Regional Geology and Petroleum Systems of the Main Reservoirs and Source Rocks of North Africa and the Middle East

Abstract

Roughly 44% of the world’s total known hydrocarbon resources are located in North Africa and the Middle East, from Algeria in the west to the Zagros region of Iran in the east. This includes more than 200 giant fields in the Middle East. North African basins, mainly in Algeria, Libya, and Egypt, contain 4% of the world’s oil and gas reserves and nearly 40 giants. The most recent giant field resides in the offshore Levantine Basin. In this chapter, the region’s geological history and petroleum systems are reviewed, including descriptions of the regional habitats and stratigraphy of the main reservoirs and source rocks. The Late Pre-Cambrian to Phanerozoic tectonostratigraphy is characterized by six major phases: (1) basement assembly and Infracambrian extension; (2) development of Cambrian to Carboniferous passive margins; (3) Late Carboniferous to Early Permian Hercynian orogeny; (4) post-Hercynian breakup of Gondwana; (5) collision with Eurasia; and (6) Red Sea rifting and the closing of Neotethys. For most of the Phanerozoic, the terranes of North Africa and Arabia were locked together at the northern rim of Gondwana. They shared a similar geodynamic history and generally similar climates. Their plate margins interacted over time with the water masses of three oceans: Proto-, Paleo-, and Neotethys. Throughout the Paleozoic, the region lay on a wide “ramp-like” passive margin facing northward toward Paleotethys. The main sediment source was the large hinterland to the south, with prevailing south to north and southeast to northwest-directed paleocurrents. Multiple marine transgressions across a low-relief continental platform were interrupted by at least four glacial events: Late Ordovician (Hirnantian), Silurian, Carboniferous, and Early Permian. Deposition of siliciclastics predominated throughout the Paleozoic whereas carbonates were much less common. The most important tectonic event was the mid-Carboniferous Hercynian composite-orogeny, which caused major uplift and erosion. The Hercynian represented the final closure of Paleotethys and produced many fault blocks and arches that would later host many of the major hydrocarbon accumulations of North Africa and eastern Arabia. The Mesozoic–Cenozoic sedimentary sequence similarly consists of eustatically and tectonically controlled depositional cycles along the newly formed passive margin of Neotethys. Triassic to mid-Cretaceous facies along North Africa are almost everywhere shallow marine, nearshore, deltaic, and continental. Neotethys reached its maximum extent in the Late Cretaceous at which time carbonate sequences dominated. In addition to the Hercynian Orogeny, two other compressional events had major consequences on North Africa–Arabia petroleum systems. The closing of Neotethys began in the late Santonian (~84 Ma) and the convergence between Eurasia and Africa–Arabia sent pulses of compressional deformation across the plate. This corresponded to the first phase of the complex Alpine orogenic cycle and caused folding, basin inversion, and strike-slip faulting along the African–Arabian Neotethyan margin (the “Syrian Arc”), and thrusting and ophiolite obduction in Oman. Compression was renewed at the end of the Maastrichtian and continued into the early Paleocene, followed by even stronger effects in the Late Eocene. Eruption of the Afar plume at about 31 Ma marked the beginning of a new phase of continental rifting that had dramatic effects on all aspects of the geology of the region. The Gulf of Aden ruptured first in the Early Oligocene, followed by the southern Red Sea in the Late Oligocene. At the Oligocene–Miocene transition, the remainder of the Red Sea north to the Gulf of Suez underwent a regional dike event and accompanying extensional faulting. Initiation of the Gulf of Aqaba—Dead Sea transform plate boundary occurred in the Middle Miocene, completing formation of the independent Arabian plate. The Neotethys Ocean also ceased to exist in the Middle Miocene following a collision between Eurasia and the Arabian plate to form the Bitlis–Zagros suture and fold belt. The Arabian plate was progressively tilted to the northeast as a result of both uplift and rifting of Arabia from Africa, and structural loading of the northeast margin by the Zagros fold belt. Recent tectonic activity is mainly concentrated along the Maghrebian Alpine Belt, the offshore Nile Delta, the Red Sea–East African (or “Afro-Arabian”) Rifts Province, the Aqaba–Dead Sea–Bekaa sinistral strike-slip fault zone, and some major intra-plate fault zones including the Guinean–Nubian, Aswan, and central Sinai lineaments. Our review of the petroleum systems of North Africa and Arabia is brief and includes only highlights of the hydrocarbon occurrences found across this broad and complex region. Based on the age of source rocks, it is possible to distinguish an Infracambrian Petroleum System, Palaeozoic-related Petroleum Systems, and linked Mesozoic–Cenozoic Petroleum Systems. The sedimentary fill contains numerous source rocks, some of them with exceptional quality and regional distribution, such as the Silurian “hot shale”. Producing reservoirs are found in both siliciclastics and carbonates. The proximity and juxtaposition of source rocks with thick reservoirs minimized the need for complex oil migration pathways, but also facilitated hydrocarbon expulsion and migration over long distances. Huge amounts of evaporites and shales are present, providing excellent lateral and ultimate top seals. Hydrocarbons are trapped in literally all stratigraphic units from the fractured Neoproterozoic basement to the youngest Pliocene–Quaternary sediments.