EFFECT OF MATURITY ON C-13 C-12 RATIOS OF INDIVIDUAL COMPOUNDS IN NORTH-SEA OILS

Carbon isotope variations in individual compounds are reported for North Sea oils of variable thermal maturity. Using the pristane/phytane ratio of the oils as an indication of source facies variation, and molecular maturity ratios to determine maturity, it is possible to differentiate the two effects. In these samples, maturity accounts for between 50 and 90% of the observed isotopic variation, all values becoming heavier with increasing maturity, typically by between 2 and 3 parts per thousand. However, source differences are also apparent with a terrestrially influenced source, possibly the Heather Formation, contributing material to the Kimmeridge Clay sourced oils. The ''Heather'' contribution is isotopically heavier than the Kimmeridge oil for most compounds, but is isotopically lighter for the isoprenoid hydrocarbons and some other branched compounds. It is also more important for lower molecular weight hydrocarbons than for higher homologues.