From Bureau of Economic Geology, The
University of Texas at Austin (www.beg.utexas.edu).
For more information, please contact the author.
Bureau Seminar, February 26, 2010
A Window to the Earth's Mantle and Type Example of a Subsurface CO2 Deposit losing Gas to Water at the Down Dip Gas/Water Contact: Bravo Dome CO2 Gas Field, New Mexico, USA.
Link to streaming video: available 02.26.2010 at 8:25am
Dr. Martin M. Cassidy
Dept. of Earth and Atmospheric Sciences, University of Houston
Dr. Martin M. Cassidy1, Dr. Chris Ballentine2, Dr. Sherwood Lolar3 and Dr. James Lawrence1.
1. Dept. of Earth and Atmospheric Sciences, 312 Science and Research Bldg. 1, University of Houston, Houston, Texas 77204-5007
2. School of Earth, Atmospheric and Environmental Sciences, University of Manchester, Oxford Road, Manchester M13 9PL. UK
3. Dept. of Geology, University of Toronto, 22 Russell Street, Toronto, Ontario M5S 3B1, Canada.
Bravo Dome CO2 gas field is a gas deposit associated with basaltic lavas in an extensional regime. The CO2 field is a structural-stratigraphic trap containing about 10TCF of 99.7% CO2 gas at a depth of 1700 to 2500 ft, 520 to 760 meters, in the Permian Tubb sandstones of 0 to 55 meters thickness on granitic basement. The reservoir is sealed above by 3 to 30 meters of massive anhydrite. The field is about 40 miles east west and 30 miles north south. The Tubb sandstone thins to zero up dip in the West and thickens downdip to the East. CO2 appears to have entered the field from below in the West and drove pore water down dip to the East. CO2 is not escaping upward but rather is escaping down dip into the water at the gas water contact. Evidence of this is the 1000 psi bottom hole pressure in the west and 600 psi bottom hole pressure at the gas water contact in the East. Compositional and isotopic analysis of CO2 and co-produced noble gases from 14 wells across Bravo Dome field were sampled with great care to avoid air contamination. Noble gases 3He, 4He, 20Ne, Ne, 36 Ar, 40Ar, and 84Kr contents range systematically across the field. In the western portion of trap, far from the gas-water contact and over impermeable granite basement, helium in the CO2 has a ratio of 3He/4He/Ra (ratio air) as high as 4.26, a mantle signature, and concentrations of other noble gases approach mantle values. Other evidences that the CO2 is of magmatic origin are the CO2 /3He ratio of 2.75 x 109 to 5.32 x 109, and ?13C values of –3.7 to -5.1 PDB. The original noble gas of mantle composition is nearly preserved in the West. In the east of the field the CO2 is contaminated with atmospheric and crustal noble gases exsolved from the water. Noble gases are also concentrated by dissolution of the CO2 into the water, leaving behind the noble gases. At the gas water contact almost 50% of the CO2 originally present in the reservoir has entered the water column. The CO2 is escaping from the deposit, not by leaking through seals, but by moving away in the down dip water.