High-Resolution Sequence-Stratigraphic Correlation and Facies Characterization of Upper Thamama (Lower Cretaceous) Reservoir Units and Non-Reservoir Zones, Abu Dhabi, United Arab Emirates

Abstract

Abstract Platform carbonates of the Lower Cretaceous Kharaib and Lower Shuaiba formations of Abu Dhabi contain three reservoir zones (Lower Kharaib, Upper Kharaib, and Lower Shuaiba Reservoir Units) bounded and separated by low porosity and very low permeability, so-called dense zones (Lower, Middle, and Upper Dense Zones). Thirteen reservoir lithofacies and eight non-reservoir (dense) lithofacies were identified from core. Depositional environments of the three reservoir units range from lower ramp to shoal crest to near-back-shoal. The dense zones were deposited in a restricted shallow-lagoonal setting. The Kharaib and Lower Shuaiba formations are part of the late transgressive sequence set of a second-order supersequence, built by three third-order composite sequences. The three third-order composite sequences are built by twenty fourth-order parasequence sets that show predominantly aggradational and progradational stacking patterns. The parasequence sets are bounded by sequence boundaries, maximum flooding surfaces, and flooding surfaces, which define reservoir sub-units or geological model units. These units are correlatable throughout Abu Dhabi. Within the Kharaib Formation, third-order transgressive systems tracts are formed by mud-dominated, moderate to high porosity but low to moderate permeability carbonates, whereas grain-dominated, high to very high porosity and permeability carbonates occur in third-order highstand systems tracts. The Lower Shuaiba Formation shows a deepening-upward trend during the third-order transgressive systems tract. The overlaying third-order highstand systems tract is dominated by deeper marine mudstone to wackestone facies that shows a subtle shallowing-upward trend, as well as an upward increase in grain-richness to skeletal, peloid wackestone to packstone. Thickness and facies changes are minor within one field and only become obvious on a sub-regional and regional view. The established sequence stratigraphic framework and sequence stratigraphy-keyed environment of deposition maps allow a better prediction of the distribution of reservoir rocks throughout the ADCO concession area. Introduction Large hydrocarbon accumulations have been discovered and produced from platform carbonates of the Upper Thamama Kharaib and Lower Shuaiba formations in Abu Dhabi (Alsharhan 1985, 1993; Alsharhan and Nairn 1993). The Kharaib Formation (Barremian and Early Aptian) contains two reservoir units. The Lower Kharaib Reservoir Unit (LKRU) and the Upper Kharaib Reservoir Unit (UKRU) are separated and encased by three zones of very low porosity and permeability. These three dense zones are called Lower Dense Zone (LDZ), Middle Dense Zone (MDZ), and Upper Dense Zone (UDZ). Thickness of the Upper Kharaib Reservoir Unit is between 150 and 170 feet and thickness of the Lower Kharaib Reservoir Unit is approximately 80 feet (Strohmenger et al. 2004, 2006, 2008).