1D and 3D Petroleum System Modelling of the Songo Songo source rocks, southern coastal offshore Tanzania.

Abstract

3D petroleum system modelling of the Songo Songo area, located southern coastal offshore Tanzania, has been constructed with PetroMod software using surfaces interpreted from 2D seismic lines. 1D modelling, in combination with measured temperature and vitrinite reflectance values, was used for model calibration. The modelling results have improved the understanding of the maturity and transformation of the source rocks in the study area. The subsidence history plots from the model indicate the basin has experienced several periods of subsidence, uplift and erosion which have affected sediment deposition, trap formation, hydrocarbon generation, migration and entrapment. A high subsidence rate was recorded in Middle Jurassic which corresponds to the period of active rifting in the East African coastal basin.

Maturity modelling indicates that the Mtumbei Formation (source rock) is at different levels of maturity ranging from immature to mature, and is potentially the main source rock in the area which has generated both oil and gas. The hydrocarbon generation started in the Early Cretaceous in the deepest part of the basin and peak oil generation began in the Early Eocene. The predicted hydrocarbon accumulations are trapped mainly in the Kipatimu Formation which is in direct contact with the Mtumbei Formation (source rock) and is sealed by the top Albian Formation which lies direct above. However, the only gas accumulation to date in Songo Songo could be accounted for by cracking of early formed oil to gas or an increase in terrestrial gas prone source rock in the main kitchens , as well as light hydrocarbons migrating vertically from deeper and more mature source rocks.