Dolomitization in the Ghawar Field: An Update Based on the Clumped Isotope Technique

Swart, Peter; Cantrell, Dave L.; Arienzo, Monica; Murray, Sean

Using the newly developed clumped isotope paleothermometer, we have determined the temperature of recrystallization and oxygen isotopic composition of the diagenetic fluids from a core taken in the Arab-D reservoir, the world's most prolific reservoir interval, in a large oil field in Saudi Arabia. These analyses show that while the dolomites and limestones throughout zones 2 and 3 of the reservoir recrystallized at temperatures of between 80 to 100oC, the carbonates in Zone 1 and at the top of Zone 2 formed at slightly lower temperatures (50-60oC). Although the δ18O of the diagenetic fluids show large variations ranging from ~ +1‰ to ~+5‰, the variations show consistent downhole changes, with the highest values being associated with the portion of the reservoir with the highest permeability and porosity. Within the samples which are only composed of calcite, there is a statistically significant trend between the δ18O of the fluid and temperature (r2=0.89). While there is a similar trend within the dolomite, the relationship has a different intercept and slope, suggesting different fluids are involved in the formation of dolomite compared to the recrystallization of the limestone and precipitation of calcite. These new data suggest a modification of the original hypothesis for the origin of dolomite which proposed dolomitization by hypersaline brines derived from overlying evaporites. Instead we propose dolomitization and recrystallization by deep-seated brines which had their δ18O elevated by water-mineral interactions at high temperatures. These fluids were mobilized during burial and permeated rocks and sediments with higher initial permeability caused either by differences in the original sediment types or changes induced during early diagenesis.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013