Abstract: Subsalt Source Rock Maturity in the Sudanese Red Sea

Carsten Geiger, John Pigott, James M. Forgotson Jr., Abdelazim Ibrahim, Hayat Yousif

Thermal modeling can demonstrate that stratal salt deposits may provide a significant heat conduit and conceptually provide a basis for hypothermal fairways of hydrocarbon exploration in regions of dominant thermal overmaturity. However, accurate evaluation of thermal maturity suppression by modeling must be geologically constrained. With respect to the Tertiary Tokar Delta of offshore Sudan, 1D tectonic subsidence analysis of boreholes in the region reveals at least two major pulses of crustal extension and associated heating (24-20 m.a. and 5.4-2.7 m.a.). Integrating the borehole geochemical information with a Tokar Delta seismic stratigraphic interpretation allows the construction of constrained 2D thermal basin models through time using Procom BMT.

The best match between the observed and modelled vitrinite reflectance values is achieved by using a two phase tectonic stretching model with pulses at 22±2 m.a. and 4±1.5 m.a. and incremental subcrustal stretching factors which vary between 2.65-2.75. Utilizing these parameters suggests the top of the oil window to occur within the Zeit Formation and bottom of the oil window to exist at the base of the Dungunab Salt. As only subsalt source rocks are observed, this model would tend to negate the possibility of the occurrence of liquid hydrocarbons. For the Tokar Delta the presently observed general high heat flow is so high that it leads in all cases to overcooked organics for a subsalt source. However, that hydrocarbons in the post-salt Zeit Formation of the Tokar Delta hav been discovered suggests significant secondary hydrocarbon migration to have occurred within the late Miocene (15.4 - 5.4 m.a.). Potential migration pathways would be along basement-induced fault conduits. If true, similar secondary migration play concepts may be applicable elsewhere in the Red Sea.

AAPG Search and Discovery Article #90956©1995 AAPG International Convention and Exposition Meeting, Nice, France