Using the clumped isotope method, the temperature of dolomite and calcite formation and the oxygen isotopic composition (σ18Ow) of the diagenetic fluids have been determined in a core taken from the Arab-D of the Ghawar field, the largest oil reservoir in the world. These analyses show that while the dolomites and limestones throughout the major zones of the reservoir recrystallized at temperatures between ca 80°C and 100°C, the carbonates near the top of the reservoir formed at significantly lower temperatures (20 to 30°C). Although the d18O values of the diagenetic fluids show large variations ranging from ca <0% to ca +8%, the variations exhibit consistent downhole changes, with the highest values being associated with the portion of the reservoir with the highest permeability and porosity. Within the limestones, dolomites and dolomites associated with the zone of high permeability, there are statistically significant different trends between the σ18Ow values and recrystallization temperature. These relationships have different intercepts suggesting that fluids with varying σ18Ow values were involved in the formation of dolomite and limestone compared to the formation of dolomite associated with the zone of high permeability. These new data obtained using the clumped isotope technique show how dolomitization and recrystallization by deep-seated brines with elevated σ18Ow values influence the σ18Ow values of carbonates, possibly leading to erroneous interpretations unless temperatures can be adequately constrained.
- Clumped isotopes
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