Assessing the Hydrocarbon Potential of Offshore Lamu Basin using Nuclear Well Logs

Abstract

Kenya’s sedimentary basins are similar to other basins in eastern Africa where petroleum has been discovered. In general, acoustic and electrical methods are used in geophysical logging for hydrocarbon exploration in the country. However, the recent discovery of oil in Turkana, a region initially dismissed as non-viable has prompted review of previous data and integration of modern nuclear logging techniques in exploration. The core objective of this study was to use nuclear well log data to assess the hydrocarbon potential in Offshore Lamu basin. The neutron log, density log, and gamma log data for Simba-1, Kofia-1 and Maridadi-1B wells were acquired from National Oil Corporation of Kenya. The log data in LAS format were subjected to well log analysis, interpretation, and presentation techniques using notepad++, ArcGIS, and Techlog software. The results were integrated with electric log data and cuttings reports whenever available for quality control and data validation. Essentially, the study determined petrophysical parameters mainly: lithology, volume of shale (Vsh), porosity, and fluid saturation to gauge the hydrocarbon reservoir characteristics of the basin. The results show, on average, three zones of interest in study area, range between 2600 m-3900 m below mean sea level. Sandstones and limestones are the predicted reservoir formations, while shale is the interpreted source rock. The porosity at the zones of interest in the three wells is placed at 21% - 35%, although there are lower values for regions viewed as tight carbonate reservoir zones. The Vsh is lower, estimated at 16% - 24% in projected reservoir zones, with high resistivity, probably due to presence of hydrocarbons. Predicted water saturation is high (30% - 60%) and exceed the recommended 30% average. This has been attributed to bound water in regions where Vsh is high, or low hydrocarbon content in clean sandstones. Estimated permeability mainly derived from porosity values ranges from 21 mD to 116 mD in single oil reservoir layers. The results for porosity and permeability, and Vsh are within the values of most oilfield discoveries. There is presence of hydrocarbons in the Lamu basin but whose quantity is undeterminable. In conclusion, the study recommends drilling of directional wells, and the use of seismic and NMR data as a determinant of bound fluids in complex lithology. Key words: nuclear logs, porosity, saturation, permeability, volume of shale