Correlation of Natural Gas by Use of Carbon Isotopic Distribution Between Hydrocarbon Components

Abstract

The natural distribution of carbon isotopes between hydrocarbon gas components is used for (1) determining a gas's maturity, (2) correlating a reservoired gas to its source, (3) correlating one reservoired gas with another, and (4) recognizing gas mixtures. Calculated separations of carbon isotopes between the normal alkane components of a natural gas have been related to source rock maturity by use of a single, continuous diagram, independent of source type. Actual data from a wide variety of geologic settings and geologic ages confirm this relationship and demonstrate its applicability to the source rock Levels of Organic Metamorphism ranging from 8 to 13, covering the entire range of oil and wet-gas generation. At greater maturities, the wet-gas components are found to undergo thermal degradation, losing their usefulness for correlation. Three examples showing indigenous gas (west Texas), non-indigenous gas (Gippsland basin, Australia), and gas mixtures from multiple sources (southeastern Alberta) illustrate exploration applications.