Abstract:

In this study, ancient fault sliding distance and sliding rate were used in a quantitative study on growth faults in the Kaikang Sag, Muglad Basin, Sudan to quantify variations in fault growth rates and activity intensities in three different rifting phases, i.e., the Early Cretaceous, Late Cretaceous, and Paleogene to Neogene. The results showed that the controlling sag faults developed in a linked multiphase multisection fashion, the controlling structural belts faults underwent linked growth at the beginning and simple growth later, whereas the controlling traps faults experienced simple growth throughout. Fault activities in the Early Cretaceous controlled the distribution of source rocks, while the three riftingsag structural cycles led to differentiation of sand and shale, forming the upper, middle and lower reservoirseal assemblages. Hydrocarbon accumulation was controlled by three main factors: trap type, fault movement intensity in the Darfur group as source rock entered into oilgenerating peak, and fault movement intensity in the NayilTendi formations when paleooil reservoir could be destroyed and readjusted. Transverse anticline in structural transform zone, horst, and antithetic fault block were favorable structures for paleoreservoir or shallow secondary oil reservoir formations, whereas consequent fault block was not favorable. The results also showed that the fault activity ratios of the Darfur group to NayilTendi formations can be used to measure the relative capability of hydrocarbon accumulation in the early period vs. oil reservoir destruction in the late period. This relative capacity, based on actual drilling data, is related to the accumulation layers in the following way: the eastern limb of the Kaikang trough hosts a prolific oil and gas accumulation zone, where under the uplift zone hydrocarbons mainly accumulated in the lower reservoirseal assemblage of the Cretaceous system, forming deposits throughout the eastern fault slope zone. In contrast, accumulation condition in the western limb of the Kaikang trough is poor in its slope and fault step zones, where the northern uplift developed multiple accumulation layers but low production, while the southern uplift faired better with accumulation mainly in the lower reservoirseal assemblage of the Cretaceous system; in the depression zone, secondary oil reservoir might be formed due to strong fault movement in the late period.

Key words: growth fault, quantitative study, ancient sliding distance, sliding rate, hydrocarbon accumulation, Muglad Basin