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Detection of Hydrocarbon Venting on the Gulf of Mexico Sea Floor from Determinations of d13C and DIC of the Water Column Overlying Seeps

AHARON, PAUL, ELLEN R. GRABER, and HARRY H. ROBERTS, Louisiana State University, Baton Rouge, LA

The selective nature of hydrocarbon accumulation in the sediments of the Gulf of Mexico slope coupled with the high costs of offshore drilling demand development of unconventional exploration techniques ahead of the drill bit. Evidence of oil and gas seeps is important in an exploration program because their presence implies the existence in the subsurface of source rocks, traps, and adequate reservoir porosity and permeability. Here we present results of a recent investigation which demonstrate the usefulness of carbon isotopes of the total dissolved inorganic carbon (DIC) in the water column to detect active hydrocarbon venting on the sea floor.

Samples from the water column above active vent sites and nonvent locations were taken with the submersibles Johnson-Sea-Link and Alvin in 1989 and 1990, respectively, during our survey of hydrocarbon vents on the northwestern Gulf of Mexico slope. In nonvent locations, d13C of the water column shows a typical "nutrient" profile with values ranging from 0.4 to 0.9(sigma) PDB, and DIC values of 2.5 to 2.7 mmole/L. Water column above active hydrocarbon vents shows anomalously negative d13C values of up to -4.5(sigma) PDB and increased levels of DIC of up to 3.1 mmole/L. The DIC enrichments coupled with the observed 13C depletions are attributed to the release of CO2 in the water column from bacterial oxidation of hydrocarbons in the seep environment. In the laboratory we have developed mass-spectrometry technique for routine determinations of both DIC and d13C of 2 mL and smaller water samples. The technique is fast (30 min/sample), reproducible (DIC = +/-2%; d13C = +/-1(sigma)), and accurate, and can be used routinely by any isotope laboratory. The isotope method outlined here can supplement organic geochemistry techniques used by the exploration companies for detection of active hydrocarbon venting on the sea floor.

 

AAPG Search and Discovery Article #91006 © 1991 GCAGS and GC-SEPM Meeting, Houston, Texas, October 16-18, 1991 (2009)