Origin of Layering in a Giant Thamama Reservoir, Offshore Abu Dhabi

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.