstone facies pass seaward into shales. Two limestone units extend across the survey area and pass rapidly into shales some 150 kilometres offshore.

Measured geothermal gradients show that thermal maturation to hydrocarbons will occur only in pre-Lower Eocene strata. Lower Eocene to Upper Cretaceous shales in Simba-1 have source rock potential, but kerogen types suggest only gas will be generated. In some of the onshore wells, there are thick dark Cretaceous marine shales which due to their bituminous nature could be regarded as favourable.

The seismic study has revealed some structural traps, which have been in existence since Mesozoic times. Two of these structural traps (anomalies 4 and 5) occur at sufficient sub-bottom depth to fall close to the oil window and have been therefore recommended for detailed study.

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Supervisor: D. J. W. Piper