Origin of
Layering in a Giant Thamama Reservoir, Offshore
Wood, Rachel A.1, J.Anthony D.
Dickson2, Hamad Bu Al Rougha3, Hesham Shebl4
(1) Edinburgh University, Edinburgh, United Kingdom (2) Cambridge University,
Cambridge, United Kingdom (3) Zadco, Abu Dhabi, United Arab Emirates (4) Zakum
Development Company (ZADCO), Abu Dhabi, United Arab Emirates
The low energy ramp sediments of the
Thamama Group often a prominent alternation of low-porosity (dense) and
high–porosity (reservoir) layers, currently assumed to follow depositional
cycles. Analysis of core from a major offshore reservoir, Abu-Dhabi, shows,
however, that the boundaries of such layers cross-cut depositional facies and
are therefore in part diagenetically mediated. Most noteworthy is that dense
layers are thicker in the reservoir crest than flanks independent of any
apparent changes in facies distribution.
Prominent and laterally extensive
hardgrounds are commonly developed within the dense layers, and these show an
irregular distribution of 13C-depleted values interpreted to be
indicative of sulphate reduction, developed subsequent to hardground formation
and burial. Further mapping of d13C and d 18O data from calcite
macrocements zones (>10mm) as well as bulk micrite show dense layers to have
d 18O values ranging from -
2.6‰ to – 5.4‰, whereas high-porosity reservoir layers show d 18O values up to - 9.7‰.
Low-porosity dense layers were therefore cemented early while high-porosity
reservoir layers contain only late burial macrocements. These results therefore
reverse current understanding that Thamama reservoirs were sealed from late
cementation by oil charge.
AAPG Search and Discover Article #90063©2007 AAPG Annual Convention, Long Beach, California