The Khuff foreshoal of Interior Oman

The Permian Khuff Formation crops out in the Haushi-Huqf area of Interior Oman. It represents the foreshoal depositional environment of the Arabian Khuff platform. To understand the architecture of the Khuff Formation subsurface hydrocarbon reservoir based on data from the outcrop scale, we present a regional correlation of four sections of the 30 to 40 m thick Khuff Formation, based on facies and biostratigraphy. We also investigate the properties of its three informal members, deposited in different environments. These vary from tidal sand-flat to barrier-beach deposits (variably reworked by waves and tidal currents) to a carbonate shelf environment in the first member and then to deeper, storm-dominated settings in members 2 and 3. These latter units consist of white to grey marlstones and bioclastic limestones (representing proximal and distal tempestites) rich in brachiopods, conodonts, foraminifers, algae, ammonoids, trilobites, gastropods, bivalves, scaphopodes, ostracodes, crinoids, corals, bryozoans and fish remains. Member 2 records outer shelf conditions below the storm wave base and it is followed by a return to depths around and above the storm wave base in the third member, with wave-reworked deposits in the topmost few metres. Vertical facies successions indicate large-scale deepening and shallowing trends. Lateral and vertical facies changes indicate that the Khuff Formation is heterogeneous in the distribution of grain-supported versus muddy beds. Microfacies and porosity analyses indicate that mudstones and packstones are prevalent, whereas grainstones are present only locally. The grain-supported bioclastic deposits occur in all the three units, without significant differences in composition and texture. Creation and modification of porosity is mainly related to primary (depositional) and eogenetic (early post-depositional) stages. Mudstone is characterised by a reduced effective porosity; packstone shows both intergranular and intragranular pore spaces filled by mud; grainstone shows intergranular and intragranular pore space filled by cements. These appear black under cathodoluminscence, supporting an early marine origin. Secondary porosity is extremely reduced or absent. No evidence of dissolution (moldic porosity) has been observed. Porosity is mainly fabric-selective. Non-fabric selective porosity (fractures) is generally rare. Most of the recorded brachiopod, conodont and foraminifer taxa range from the Roadian to the Capitanian, with conodonts suggesting a Wordian-Capitanian age and foraminifers extending also into the Late Permian. Strontium data seem to be more consistent with an early to middle Guadalupian age. Palynomorphs from the basal Khuff clastics, a few metres below the base of the Khuff Formation, yield well-preserved palynomorphs indicating the Arabian palynological Biozone OSPZ6, thus supporting a Wordian-Capitanian age. The Khuff Formation records a major transgression that is recognised all along the Neotethyan shores in Guadalupian (probably Wordian) times and coincides with the final drowning of Neotethys rift shoulders and with global climate warming. This transgression was probably diachronous, younging to the north and west, as suggested by the palynological composition of the basal Khuff clastics across Pakistan, Oman, Saudi Arabia and Jordan.