AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Late-Stage Diagenesis and Bacterial Sulfate Reduction in the 1st Eocene Reservoir, Wafra Field, Partitioned Zone, Kuwait
(1) Chevron Energy Technology Company, Houston, TX.
(2) Department of Earth Science, Cambridge University, Cambridge, United Kingdom.
The 1st Eocene reservoir at Wafra Field in the Partitioned Zone between Kuwait and Saudi Arabia is very porous dolomite with minor gypsum and anhydrite. Gypsum and anhydrite commonly occur as evaporite nodules. Some evaporite nodules have been leached forming vugs in intervals scattered from the top of the reservoir to below the oil-water contact. Vugs are partially filled with calcite cement. Minor amounts of native sulfur are also associated with the alteration of evaporite nodules. Dissolution of evaporite nodules as well as precipitation of calcite cements and sulfur are petrographically late. Isotopic analyses of the calcite cements yield δ13C values ranging from -17.1 to -34.9‰ (PDB) and δ18O values of -7.7 to -11.2‰ (PDB). The host dolomites have δ13C values that cluster between -2.5‰ and +2.5‰ (PDB) and δ18O values that cluster between -1‰ and +3‰ (PDB). The δ18O compositions of the calcite cements are consistent with precipitation from existing formation waters (-7 to -4‰, SMOW) at initial reservoir temperatures (35°C). The δ13C values are so low that they require substantial input from an organic carbon source which is apparently oil. The 1st Eocene oil at Wafra is sulfur-rich and biodegraded. Produced oils have API gravities of 17-19°. Production data indicate formation water moves through the reservoir which is currently at depths of ~1,000 to 1,300 feet below the surface. Bacterial sulfate reduction (BSR) associated with waters moving through the reservoir was apparently responsible for dissolving sulfate nodules, precipitating calcite cement and native sulfur, biodegrading oil, and contributing sulfur to the oil.