ABSTRACT The glaciogenic Late Carboniferous-Early Permian Al Khlata Formation represents the lower unit of the Haushi Group, and is associated with the third known glacial episode to occur on the Arabian Plate (AP G3). The Al Khlata Formation outcrops in the Haushi-Huqf region, but is predominantly sampled in the subsurface of south and southcentral Oman. The subsurface Al Khlata Formation (abbreviated as AK) is divided by Petroleum Development Oman (PDO), based on palynological dating, into three ‘Production’ units (from oldest to youngest): AK P9, AK P5 and AK P1. Units AK P9, AK P5 and AK P1 respectively correspond to three broad spore and pollen assemblage zones: (1) PDO Palynozone 2159 (Late Carboniferous, late Westphalian/late Moscovian–early Stephanian/middle Kasimovian); (2) PDO Palynozone 2165 (late Stephanian/late Kasimovian to Early Permian, middle Asselian); and (3) PDO Palynozone 2141 (Early Permian, Asselian and Sakmarian). Further refinement to these palynozones have been developed locally for various Al Khlata reservoirs. The lower boundary of the Al Khlata Formation, the pre-Al Khlata unconformity, cuts Devonian (Misfar Formation) to Proterozoic formations. The upper boundary is broadly conformable with the overlying Gharif Formation (Depositional Sequence Arabian Plate Permian 10, AP DS P10). The boundary is generally picked at the base of the lowermost Gharif sandstone overlying the Rahab Member of the Al Khlata Formation. In northcentral Oman, the Rahab Member is not clearly developed and correlation of the boundary interval can only be achieved through palynological age dating. The Al Khlata Formation was deposited in proglacial environments during deglaciation phases. It comprises glacio-fluvial, glacio-deltaic and glacio-lacustrine lithofacies associations with a wide range of thicknesses (total Al Khlata thickness: 100–800 m or 328–2,624 ft). Glacio-lacustrine and glacio-deltaic deposits are volumetrically dominant. The spatial extent of these heterogeneous deposits is poorly constrained due to sparse well control, especially beyond southcentral Oman. Four Al Khlata Depositional Sequences (DS) are considered to be regionally mappable in Oman’s subsurface and are numbered according to the Arabian Plate Sequence Stratigraphic framework: Carboniferous 30 (AP DS C30), Carboniferous-Permian (AP DS CP), Permian 6 (AP DS P6) and Permian 8 (AP DS P8). The AP G3 ice age affected the whole southern hemisphere and lasted several tens of millions of years. The sequences of the Al Khlata Formation represent a selection of deposits from several glaciations and warm interglacial periods in Oman. Only a fraction of the sediments, originally deposited during a single glacial advance and meltdown, are preserved.

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.