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Passive Continental Margin Basin Evolution and Giant Gas Discoveries in Offshore East Africa

Abstract

Passive continental margin basins in offshore east Africa (PCMBOEA) include Somali, Lamu, Tanzania, Ruvuma, Mozambique, and Zambezi Delta Basin with total areas over 2,000,000 km2. Gas discoveries dates back to 1970s, however, since 2010 about 141 Tcf gas discoveries make it become a hot area. The basin evolution mainly involves three stages: 1) The NE-SW trending intracontinental Karoo system was developed in Permo-Triassic with a range of facies from fluvial to lacustrine to deltaic; 2) The initiation of Gondwana break-up occurred in Middle Jurassic, resulting in the southward drift of Madagascar away from east Africa and a passive continental margin development along the western coast of Madagascar and eastern coast of Africa from Middle Jurassic to Early Cretaceous represented almost by continental- transitional to shallow marine deposits, followed by development of widespread marine transgression; 3) After the end of the sea-floor spreading between Madagascar and Africa and the initiation of India's separation from Madagascar in late Cretaceous, a passive margin developed along the eastern coast of Africa. Several potential lacustrine and marine organic-rich source rocks are present in the PCMBOEA, ranging in age from Triassic to middle Jurassic to Tertiary respectively. Cretaceous sandstone can represent a regional proven reservoir, while Tertiary deltaic sandstones or limestone is a local proven reservoir. Permo-Triassic Karoo Group sandstone and Jurassic sandstone or limestone is considered as the potential reservoirs. Cretaceous siltstone and shale is a regional seal and Jurassic evaporates if present may locally provide effective seals, whose quality deteriorates towards the east Africa coast. Three distinct structural phases corresponding to the basin evolution are recognized: E-W Karoo rifting, S-N extension during the early stages of Gondwana break-up, followed by Tertiary extension and listric fault growth. Hydrocarbon generation from different levels of source rocks probably occurred at various times throughout late Cretaceous and Tertiary times. Then vertical and lateral migration with a relatively short and long distance respectively lead to the hydrocarbon accumulation within the drift sequence such as delta sand body and deep-sea turbidite deposits. Area 4 operated by EniEAfrica, is the largest gas discovery with total recoverable reserves over 10,000 mmboe, whose main reservoir is Paleocene-Oligocene sand in such setting.