Relating Chemical Speciation of Pore Fluids to Initial Seawater Chemistry and to Diagenetic Reaction Pathways: SrRSA Data Compilation, Gulf of Mexico, U.S.A.

Abstract

The oil and gas fields of the deepwater Gulf of Mexico (GoM) vary in both size and extent as well as the level of connection to regional aquifer systems. Providing reasonable aquifer constraints and compartmentalization assessments to management is important to development planning and can be challenging when penetrations are limited to the oil column. Here we present new strontium isotope records from Residual Salt Analysis (Sr-RSA), from multiple Miocene-aged reservoirs located in the Green Canyon and Mississippi Canyon protraction areas of the GoM. Collected from residual salts of core samples, these samples provide strontium ratios characteristic of residual pore water in the reservoir compartment where samples were collected. Analyses in eighteen discrete reservoirs from twelve fields yield information about compartmentalization at both local and regional scales. Samples from the Green Canyon area have a relatively small range of variation (0.70762 to 0.71026), close to the expected ratios of Miocene seawater. In contrast, a much wider range of variation is observed in samples from the Mississippi Canyon area (0.70811 to 0.71369).

Initial pore-water strontium isotope ratios should reflect the strontium composition of seawater at the time of sand deposition. Within the reservoir, pore-water strontium ratios can evolve over time through interaction with strontium bearing minerals. Two factors are proposed as the dominant influences on the magnitude of change over time: regionally connected aquifer volume and reservoir temperature. At low temperature, processes such as mineral leaching contribute relatively small amounts of additional strontium to porewater over time. At temperatures exceeding 105°C, however, diagenesis-induced reactions with rubidium-rich minerals such as clays, feldspars, and micas introduce highly radiogenic strontium. Large scale, widespread sands (e.g., Green Canyon basin floor fans) with large regional aquifer volumes are effectively buffered against this effect, therefore initial pore fluid compositions remain close to those of Miocene seawater. In restricted mini-basins (e.g., Mississippi Canyon slope settings, with small lobes and aprons) or areas of structural compartmentalization pore-waters are more likely to become enriched in ⁸⁷Sr in reservoirs affected by diagenesis, producing a strong correlation between ⁸⁷Sr/⁸⁶Sr and temperature.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018