Formation of the stratigraphic hydrocarbon accumulations of the Upper Yabus Formation in the Northern Sub-basin of the Melut Basin, South Sudan

Abstract

The Melut Basin is an important petroliferous rift basin in Africa, and a large number of structural hydrocarbon accumulations of the Paleogene Yabus and Samma Formations have been discovered in it. However, there are few published research results and exploration discoveries on stratigraphic hydrocarbon accumulations in the Melut Basin. Based on information from the evaluation of the lithology assemblage, oil-source, migration pathway, preservation condition and the detailed analysis of typical exploration cases, the favorable exploratory interval, formation conditions and accumulation characteristics of the stratigraphic hydrocarbon accumulations have been studied in the northern Melut sub-basin. The sedimentary facies study indicated that the Upper Yabus Formation developed widespread delta front deposits with the lithology assemblage of “sand wrapped by mud”, which was conducive to forming depositional stratigraphic traps dominated by facies change or depositional pinchout. The Adar Formation is a set of mudstone deposition with a thickness of 200–600 m above the Yabus Formation and well sealed the Yabus and Samma hydrocarbon accumulations as the regional top seal. The parameters of organic matter, and the thickness and distribution of source rocks showed that the Al Renk Formation has good source rock quality with an average TOC value of 2.08% wt. and type II kerogen prone to oil generation. The thermal modelling revealed that the Al Renk source rocks continuously generated and expelled hydrocarbon since the Late Cretaceous to the present and provided sufficient oil source for the Upper Yabus stratigraphic traps. The tectonic evolution research showed that the multi-stage rifting cycles made the Melut Basin develop numerous normal faults, which provided essential vertical migration pathways for the hydrocarbon charging of the Paleogene stratigraphic traps. Among these faults, the basinal border faults and the intra-basinal late-rifting faults are the primary hydrocarbon conduit faults. The details of exploration cases showed that the hydrocarbon conduit faults are the key controlling factors for the formation of the Upper Yabus stratigraphic hydrocarbon accumulations, and the stratigraphic traps jointly dominated by lithology change (depositional pinchout) and hydrocarbon conduit faults have low hydrocarbon filling risk. The sandstones of the Upper Yabus Formation are shallow (700–2500 m) and have high porosity with a value of greater than 20% and high oil production of a few hundred to more than 1000 barrels per day. Therefore, the Upper Yabus Formation develops favorable petroleum geology conditions for the formation of stratigraphic hydrocarbon accumulations, and exploring such stratigraphic traps is of great significance to the increasing exploration in the mature northern Melut sub-basin.