Petrographic and Geochemical Characterization of the Frio Formation, Texas: Implications of CO2 Sequestration
The Gulf Coast Carbon Center (GCCC), a branch of the Bureau of Economic Geology of the University of Texas at Austin, has conducted CO2 sequestration experiments in the Oligocene, Frio Formation at the South Liberty Oil Field, Dayton, Texas. Petrographic examination of core samples from the Frio “C” and “B” sandstones, ranging in depth from 1500m-1657m, classifies the sandstones as poorly cemented, subangular to subrounded, subarkoses with mean composition of Q70F24L6. Detrital grains are dominated by quartz, plagioclase, K-feldspar, and volcanic rock fragments. Matrix increases with depth. Measured core plug mean porosity is 32% (±3) and mean permeability is 1513md (±872). Point count porosity, dominated by primary intergranular porosity, is 24% (±1). Formation waters, sampled during the sequestration experiment, exhibited a rapid decrease in pH and increases in alkalinity and dissolved metals (Ca, Fe, Mn, Zn, Pb, & Mo). In an effort to identify the source of ions in solution, XRD and SEM analyses were completed. XRD and SEM analyses identify smectite/illite clay coats with rare amounts of kaolinite and calcite. SEM with EDAX analyses identified authigenic pyrite as framboids and isolated octahedra and lesser amounts of quartz overgrowths, feldspar overgrowths, and barite. Dissolution of feldspar is also observed. EDAX analysis of clay grain coats identifies Fe, Si, O, Al, K, Na, and Mg and BSE identifies pyrites (≤ 1µm) intergrown with the clays. Rock-water interaction experiments will be conducted, at surface temperature and pressure, to identify ions in solution as a result of decreasing pH. These data are essential in understanding the chemical changes occurring in the formation and assisting in a model simulation of the Frio sandstone’s chemical reactive properties, all in response to increased CO2 concentrations. This research supports the GCCC’s CO2 sequestration efforts, assessing the Frio Formation as a repository for anthropogenic CO2, and ultimately, atmospheric CO2 reduction.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009