Carbon isotope geochemistry of hydrocarbons in the Cerro Prieto geothermal field, Baja California Norte, Mexico.
Hydrocarbon abundances and stable-isotopic compositions were measured in wells M5, M26, M35 and M102, which represent a range of depths (1270-2000 m) and temperatures (275-330 degrees C) in the field.  In order to simulate the production of the geothermal hydrocarbons, gases were collected from the pyrolysis of lignite in the laboratory.  This lignite was obtained from a well which sampled rock strata which are identical to those occurring in the field, but which have experienced much lower subsurface temperatures.  In both the well and the laboratory observations, high-temperature environments favored higher relative concentrations of methane, ethane and benzene and generally higher delta 13C-values in the individual hydrocarbons.  The best correlation between the laboratory and well data is obtained when laboratory-produced gases from experiments conducted at lower (400 degrees C) and higher (600 degrees C) temperatures are mixed.  This improved correlation suggests that the wells are sampling hydrocarbons produced from a spectrum of depths and temperatures in the sediments.
Des Marais
D J
DJ
Ames Research Center, Moffett Field, CA 94035, USA.
Stallard
M L
ML
Nehring
N L
NL
Truesdell
A H
AH
D002245:Carbon Dioxide / Q000302:isolation & purification; D002247:Carbon Isotopes; D005579:Fossil Fuels; D005740:Gases / Q000032:analysis / Q000737:chemistry; D019015:Geologic Sediments / Q000737:chemistry*; D005844:Geology / Q000379:methods*; D006838:Hydrocarbons / Q000032:analysis / Q000737:chemistry*; D008800:Mexico; D013696:Temperature; D013816:Thermodynamics
D002247:Carbon Isotopes; D005579:Fossil Fuels; D005740:Gases; D006838:Hydrocarbons; D002245:Carbon Dioxide