Carbon isotopic composition of an isoprenoid-rich oil and its potential source rock

Abstract

Despite the multiple controls on the carbon isotopic composition of organic matter, δ13C values for saturated and aromatic hydrocarbon fractions for over 300 Norwegian North Sea (Central Graben) oils range from −26 to −31‰ and from −24 to −30‰, respectively. However, for one oil (well 2/2-5), δ13C values for the saturate and aromatic fractions are −21.5‰ and −22‰, significantly enriched in 13C relative to the isotopic compositions of all oil fractions investigated. The saturate hydrocarbon fraction of the 2/2-5 oil is comprised predominantly of low-molecular-weight (<C20), primarily regular, acyclic isoprenoids. Also present are C36–C40 irregular acyclic isoprenoids; these compounds are composed of various combinations of tail-to-tail condensed C15–C20 regular isoprenoid subunits, resulting in pseudo-homologous series with six to nine methyl groups. The isotopic compositions of both high- and low-molecular-weight isoprenoids are around −19‰. This is in marked contrast to n-alkane δ13C values (ca. −31‰), and it is clear that isoprenoid δ13C values dictate the δ13C value of the saturate fraction. The same 13C-enriched isoprenoids are present in a thin (<1 m) immature oil shale facies (hudlestoni biozone) of the Kimmeridge clay formation (KCF; Dorset, UK). Moreover, the relatively immature KCF sediment contains unique high-molecular-weight isoprenoid thiophenes and thianes, whose structures indicate that sulfur has reacted with functional groups in the isoprenoid precursor's interiors. This suggests that these novel high-molecular-weight isoprenoids were probably biosynthesized as such. Reasons for their profound 13C-enrichment and predominance are discussed. Although the KCF facies, located in the southern UK, cannot be a direct source of the North Sea 2/2-5 oil, a related North Sea facies seems plausible since an isotopically enriched TOC zone has been reported in a well also from the Central Graben.