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

Despite the multiple controls on the carbon isotopic composition of organic matter, delta C-13 Values for saturated and aromatic hydrocarbon fractions for over 300 Norwegian North Sea (Central Graben) oils range from -26 to - 31 parts per thousand and from -24 to -30 parts per thousand, respectively. However, for one oil (well 2/2-5), delta C-13 values for the saturate and aromatic fractions are -21.5 parts per thousand and -22 parts per thousand, significantly enriched in C-13 relative to the isotopic compositions of al l oil fractions investigated. The saturate hydrocarbon fraction of the 2/2- 5 oil is comprised predominantly of low-molecular-weight (< C-20), primaril y regular, acyclic isoprenoids. Also present are C-36-C-40 irregular acycli c isoprenoids; these compounds are composed of Various combinations of tail -to-tail condensed C-15-C-20 regular isoprenoid subunits, resulting in pseu do-homologous series with six to nine methyl groups. The isotopic compositi ons of both high- and low-molecular-weight isoprenoids are around -19<parts per thousand>. This is in marked contrast to n-alkane delta C-13 values (c a. -31 parts per thousand), and it is clear that isoprenoid delta C-13 Valu es dictate the delta C-13 Value of the saturate fraction. The same C-13-enr iched isoprenoids are present in a thin (< 1 m) immature oil shale facies ( hudlestoni biozone) of the Kimmeridge clay formation (KCF; Dorset, UK). Mor eover, the relatively immature KCF sediment contains unique high-molecular- weight isoprenoid thiophenes and thianes, whose structures indicate that su lfur has reacted with functional groups in the isoprenoid precursor's inter iors. This suggests that these novel high-molecular-weight isoprenoids were probably biosynthesized as such. Reasons for their profound C-13-enrichmen t and predominance are discussed. Although the KCF facies, located in the s outhern 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 h as been reported in a well also from the Central Graben. (C) 2001 Elsevier Science Ltd. All rights reserved.