Print this pageInterpretation of Surface Sediment Methane Carbon Isotopes for Detection of Subsurface Hydrocarbons
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. Unfortunately, this assumption may not be valid for many methane isotopes collected in recent surface sediments due to (1) mixing of gases having multiple origins and (2) alteration due to secondary processes.
Examination of a relatively large data set demonstrates the strong correlation of total methane gas volume to the corresponding isotopic composition. Based upon a comparison of gas volume versus methane isotopes, three types of isotopic compositions have been defined: Type I, small volumes of methane gas with very heavy isotopic compositions; Type II, large volumes of methane gas with very light isotopic compositions; and Type A, average to above-average volumes of methane with moderate isotopic composition.
In order to assess the reliability of methane isotope values for surface prospecting, a large data set from deep core samples must be collected. A volume versus isotopic analysis used in combination with the hydrocarbon composition data can aid in determining the significance of the isotopic composition and subsequent interpretation of origin. Those samples which contain both Type A isotopic compositions and anomalous higher molecular weight hydrocarbons will provide the most reliable information on the origin of migrated gas.
AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.