A NATURAL ANALOGUE IN WYOMING FOR GEOLOGIC CO-SEQUESTRATION

Geologic sequestration of combustion products from coal-fired power plants will be a critical component of carbon capture and storage (CCS). Coal combustion generates SO_x, NO_x, and other constituents in addition to CO₂, thus geologic sequestration of multi-component (CO₂-SO_x-NO_x) gasses (co-sequestration) will be required. Natural analogues provide a means of understanding and predicting behavior in carbon repositories.

The Moxa Arch is a major north-south trending structure located in southwestern Wyoming. CO₂ and CH₄ were emplaced in Paleozoic rocks, including the 1000 feet thick Mississippian Madison Limestone, through natural processes. Concentrations of CO₂ in these formations ranges from 66 to 95% and are as low as 15% in gas producing areas outside the Moxa. Temperature and pressure are high enough for the CO₂ to be supercritical. CH₄ (22%), N₂ (7%), H₂S (4.5%) and He₂ (0.5%) comprise the balance of the gas. Twelve samples were collected from core of three wells completed in the Madison Limestone across the southern end of the Moxa Arch. These samples are being analyzed with standard techniques to understand the mineralogy and petrology of this natural analogue. The Madison Limestone in these samples is actually a dolostone. Secondary dolomite, quartz and illite fill some of the pores, and secondary carbonate, quartz and illite occur as fracture fillings. In many samples there appears to be multiple generations of carbonates, and the recrystallized edges of dolomite grains lining pores exhibit alternating bands of iron-rich and iron-poor dolomite. Analcime, not dawsonite, is identified in core from all three wells. Secondary anhydrite is prevalent as pore fillings and native sulfur is present in one sample. Co-sequestration (CO₂ + SO₂) may produce geochemical reactions yielding a similar sulfur-bearing mineral assemblage and gas composition. Thus the Madison Limestone serves as a natural analogue for co-sequestering CO₂ and SO₂ in a carbonate reservoir.