Modeling Petroleum Generation in the Southern Muglad Rift Basin, Sudan

ABSTRACT Maturity and petroleum generation have been modeled in a 120-km-long northeast-southwest cross section of the Southern Muglad basin, Sudan. The section passes close to the Unity and Kaikang areas, which provide well control on the structure interpreted from gravity and seismic reflection profiles. Modeling of geotherms from corrected bottom-hole temperatures indicates present heat flows around 60 mW/m2 in the well control areas. Using measured vitrinite data as calibrant, maturity can be modeled equally well with a constant paleoheat flow similar to the present day or with a complex heat flow pattern related to rifting. A geologically realistic complex heat flow model was employed to calculate the timing of generation and expulsion from the Lower Cretaceous lacustrine source rocks of the Abu Gabra and Sharaf formations. Kinetic (Easy% Ro) calculation of vitrinite maturity indicates a projected oil preservation depth limit around 4000 m shallowing to around 3500 m northeast of Unity near the basin margin. Modeling of hydrocarbon evolution in the main hydrocarbon kitchen areas indicates an early phase of oil expulsion from lower source units between 110 and 95 Ma prior to Upper Cretaceous reservoir deposition. Oil in Darfur Group reservoirs of the Unity field probably was sourced initially from the upper Abu Gabra in the main kitchen area between 80 and 60 Ma and may have remigrated from deeper reservoirs during Tertiary rifting. Subsequent oil charge probably migrated from the lower Abu Gabra in minor kitchens northeast of the Unity-Heglig ridge, which may have expelled oil until Miocene uplift. The petroleum system probably is relatively inactive at present. Volumetric calculations for the Sharaf and Abu Gabra source formations suggest that around 1012 bbl of oil and 1014 m3 of gas have potentially charged reservoirs, although much may have been expelled prior to trap development. Apparent lack of significant gas accumulations may reflect leakiness of traps during rift-related fault rejuvenation.