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ABSTRACT

The Gharif Formation of the Haushi Group in subsurface Interior Oman is underlain by the glaciogenic sediments of the Al Khlata Formation, and is progressively transgressed by the marginal to fully-marine Khuff Formation. The Gharif Formation consists of three members that are interpreted as depositional sequences (DS). The Early Permian (Artinskian) Lower Gharif Member (about 65 m, 213 ft thick) corresponds to DS Permian 10 (DS P10) and consists of Gharif cycles 1 and 2. Gharif Cycle 1 contains Arabian Plate Maximum Flooding Surface MFS P10 in the ‘Marine Flooding Shale’ unit; Gharif Cycle 2 is characterised by the ‘Haushi Limestone’ in central and north Oman. The Middle Gharif Member (about 85 m, 279 ft; DS P13) corresponds to Gharif cycles 3 and 4; Cycle 3 is also Artinskian in age. The Upper Gharif Member (about 90 m, 295 ft; DS P15) consists of Gharif cycles 5-8 and by stratigraphic position it ranges in age from late Artinskian to early Wordian. Whereas Gharif cycles 5-7 are interpreted as Depositional Sequence 15, uppermost ‘estuarine’ Gharif Cycle 8 is only tentatively assigned to DS P15. The base of Gharif Cycle 8 is an apparent sequence boundary and this cycle may be related to the Khuff transgression. The Gharif Formation was deposited in environments ranging from continental to marginal marine, and its lithofacies can be organised in a hierarchical reservoir scheme. The boundaries between the three members and some of the intra-member cycles are sequence boundaries; characterised by incised channels (20-50 m deep) encountered in wells and sometimes imaged in seismic sections.

The Khuff Formation of the Akhdar Group reaches a maximum thickness of 1,110 m (3,641 ft) in northern Oman, where it is dominated by carbonates with very little shale. Towards southeast Oman, the Khuff becomes progressively truncated (geographically and stratigraphically) and interspersed with, and finally dominated by, mainly ‘red bed’ clastic facies. In south Oman, the Khuff and Gharif red beds are palynologically barren and lithologically indistinguishable. The Khuff Formation is divided into three members and eleven transgressive-regressive cycles or possible depositional sequences. The Lower Khuff Member (up to 325 m, 1,066 ft thick) is interpreted as three depositional sequences (DS P17-19) that are Wordian and younger in age. DS P19 contains the Khuff Marker Limestone (KML) that is interpreted as Maximum Flooding Interval MFI P19; it provides a semi-regional datum in the red bed-dominated Lower Khuff Member in south Oman. The Middle Khuff Member (up to 640 m, 2,099 ft) is nearly equally bisected by the regional Middle Khuff Anhydrite (Khuff-D Anhydrite of Saudi Arabia). The Middle Khuff Member appears to consist of six depositional cycles: three below (DS P20, P23 and P27) and three above the Middle Anhydrite (DS P30, P35 and P40). The boundary between the Middle and Upper Khuff members is generally abrupt with claystones overlying carbonates. The first down-hole Permian microfauna occur within the Middle and Lower Khuff members; in contrast, microfauna is absent in the Upper Khuff Member. The Upper Khuff Member (up to 160 m, 525 ft) is Triassic in age and appears to consist of two depositional cycles (DS Tr10 and DS Tr20). The top Khuff carbonate boundary is conformable with the base of the Sudair Formation shale in north Oman; and unconformably overlain by progressively younger formations in south Oman.

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