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7th Middle East Geosciences Conference and Exhibition
Manama, Bahrain
March 27-29, 2006
Reservoir
Characterization
of Upper
Thamama (Lower Cretaceous) Reservoirs of Onshore and Offshore Abu Dhabi, United Arab
Emirates
1 Abu Dhabi Company for Onshore Oil Operations (ADCO), Abu Dhabi, United Arab Emirates, [email protected]
2 ExxonMobil Exploration Company (EMEC), Houston, TX
Important hydrocarbon accumulations occur in Upper Thamama platform carbonates
of the Kharaib (Barremian and Early
Aptian) and Shuaiba (Aptian) formations of Abu Dhabi. These formations contain three
reservoir
units bounded by low
porosity/permeability dense zones.
Core descriptions of 25 wells were used to establish a sequence-stratigraphic framework of the Upper Thamama applicable
to reservoir
units and dense zones alike. Thirteen
reservoir
and eight non -
reservoir
(dense) lithofacies are identified from
texture, grain type, sedimentary structure, and lithology. Depositional environments of
reservoir
units range from lower ramp
to shoal crest to near-back-shoal. Dense zones (locally with features indicative of very shallow water deposition and
exposure) were deposited in a restricted shallow-lagoonal setting.
The Kharaib Formation is a second-order, late transgressive sequence set, built by several third-order composite
sequences. The Lower Shuaiba is one third-order composite sequence, deposited during a second-order transgression.
These third-order composite sequences consist of fourth-order parasequence sets. This framework provides insight into
distribution of “higher” and “lower” quality reservoir
. In the middle Kharaib
reservoir
, mud-dominated, low to moderate
porosity/permeability rocks were deposited during a third-order transgression. Higher porosity/permeability grain-dominated
rocks occur in a third-order highstand.
Thickness and facies changes are minor within one field and only become obvious with a regional view. The lower Kharaib
reservoir
unit thins by nearly one-half from west to east, most likely due to lap-out. A facies change follows this trend:
orbitolinid-rich skeletal wackestone/packstone (west) becomes bioturbated wackestone/packstone (east).
The stratigraphic framework established in this study gives insight on distribution of reservoir
rocks within the ADCO
concession. Further, a more “regional” view of these formations reveals facies, stratigraphic geometries, and thickness
variations not obvious within one field.