Source and Thermal Maturity of Bitumen in Precambrian Nonesuch Formation Inferred from Isotopic Compositions of Individual Normal Alkanes

HIESHIMA, G. B., Indiana University, Bloomington, IN

As part of a continuing study of organic geochemistry of the 1.05 Ga Nonesuch Formation in the North American Mid-Continent rift, isotopic compositions of individual normal alkanes were measured using compound-specific isotopic analysis. This is a relatively new technique that can be used in conjunction with traditional biomarker analysis in interpretation of thermal maturity, oil-source rock correlation, and paleoecologic reconstruction.

Carbon-isotopic compositions of individual normal alkanes from C(15) to C(25) in five bitumens and one oil from Nonesuch strata vary from -28.8 to -34.5% (versus PDB). Within a given sample, isotopic compositions of n-alkanes span a range of 2 to 4%. Homologues from C(15) to C(19) are isotopically light (depleted) by approximately 0.50 relative to higher homologues from C(20) to C(25). Two sources of normal alkanes are interpreted from the distribution of isotopic compositions. Based on chain length, the series from C(15) to C(19) may have been derived from algal components. Isotopically heavy, higher homologues are suspected to have been derived from bacterial sources.

The pattern of isotopic compositions of bitumens is similar to that of the oil, demonstrating an oil-source rock relationship.Superimposed upon this primary, bimodal distribution of normal alkanes is the influence of thermal maturity on isotopic composition. Normal alkanes in samples of higher thermal maturity are enriched isotopically by 0.25 to 2% relative to those of lower thermal maturity. Isotopic enrichment with increasing thermal maturity is interpreted to be the result of either addition of isotopically heavy components derived from kerogen and/or preferential loss through cracking of isotopically light components in bitumen.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)