Source rock maturation and hydrocarbon generation evaluation using 3D Basin modelling
Nakisita, Betty Mpongo
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The Semliki basin is located in the Albertine Graben which is part of the western arm of the East African Rift System (EARS). The basin demonstrated existence of a Petroleum System (Kiconco, 2005 and PEPD, 2012). Although an oil seep has been found in Semliki basin, the major hydrocarbon discovery in the wells is natural gas. This therefore created a need for an explanation for the hydrocarbon nature observed. The evaluation of the source rock maturity and hydrocarbon generation of the Semliki basin was done using 3D basin modelling approach. The research utilized the existing 2D Seismic data and well completion reports from Semliki basin provided by the Directorate of Petroleum and geochemical data abstracted from literature. The methods involved interpretation of faults and horizons using Schlumberger Petrel software to acquire surfaces which in turn were used as input data for PetroMod software to generate the final hydrocarbon maturation and generation model. The results showed that the basin has generally experienced several episodes of subsidence at both slow and rapid rates. The deposition of the source rock commenced in Early Miocene (12 Ma) and continued at a gentle subsidence until Mid-Miocene (9 Ma). This was followed by a series of rapid subsidence (9-7.5 Ma; 5-4.5 Ma; 4.5-3 Ma; 2-1Ma) and slow subsidence episodes (7.5-5 Ma; 3-2 Ma; 1 Ma –present). A total burial depth of 2500 m was reached at Pleistocene. The maximum temperature reached by this interod is 140oC and the kerogen is currently at the Early to Late Oil window maturity. The transformation ratio shows that source rock in the deeper parts of the basin has completely transformed to hydrocarbons, while those in the shallower parts of the basin are immature or have only attained the early oil window. The Kasande source rock in the Semliki basin started generating hydrocarbons during Pleistocene. Almost half of the source rock has generated but at different maturity levels, whereas, the other half has kerogen which has not yet expelled any hydrocarbons. Therefore, the source rock is still generating to date. This research has provided a better understanding of the key controls to the hydrocarbon generation in the Semlki basin.