USING A NATURAL ANALOGUE TO EXAMINE GEOLOGIC CARBON SEQUESTRATION

Capture and storage of carbon dioxide in underground geologic formations (geologic carbon sequestration) is currently one of the most promising new technologies for reducing or mitigating anthropogenic carbon dioxide emissions. A number of scientific challenges are associated with injection and storage of large amounts of carbon dioxide in geologic formations. One of these challenges is to understand the geochemical reactions that can occur in reservoir rocks and cap-rocks among aqueous fluids, supercritical carbon dioxide and¬ other gasses. Flue gasses from coal fired power plants, for example, contain sulfur dioxide and nitrous oxide compounds in addition to carbon dioxide. Natural analogues for geologic carbon sequestration are natural systems where carbon dioxide has been stored over geologic time scales. By studying these analogues we can determine important geochemical reactions among the host and cap rocks, aqueous fluids, and stored gases. The Moxa Arch is a structure located in the southern end of the greater Green River Basin, Wyoming. Carbon dioxide and methane were emplaced in Paleozoic rocks, including the 1000 feet thick Mississippian age Madison Limestone, through natural processes. The Arch is, therefore, an excellent natural analogue for the sequestration of carbon dioxide. Concentrations of carbon dioxide in the emplaced gas in these formations vary in the region of the Moxa Arch from 66 to 95% and are as low as approximately 15% in gas producing areas outside of the Moxa Arch. Methane (22%), Nitrogen (7%), hydrogen sulfide (4.5%) and helium (0.5%) comprise the balance of the gas. Twelve samples were collected from core of three wells completed in the Madison Limestone near the southern end of the Moxa Arch. These samples are being analyzed with optical microscopy, SEM, EPMA, and x-ray diffraction techniques to understand the petrology of this natural analogue. Thus far we have determined that the Madison Limestone in these samples is actually a dolostone. Secondary dolomite, anhydrite, analcime, quartz and illite fill some of the pores, and secondary carbonate, quartz and illite occur as fracture fillings. Elemental sulfur is present as a secondary mineral in one sample. Thus the Madison Limestone serves as a natural analogue for sequestering carbon dioxide in a dolomite reservoir.