THE ORIGIN OF BARROW SUB-BASIN CRUDE OILS: A GEOCHEMICAL CORRELATION USING LAND-PLANT BIOMARKERS

Abstract

New biomarker technology has been developed to provide insight into the age range of the source rocks of crude oils and condensates from the Barrow Sub-basin. These methods are based on a biomarker or chemical fossil fingerprint provided by the land-plants that were present at the time of deposition of the source rocks. The changing distribution of plants through time, in response to factors such as climate change, provides a time-based fingerprint that can be compared with that from the crude oil. The plant fingerprint was established using samples from the Koolinda-1 exploration well which range in age from Middle to Upper Jurassic. The distributions of the key marker compounds were measured in a suite of 25 crude oils and condensates. Sixteen of the crude oils that were analysed in this study correlated closely with Koolinda-1 sediments from a narrow Oxfordian age-range, more precisely the W. spectabilis dinoflagellate zone. The observed correlation suggests that these oils were generated from time-equivalent source rocks. Four oils correlated well with slightly older sediments. Their relatively high values for the total higher plant input suggest that their source rocks were closer to their respective supplies of plant material than the Koolinda-1 sediments. Five condensates did not fit the higher plant fingerprint of the Jurassic in Koolinda-1. These oils might have been generated from source rocks older than those analysed at Koolinda-1.