AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Geochemical Characterization of the Donovan Sand Reservoir for CO2 Sequestration, Citronelle Oil Field, Southwest Alabama
(1) West Virginia University, Morgantown, WV.
(2) University of Alabama, Tuscaloosa, AL.
Injecting supercritical CO2 fluid into a reservoir can drive porosity development and/or mineral trapping reactions that then alter the sequestration capacity of the reservoir. To address this concern, this study investigates the mineralogy and geochemical composition of the Donovan Sand, a long-lived producing hydrocarbon reservoir in the Citronelle field, southwestern Alabama. The Donovan sand is the target of on-going supercritical-CO2 injection experiment the Citronelle field. Forty-five sandstone samples were taken from 2 bore-hole cores, one from the injection well and one from an outlying well, in order to perform petrographic and geochemical characterization of the reservoir matrix. Thin-sections reveal arkosic sandstone composition. Primary framework grain types consist of monocrystalline and polycrystalline quartz and plagioclase, with muscovite as an important accessory phase. Lithic fragments are very rare, though out-sized, rounded mud/argillaceous rip-up clasts (up to pebble-sized) are common in some facies. Cement is composed of abundant calcite, with dolomite and anhydrite also present as local cements. Pore types range from interstitial porosity to moldic porosity resulting from feldspar dissolution. Porosity is highly variable throughout the reservoir, with some samples exhibiting porosity in excess of 10% whereas others exhibit no porosity. Additional core material was taken from 17 sample spots and analyzed by XRF at the Analytical Geochemistry Laboratory at the University of Alabama. Normalized average CaO and MgO abundances are 6% and 2% respectively, and show standard deviations that are 60-100% of oxide abundance. These standard deviations are elevated compared to other mobile major elements, such as K2O and Na2O. CaO and MgO show positive covariation, excepting 3 samples with elevated CaO abundances and low MgO abundances. These data suggest that overall, CaO and MgO are proxies for calcite and dolomite secondary mineral phases and that these phases are highly variable throughout the reservoir. TiO2 abundances show some positive covariation with Fe2O3 (R2= 0.4544) that may reflect presence of ilmenite alteration after hematite as a local phenomenon with minimal Fe loss. Reservoir geochemistry will be compared with post-injection formation fluid geochemistry and calibrated to reproduce geochemical interactions and multi-scale mass transfer processes operating in the reservoir.