Occurrence of Authigenic Pyrite in the Upper Frio Formation, Texas: Potential Impact on CO₂ Sequestration

Jeffry D. Grigsby and Kelli A. McGuire
Ball State University, Department of Geological Sciences, Muncie, IN 47306, [email protected], [email protected]

The Oligocene upper Frio Formation, located at South Liberty Oil Field, Dayton, Texas, is a poorly cemented, subangular to subrounded subarkose with a mean composition of Q₇₀F₂₄L₆. Detrital grains are dominated by monocrystalline quartz and plagioclase, with lesser amounts of K-feldspar and volcanic rock fragments. Grain-coating illite/smectite is the dominant authigenic phase, however pyrite is ubiquitous throughout the sandstone. Pyrite occurs as euhedral cubes and as framboids. In both forms, pyrite postdates illite/smectite, grows into primary pore spaces, and is also found scattered throughout depositional matrix.

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 upper Frio at Liberty field. Measured core plug mean porosity is 32% (±3) and mean permeability is 1513md (±872), making this an ideal target formation for CO₂ sequestration. Formation waters, sampled during the sequestration experiment, exhibited a rapid decrease in pH and increases in HCO3 - and dissolved metals (Ca, Fe, Mn, Zn, Pb, & Mo).

Electron probe microanalysis of pyrite in the upper Frio sandstones found Mn contents ranging from ~0-4% and averaging 1.6%, with framboidal pyrite being most enriched in Mn. It is proposed that alteration of pyrite during CO₂ sequestration resulted in the release of Mn and Fe into the formation waters, and may potentially be responsible for other metals reported such as Zn, Pb, and Mo. However, concentrations of these elements in pyrite were below the detection limits of the electron microprobe.

AAPG Search and Discovery Article #90095©2009 AAPG Eastern Section Meeting, Evansville, Indiana, September 20-22, 2009