Petroleum Systems and Basins of Yemen
Yemen is situated in the southwestern part of the Arabian Peninsula and both contains onshore and offshore sedimentary basins, all of which developed during discrete time intervals in the Paleozoic, Mesozoic, and Cenozoic. Two onshore sedimentary basins, Sab’atayn and Say’un-Masilah, where oil was discovered in 1984 and 1991 respectively, are currently the only petroleum-producing basins in Yemen, while the other basins, including the onshore Paleozoic and offshore Cenozoic basins, remain little-explored. The Paleozoic basins include the Rub’ Al-Khali Basin (Infra-Cambrian-Permian) to the north of the west-east striking Hadramawt Arch, and the San’a Basin dominated by the Paleozoic Wajid clastics and Akbarah clay at the base and extending to Upper Jurassic marine sediments (a possible petroleum system) and a terrigenous Cretaceous cover. The Southern Suqatra (Socotra) Basin in the Gulf of Aden, filled with continental to marine rift-sediments of Permo-Triassic age, remains to be explored thoroughly. The fragmentation of Gondwana, accompanied by the separation of India/Madagascar from Afro-Arabia during Late Jurassic–Early Cretaceous times, initiated major extensional basins in Yemen including the Siham-Ad-Dali’, Sab’atayn, Balhaf, Say’un-Masilah, and Jiza’-Qamar Basins. The location and the NW-SE orientation of these basins appear to have been controlled by the latest Neoproterozoic structural grains associated with the Najd strike-slip fault system that extends from western Saudi Arabia into Yemen. The well-explored Sab’atayn and Say’un-Masilah Basins, filled with syn- and post-rift sediments, share many similarities in source rocks (Kimmeridgian-Tithonian Madbi Shale and Berriasian Sa’ar Shale) and reservoir rocks (Kimmeridgian-Tithonian Madbi carbonates and clastics and Berriasian-Valanginian carbonates and clastics); however, Tithonian evaporite beds are absent in the latter. The intra-salt sandstones and sub-salt turbidites offer significant oil accumulations in the Sab’atayn Basin, while the sandstones of the Qishn Formation (Lower Cretaceous) are the main reservoir in the Say’un-Masilah Basin. Fractured Pre-Cambrian granite gneiss also serves as an important oil reservoir in upthrown blocks in both Sab’atayn and Say’un-Masilah Basins. The initial reservoir pressures in the Sab’atayn Basin are gas-driven while those in the Say’un-Masilah Basin are water driven. The less explored Siham-Ad-Dali’, Balhaf, and Jiza’-Qamar Basins, located respectively in western, central, and eastern Yemen, also contain thick sediments of Late Jurassic and Cretaceous age, but with varying depositional environments, being more continental in the western and central parts of Yemen and fully marine in the Jiza’-Qamar Basin. These frontier basins extend offshore beneath the Cenozoic sediments deposited during the Gulf of Aden rift subsidence. The onshore Jiza’-Qamar Basin has already demonstrated good gas potential in the Upper Cretaceous coal-shale source rock and associated limestone reservoir; the Balhaf Basin contains potential source and reservoir rocks at several stratigraphic levels; and the Siham-Ad-Dali’ Basin has highly bituminous Jurassic rock as well as good reservoir potential in its Jurassic sandstone units. The Cenozoic rift basins of Yemen also remain exploration frontiers. They developed during the Oligocene/Miocene rifting phases of the Gulf of Aden (the Mukalla-Sayhut, Hawrah-Ahwar, and Aden-Abyan Basins) and the Red Sea (the Tihamah Basin), and were filled with thick sedimentary successions marked by several stratigraphic breaks. These sediments range from continental and evaporitic to shallow- and deep-marine facies, and are injected or covered by alkali-basalt volcanic rocks. In contrast to the Cenozoic rift basins of the Gulf of Aden and the Red Sea, no volcanic activity was associated with the development of the five sub-parallel Mesozoic extensional basins onshore Yemen.