Stable Isotope Signatures in Carbonates as a Tool for Hydrocarbon Exploration

Anne Mueller, Department of Earth Sciences, The University of Queensland, Brisbane, Qld 4072, Australia, phone: +61 7 336 58 379, a.mueller@uq.edu.au, Helen McGregor, Research Center for Ocean Margins, Universitat Bremen, FB5 Geowissenschaften, Postfach 330440, D-28334 Bremen, Germany, Michael K. Gagan, Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia, and Janice M. Lough, Australian Institute of Marine Science, PMB 3, Townsville MC, QLD 4810, Australia.

Early marine aragonite cements are commonly precipitated from pore waters at the basal portions of coral skeletons. Inorganic Mg-calcite may also be added to the coral skeleton during early diagenesis in the marine environment. The progressive addition of early diagenetic inorganic aragonite and calcite toward the base of massive corals in Western Australia and Papua New Guinea produces an apparent increase in density and an increase in 13C. Both diagenetic aragonite and Mg-calcite precipitated in the marine environment are enriched in 13C relative to coral aragonite. A comparison of the change in density and change in 13C values in our corals leads to sound interpretations of the relation between the nature and degree of diagenesis and their effect on the 13C values. Additional consideration of the relationship between 13C and 18O signatures confirmed the above interpretations. Calcite has almost the same density as aragonite (2.71 cf 2.93 g/cm3), and thus the addition of similar amounts of secondary calcite in the Papua New Guinea coral and secondary aragonite in the Ningaloo Reef coral would produce the same density increase. An apparent increase in density of about 25% due to infilling of pores in the coral skeleton would be accompanied by a ~1.6‰ decrease in 13C in the coral affected by the precipitation of secondary inorganic aragonite as the latter is enriched in 13C, relative to coral aragonite. Inorganic Mg-calcite precipitated in equilibrium with seawater is also enriched in 13C relative to pristine coral aragonite (O'Neil et al. 1969), though not to the same extent as inorganic aragonite. Despite this, 13C would still show a decrease of ~0.7‰ towards the present.

Carbonate Reservoirs: Depositional Systems and Reservoirs
2006 AAPG International Conference and Exhibition, (November 5-8, 2006) Technical Program