Published 1996 as part of Memoir 66
Copyright © 1996 The American Association of Petroleum Geologists.   All Rights Reserved.
 

In surface prospecting for hydrocarbons, it is often assumed that the carbon isotopic composition of methane can provide information on the origin of the gas. This assumption may not be valid for methane carbon isotopes extracted from surficial marine sediments because of (1) mixing of gases with multiple origins and (2) alteration resulting from secondary processes. Examination of a relatively large data set reveals the correlation of methane concentrations with carbon isotopic compositions. Three groups have been defined based on a comparison of molecular concentrations to methane carbon isotopes: type I--small concentrations of methane with heavy isotopic compositions; type II--large concentrations of methane with very light isotopic compositions; and type A--average to above average concentrations of methane with intermediate isotopic compositions.

A large data set from deep core samples must be collected to assess accurately the reliability of methane carbon isotope values to deduce the origin of gas contained in surface sediments. A plot of methane concentration versus d¹³C is used in combination with the hydrocarbon composition data to help determine the significance of the isotopic composition and subsequent interpretation of origin. Samples that contain both type A isotopic compositions and anomalous higher molecular weight hydrocarbons provide the most reliable information on the origin of migrated gas.