ST. JOHNS CO2 GAS FIELD, AN EXAMPLE OF A LEAKING DEPOSIT

On the western border of south central New Mexico and into Arizona, USA is St. Johns CO₂ gas field of 2.4 trillion cubic feet gas in place. This deposit is close to the Pleistocene to recent age Springerville basalt volcanic field that sourced some of the mantle CO₂ (as evidenced by ³He/4He Ra of 0.424 to 0.631). This volcanic field is on the Jemez Lineament, a NE-SW trending lineament more than 300 km in length along which are several volcanic fields and CO₂ deposits At the North east end is the Tertiary age Raton basaltic Volcanic field. There is found the Bravo Dome CO₂ field that has been discussed earlier in this oral session.

However, unlike the well-sealed Bravo Dome field slowly losing CO₂ to down dip water, much of St Johns field CO₂ has leaked to the surface. The evidence for this is important.

St. Johns field is an anticline about 33 miles long NW-SE and 20 miles NE-SW,

sealed on the Northwest by a normal fault down to the west. The CO₂ is trapped in Permian sandstones and limestones in three vertically separate reservoirs. The most porous reservoir had a bottom hole pressure of approximately 400 PSI at a reservoir depth of 4815 feet below surface. The reservoir is severely under pressured by about 2000 PSI. That pressure has been lost as the CO₂ dissolves in water at the bounding fault and flowed to the surface. The percentage of helium measured as 0.64% in the same well was reported by a previous operator to increase to 1.6% in the north of the field. The helium has low solubility in water concentrating in the remaining CO by partitioning into the CO₂. Discussion of the use of noble gases to aid in defining the history of the field will be presented.

The most spectacular evidence of loss of CO₂ to the surface is extensive Travertine deposits. Numerous travertine mounds of calcium calcite and aragonite cover an area of more than 300 square sq. km overlying the west edge of the field in the vicinity of Lyman Lake. These will be illustrated and compared to the Travertine of the Crystal Geyser (an active cold CO₂ Geyser) of Utah.

In summary: natural deposits of CO₂ can be ephemeral and of varying life span in the subsurface. One model does not fit all.