--> Abstract: Lithofacies Attributes, Reservoir Qualities and Depositional Setting of the Oligocene Carbonates, Al-Khod Area, Oman, by Abdulrahman Al-Harthy, Osman S. Hersi, and Abdul Razak Al-Sayigh; #90077 (2008)

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Lithofacies Attributes, Reservoir Qualities and Depositional Setting of the Oligocene Carbonates, Al-Khod Area, Oman

Abdulrahman Al-Harthy*, Osman S. Hersi, and Abdul Razak Al-Sayigh
Sultan Qaboos University, Oman
*[email protected]

The Oligocene strata of the Al-Khod area consist of cyclic sedimentation of basal siliciclastic-dominated units that grade upwards to (locally sandy to pure) carbonates. The siliciclastic units (less than 5% of each cycle) vary from poorly-cemented granular conglomerates to coarse-grained sandstone lithofacies (Lf1). The dominant carbonate portion of each cycle is divisible into two parts: a lower biostromal (c. 20–25% of each cycle) and an upper biohermal coral reef facies (c. 70–75%). The biostromes are characterized by inter-beds of laterally discontinuous to continuous lithofacies of algal-foraminiferal packstone/grainstone (Lf2), algae-dominated, meter-scale biohermal mounds (Lf3) and peloidal wackestone to floatstone with large Nummulites and Assilina shells (Lf4). Large, partially to completely leached-out mollusks are present throughout the biostromal lithofacies. The biohermal coral reef facies (Lf5) is represented by massive coral framestones (e.g. Stylopora, Poccillopora, Platygyra, Lobophyllia and Galaxea) with subordinate, localized algal mounds. The stacking nature of the strata suggests intermittent basin subsidence with hinterland uplifting. The latter acted as loci of short-term siliciclastic provenance, whereas the former promoted creation of accommodation space for the prevailing carbonate lithofacies, which evolved from initial stages of stabilization and colonization (biostromal facies) to more developed stages of diversification and domination (biohermal coral reef facies). The diagenetic features of the carbonate facies include marine and shallow burial calcite cementation, micro- and megaquartz cements, meteoric dissolution and localized dolomitization. Amounts of preserved pore spaces vary among the various lithofacies and include large (cm-scale) vuggy (mainly due to mollusk and coral dissolution), inter- and intra-granular and fracture porosity. The coral framestones show the best reservoir qualities with porosity as high as 15–20% and thus suggest the existence of similar porous reefs offshore the Al-Batinah Coast of northeastern Oman.

 

AAPG Search and Discovery Article #90077©2008 GEO 2008 Middle East Conference and Exhibition, Manama, Bahrain