--> ABSTRACT: Strontium Isotopes in Oil Field Brines: Potential Fingerprints of Reservoir Heterogeneity, by R. J. Barnaby, S. C. Ruppel, W. M. Fitchen; #91020 (1995).

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Strontium Isotopes in Oil Field Brines: Potential Fingerprints of Reservoir Heterogeneity

R. J. Barnaby, S. C. Ruppel, W. M. Fitchen

Because strontium isotopes do not fractionate during geological processes, the 87Sr/86Sr ratio of oil field brines provides a natural tracer for subsurface fluid studies. The strontium isotopic composition of formation waters can be a useful tool to evaluate reservoir interconnectivity and compartmentalization and to monitor water injection. Regional investigations of subsurface fluids have used strontium isotopes to delineate hydrologic flow regimes, identify fluid sources, and assess geochemical and fluid migration history. However, the utility of strontium isotopes to resolve more detailed hydrodynamic issues within a producing oil field or reservoir has not been demonstrated.

To test the potential of this technique, we selected several, non-waterflooded oil fields in West Texas and eastern New Mexico that are producing from carbonate reservoirs. Geological characterization of the reservoirs using well log, core, and production data provided a framework for geochemical investigations. Water samples were collected from multiple wells producing from each reservoir to evaluate intrareservoir geochemical variation and the influence of reservoir heterogeneity on 87Sr/86Sr ratios.

In Martin field of the West Texas Permian Basin, six reservoirs were sampled; the Ellenburger, Wichita, lower and upper Clear Fork, Tubb, and San Andres. All formation waters are Na-Cl type brines with total dissolved solids of 40,000 to 210,000 mg/L. Strontium content ranges from 40 to 250 mg/L, and 87Sr/86Sr ratios are 0.70753 to 0.71468. Reservoir brines exhibit discrete populations on plots of 87Sr/86Sr versus total strontium, indicating that each reservoir is hydrologically isolated from the others. Brines are enriched in 87Sr relative to host carbonates; this radiogenic strontium was acquired from extraformational siliciclastics, probably during hydrocarbon migration from basinal source rocks. Ongoing studies are evaluating ore subtle intrareservoir 87Sr/86Sr variations and their relation to reservoir heterogeneity and compartmentalization.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995