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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 1.

Study area location.

Figure 2.

Block 52 offshore stratigraphic summary chart.

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 3.

Well location, Circle 2006 and 2011 2D seismic control and bathymetry map.

Figure 4.

Example of Area A structural interpretation.

Figure 4.

Example of Area A structural interpretation.

Figure 5.

Type seismic line to show inverted half graben architecture.

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.

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