High-resolution Sequence Stratigraphy and Reservoir Characterization of Upper Thamama (Lower Cretaceous) Reservoirs of a Giant Abu Dhabi Oil Field, United Arab Emirates

Abstract

Important hydrocarbon accumulations occur in platform carbonates of the Lower Cretaceous Kharaib (Barremian and early Aptian) and Shuaiba (Aptian) formations (upper Thamama Group) of Abu Dhabi. The Kharaib and Lower Shuaiba formations contain three reservoir units separated by three low-porosity and low-permeability dense zones. From base to top, the thickness of the reservoir intervals range from approximately 80, 170, to 55 ft (24, 51, to 16 m), respectively, for the Lower Kharaib, Upper Kharaib, and Lower Shuaiba Reservoir Units. Core and well-log data of a giant oil field of Abu Dhabi, as well as outcrop data from Wadi Rahabah in the Emirate of Ras Al-Khaimah were used to establish a sequence-stratigraphic framework and a lithofacies scheme, applicable to all three reservoir units and the three dense zones. The Lower and Upper Kharaib Reservoir Units, as well as the lower, middle, and upper dense zones are part of the late transgressive sequence set of a second-order supersequence, made up of two third-order composite sequences. The overlying Lower Shuaiba Reservoir Unit belongs to the late transgressive sequence set and the early highstand sequence set of this second-order supersequence and is made up of one third-order composite sequence. The three third-order composite sequences are composed of 19 fourth-order parasequence sets that show predominantly aggradational and progradational stacking patterns, typical of greenhouse cycles. Conventionally, composite sequence boundaries are placed at or near the base of the three dense zones. As an alternative scenario, the possibility that the major composite sequence boundaries actually occur on top of these dense zones is discussed. On the basis of faunal content, texture, sedimentary structures, and lithologic composition, 13 reservoir lithofacies and 8 nonreservoir (dense) lithofacies are identified from core. Similar lithofacies are identified in time-equivalent rock exposures studied in Wadi Rahabah. Depositional environments of reservoir units range from lower ramp to shoal crest to near-back shoal open-platform deposits. Dense zones were deposited in an inner-ramp, restricted shallow-lagoonal setting. Intensively bioturbated wackestone and packstone, and interbedded organic- and siliciclastic-rich limestone, characterize the dense zones. Locally, mud cracks, blackened grains, and rootlets are observed. Outcrop analogs of subsurface reservoirs allow for a detailed investigation of facies architecture and structure of carbonate bodies. Integration of subsurface and outcrop data (e.g., low-angle clinoforms that cannot be seen in core data) leads to more insightful and realistic geological models of subsurface stratigraphy. Geological model realizations based on core, outcrop, well-log, and seismic data constrain fluid flow-simulation models. Results mimic known behavior in analogous producing fields, and the process of going from rock data to simulation provides a useful training tool for reservoir characterization methods and techniques.