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Abstract: Reservoir Characterisation of the (M/T) Thamama Structure (Offshore Abu Dhabi) Based on Petrography and Geochemical Analysis

C. Javaux, R. Swennen, T. H. Hassan, S. R. Azer

Oil accumulation in the (M/T) Thamama structure (offshore Abu Dhabi) occurs in nine reservoirs (from bottom to top Thamama IA, II (principal reservoir), IIIA, IIIC, IIIF, IIIG1, IVA, IVB, IVC). They are located in the upper part of the Thamama Group (Early Cretaceous).

In order to characterize the diagenetic processes leading to the present reservoir properties, results from different geochemical approaches were integrated after petrographical sample selection.

^dgr13C and ^dgr18O-isotope analysis provided data on the nature of the diagenetic fluids from which the different minerals precipitated and to some extent on precipitation temperature. Additional information was deduced from fluid inclusion analysis. This allowed to infer also the timing of the different diagenetic processes and to put them into their geological context. Combination of these data enabled to reconstruct the diagenetic fluid evolution path through time. 87Sr/86Sr analysis allowed to assess for some cements the absolute precipitation age and the involvement of radiogenic fluids.

During early diagenesis (shallow burial), mixed highly reactive fluids at the end of deposition of the Shuaiba Formation caused dissolution and recrystallisation. During the intermediate stage of diagenesis, i.e. at the end of sedimentation of the Mishrif Formation, saturated meteoric fluids were channeled along fractures and caused cementation by non-ferroan calcite at a maximum inferred burial depth of 700 m. During the late diagenesis ( > 1200m) burial fluids rich in organic acids and CO2 affected the reservoir during the middle to late Tertiary. These diagenetic fluids, which were likely expelled from the deeper basin, gave rise to cementation by ferroan calcite, saddle dolomite and minor products such as anhydrite kaolinite, etc. ...

AAPG Search and Discovery Article #90956©1995 AAPG International Convention and Exposition Meeting, Nice, France