AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Carbonate Clumped Isotope Thermometry: A Tool for Investigating Carbonate Burial Diagenesis
(1) Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA.
(2) Chevron Energy Technology Company, San Ramon, CA.
The diagenesis of carbonate sediments controls the porosity and permeability of carbonate rocks and preservation potential of primary isotopic signals. Despite its importance, the conditions and mechanisms of diagenesis have been difficult to characterize. Clumped isotope thermometry allows new insights into both the temperature and source fluids of diagenetic events recorded in ancient carbonates. This thermometer depends on the thermodynamically controlled, temperature dependent abundance of carbonate groups containing both 13C and 18O. Temperature estimates from clumped isotope thermometry are always accompanied by a measurement of carbonate δ18O, and so can be used to calculate the oxygen isotopic composition of the waters that diagenetic carbonates precipitated from or equilibrated with.
The first element of this two-part study explores how the burial and exhumation history modifies the primary distribution of O and C isotopes in carbonates from the Nafun and Ara Groups of Oman. Temperatures of samples recovered from depths of 2 to 6 km generally do not correspond to the modern geothermal temperatures (i.e., they are lower than the temperatures at which they were collected). Additionally, subsurface Nafun samples are both cooler (~40-60°C) than Ara Group samples (~60-100°C) and have similar temperatures to outcrop samples (~40-65°C) exposed on the central Oman high. Ara samples from surface-piercing salt domes are equally hot or hotter (~80-120°C) than subsurface Ara samples. These results suggest the Nafun Group preserves events that occurred early in its burial history while the Ara Group carbonates were susceptible to recrystallization at higher temperatures.
Subsurface samples from the Tengiz and Karachaganak fields in Kazakstan complement the Oman dataset because they also preserve low temperatures (35-60°C) that do not reflect the geotherm and are similar to the Nafun Group. Early results from secondary cements indicate that the temperatures and fluid compositions of these phases are distinct from the host carbonates. A calcite vein records temperatures ~20°C hotter than the host carbonate and precipitated from a fluid with a higher δ18O value. These results suggest 1) distinct temperature differences exist that are likely related to burial history and 2) co-occurring carbonates of different textural types can be shown to precipitate from fluids with distinct temperatures and isotopic compositions.