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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.

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