Inferences for sources of oils from the Norwegian Barents Sea using statistical analysis of biomarkers

Abstract

Conventional biomarker age indicators may not always give conclusive inferences for the age of an oil’s source rock, particularly with regard to condensates, in which most of the key biomarkers are often too sparse to determine. It is possible that other compounds, reflecting changes in primary producer assemblages or depositional environment, can provide age indications in basins that have experienced a progressive change in conditions. The Norwegian Barents Sea provides an opportunity to evaluate this approach, although the gradual changes are punctuated by intervals of stratification, OAEs and extinctions. Partial least squares analysis was used to identify a number of apparently suitable parameters, to establish an age calibration from source rock extracts and then to apply it to a variety of oils and condensates from the Barents Sea. The resulting source age inferences are consistent with conventional age parameters for the normal oils, providing a degree of confidence in their applicability to the problematic condensates. In the main part of the Hammerfest Basin, oils of Jurassic origin dominate, whereas contributions from Triassic sources are suggested towards the margins, particularly in pre-Jurassic reservoirs. To the east, on the Norsel High and in the Nordkapp Basin, the main oil sources are suggested to be Early Triassic, although a Palaeozoic, possibly Permian, source is likely on the margin of the Finnmark Platform in well 7128/4-1. Among the novel parameters, the relative abundances of phytanyl arenes and diadrimanes appear to be controlled by diagenetic conditions.