THE SPECIAL ROLE OF STABLE ISOTOPES IN ASSESSING HYDROCARBON OCCURRENCES

Abstract

13C/12C measurements on carbonates associated with crude oils reveal that, although much of this carbonate may represent primary marine deposition, frequently the carbonate has resulted from a secondary biological oxidation of the crude oil itself. Complete utilization of the oil, major changes in the chemical composition of residual oil, or severe reduction of reservoir porosities may eventuate.

Where carbonates occur with coals, isotopic evidence shows that any primary siderite arises as a result of early fermentation processes. However, the final isotopic composition of secondary calcite appears to depend on the establishment of isotopic equilibration between carbon dioxide and methane within the coal seam. Biogenic carbonates isotopically similar to those resulting from the oxidation of crude oils have not been commonly detected in coal.

Variations in the D and 13C contents of components of plants are well established. Invariably, the lipids are most enriched in both the 'lighter' isotopes. Since the lipids are regarded as being a major source of liquid hydrocarbons, isotopic measurements promise to afford a valuable additional method for following hydrocarbon genesis. Early isotopic data from Victorian brown coals and Bass Strait crudes illustrate that isotopic differences generated during plant growth remain well preserved; however, coal extracts and crude oils appear to be isotopically dissimilar.