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Abstract: Application of 87Sr/86Sr, ^dgr18O and ^dgr13C Isotopes to Diagenesis, Correlation and Connectivity of a Fractured Chalk Reservoir, the Sidi El Kilani Field, Tunisia

Euan W. Mearns, Mark Bramwell, John J. Mcbride

Strontium Stratigraphy Analysis of the primary matrix chalk of the Abiod Formation reservoir in the Sidi El Kilani Field indicate a Campanian to Maastrichtian age (Upper Cretaceous). A resolution of ±1Ma has been achieved and results suggest that there are no major stratigraphic breaks in the studied sequences.

Sr-O-C isotope data from early, fracture-filling calcite cements suggest they may have formed by the redistribution of CaCO3 from underlying carbonate sequences and may have precipitated at temperatures in the region of 35-55°C.

The 87Sr/86Sr isotope ratios of formation waters determined by residual salt analysis (SrRSA) suggest that the chemical evolution of waters during reservoir filling was controlled by the influx of basinal waters as opposed to in situ water-rock interaction. Late, fracture-filling dolomite and barite cements have Sr-O-C isotope characteristics consistent with precipitation from these migrating basinal fluids at temperatures similar to current reservoir conditions (70-75° C).

Sr RSA results suggest that the reservoir section in two of the wells may have been in direct lateral communication at the time of oil emplacement. These wells however are separated by a strike-slip fault. The SrRSA results therefore suggest that the fault is a partial barrier which has restricted pressure equilibration in the relatively short timescale of oil production, but which may have allowed homogenisation of Sr isotope ratios in formation water.

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