Abstract The Red Sea and Gulf of Aden sedimentary basins are developed along the African and Arabian conjugate margins and are characterized by Late Tertiary rifts filled with siliciclastic, carbonate, and thick evaporite successions north of the Babel-Mandeb Strait in the Red Sea. Geodynamic models for the development of the Red Sea–Gulf of Aden continental margins include simple shear mechanisms associated with mantle exhumation, as described in the Iberian margin, and pure shear mechanisms, with continental breakup associated with magmatic intrusions and development of organized oceanic crust in some segments of the axial trough. The rifted continental margin in the southern segment of the South Atlantic is characterized by several Mesozoic rifts that extend from onshore to offshore Brazil, Uruguay, and Argentina; the onshore rift-border faults in Argentina are at high angle to the continental margin basins. These rifts and also the Pelotas basin in southern Brazil are essentially devoid of evaporites, which mainly occur northwards of the Florianópolis Fracture Zone. A mantle plume before continental breakup is interpreted to cause the massive volcanic outpouring both in the Red Sea–Gulf of Aden continental margins (Afar plume) as well as in the region between the Pelotas and Santos basins in Brazil (Tristão da Cunha plume). The basalts associated with the continental breakup include seaward-dipping wedges in the transition from continental to oceanic crust, and volcanic eruptions probably formed barriers isolating oceanic basins from an incipient gulf developed on continental crust with synrift sedimentation. Episodic marine incursions resulted in accumulation of thick layers of massive evaporites that were deposited before the development of active oceanic spreading centers. The oceanic ridges split the salt basins initially with localized igneous intrusions and subsequently by organized oceanic crustal spreading, with allochthonous salt flows advancing towards the axial trough and covering the volcanic basement.

Abstract 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.

Abstract An undrilled and structurally inverted half graben basin has been identified on the North Eastern margin of the Gulf of Aden in Block 52, Offshore Southern Oman. This block (Fig. 1) was awarded to Circle Oil in September 2005 and originally covered an area of 90,760 sq km. The known stratigraphy of the area ranges in age from Precambrian to Tertiary, based on onshore outcrops, offshore well penetrations, and seismic interpretation. Much of the Paleozoic is absent in the offshore wells. The identified principal reservoir intervals are Paleogene, Hadhramaut, and Upper Cretaceous Aruma Group carbonates (Fig. 2). Potential source rocks include the basal Hadhramaut Group shales, intra-Aruma Group shales, organic-rich units in the basinal Jurassic Sahtan Group, and the Infracambrian middle to upper Huqf Super Group. Sealing units for potential traps are provided by extensive shale and evaporate units of the Miocene Fars, intra-Hadhramaut, and intra-Aruma groups. Figure 1. Study area location. Figure 2. Block 52 offshore stratigraphic summary chart. Three near-shore wells have were drilled on the block (Fig. 3); all had either oil or gas shows, and there was evidence of a working petroleum system from active offshore seeps. SQB-1, drilled by Amoco in 1979, encountered minor oil and gas shows in Cretaceous Natih carbonates and reached TD of 3314 m in Precambrian igneous basement. KM-1, drilled in 1982 by Amoco, encountered minor gas shows in Infracambrian clastics, though no significant Mesozoic reservoirs were penetrated and the well reached TD at 2178 m in the Precambrian. Well SQBS-1 was drilled by PDO in 1991 and encountered minor gas shows in the Paleogene Hadhramaut carbonates; the well reached TD at 1950 m in crystalline basement. These three wells were drilled on basement highs (Fig. 4) and do not reveal the full story of the basin potential (Fig. 5). In addition, eight shallow Ocean Drilling Program (ODP, 1987) wells were drilled on the outer shelf of Sawqirah Bay to test the shallow stratigraphy as part of global academic research. Figure 3. Well location, Circle 2006 and 2011 2D seismic control and bathymetry map. Figure 4. Example of Area A structural interpretation. Figure 5. Type seismic line to show inverted half graben architecture. The undrilled half-graben basin is about 100 km long by 30 km wide, located 110 km offshore from Sawqirah Bay, and is oriented northeast-southwest, subparallel to the coast. Although over 10,500 line km of legacy 2D seismic existed and a further 6,200 line km were acquired by Circle Oil in 2006, only a few regional 2D seismic lines had been shot with extensions into this area, showing few indications of the inversion structures. Circle Oil acquired a detailed survey of 2640 line km in 2011 over this area to understand better the potential prospectivity seen on the older 2D coverage. The 2D seismic surveys had been used to delineate the extent and evolution of this basin, which formed in the Late Cretaceous as an early phase of opening of the Gulf of Aden. Deposition continued in this extensional phase through the Paleogene, with up to 3 seconds TWT of sedimentary fill observed in the deep northwest margin of the half graben; the fill thins seawards to the southeast. A significant compressional phase in the Oligo-Miocene resulted in basinal inversion and the creation of a northeast/southwest-trending structural ridge. A chain of en echelon culminations form prospective exploration targets of individual anticlinal closures of up to 43 sq km. Water depths over this prospective area range from 724-956 m and target depths for Paleogene–Late Cretaceous reservoirs are 1490-2850 m.