Compound specific isotope correlation (CSIC) using gasoline range hydrocarbons : a novel approach for petroleum exploration

Date

1999

Authors

Harris, Scott Allan

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Abstract

Most oils and condensates contain gasoline-range hydrocarbons (Cs-C10) that potentially can be used as geochemical indicators or signals. The abundance and mobility of this hydrocarbon fraction is particularly useful for reservoir and oil-oil correlation. Previous correlation techniques based on biomarkers, structural or molecular composition of oils sometimes fail because of alterations to the oils at high maturities (condensates), bi ode gradation or oxidation. Compound Specific Isotope Correlation (CSIC) uses the stable carbon isotope ratios of individual hydrocarbons in the gasoline range fraction as a diagnostic fingerprint that can be used to correlate oils to each other or to potential sources. CSIC takes advantage of an improved method developed for molecular and stable isotope analysis of gasoline range hydrocarbons using Solid Phase MicroExtraction (SPME). Analytes are sampled from the headspace above oil droplets and analyzed by gas chromatography and continuous-flow isotope ratio mass spectrometry (CF-IRMS). Analytical considerations with this sampling technique include equilibration times, vapor pressure/temperature effects, and the presence of a complex matrix. Instrumentation factors such as column/split flows are also considered. The results are compared with an alternative purge and trap method. Molecular analyses prove reproducible between the two methods, however, stable carbon isotope ratios exhibit a non-systematic depletion and enrichment of 0.2 - 2.0%0 for 16 pre-selected compounds. A sample suite of 27 oils primarily of Devonian age from the Western Canada Sedimentary Basin tests both the technology and technique of CSIC. Oils are correlated to each other using hierarchical cluster analysis of 813C signatures for 15 gasoline range compounds, taking into consideration the geologic setting where reservoirs are located. From this process, 6 groups of oils were generated and most can be attributed to a common source rock, the Upper Devonian Duvernay formation. It is unclear if Nisku formation source beds have contributed partially or completely to some of these oils. Comparison of isotope ratios from different compound classes in the gasoline range may provide information on the origin of oils and light hydrocarbons. Straight­ chain and branched alkanes display both a molecular and isotopic correlation to each other, while cyclic and aromatic compounds show more variable results. The data in this study support a steady-state or kinetic mechanism for oil generation, in contrast to previous thermodynamic models. Systematic isotopic offsets of individual compounds with similar carbon number or chemical structure also indicate some form of source control on isotopic composition of oils. LMW compounds are generally enriched in 13C relative to heavier compounds. Thermochemical Sulfate Reduction (TSR) appears to result in pronounced 13C enrichment for several compounds relative to unaltered samples.

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