Unusual polycyclic aromatic hydrocarbons in Lower Cretaceous Ostracode Zone sediments and related oils of the Western Canada sedimentary basin* MAOWEN LI 1 , C. L. RIEDIGER 2 , M. G. FOWLER 1 and L. R. SNOWDON 1 1 Geological Survey of Canada (Calgary), 3303-33 St. N.W. Calgary, Alberta, Canada T2L 2A7 and 2 Department of Geology and Geophysics, University of Calgary, Calgary, Alberta, Canada T2N 1N4 (Received 25 November 1996; returned to author for revision 26 February 1997; accepted 7 April 1997) AbstractÐAnalyses of the aromatic hydrocarbon fractions of extracts of Lower Cretaceous Ostracode Zone sediments from the Western Canada sedimentary basin indicate the predominance of polycyclic hydroaromatic components. Most of these have not been previously reported in the literature, but are obviously diagenetically related to a new class of molecule whose natural distribution and biological signi®cance remain to be determined. The components whose molecular structures have been con®rmed, together with those for which tentative structural assignments are given, provide strong support for a pathway of progressive aromatisation of the presumed polyprenol precursors. These compounds appear to be speci®c markers for the Ostracode Zone sourced oils in the Lower Mannville Group reservoirs of the Provost area and may serve as excellent oil-source correlation parameters. # 1997 Elsevier Science Ltd. All rights reserved Key wordsÐDiagenetic aromatisation, polycyclic hydroaromatic hydrocarbons, polyprenols, source rocks in Western Canada, sedimentary basin in Western Canada INTRODUCTION Studies of the extracts of ®ne-grained sediments in the Lower Cretaceous Ostracode Zone from the southern Alberta portion of the Western Canada sedimentary basin (WCSB) have shown them to contain abundant molecular fossils with unusual chemical structures (Riediger et al., 1993, 1995a,b). In a recent paper (Li et al., 1996), we reported the occurrence of a wide range of polycyclic alkanes in the aliphatic hydrocarbon fractions of Ostracode Zone sediment extracts, whose natural distribution and biological and geochemical signi®cance remain to be determined. These compounds have not been detected in sediment extracts of other petroleum systems in the WCSB and, hence, appear to be speci®c markers for Ostracode Zone sourced oils in southern Alberta. Some of these polycyclic com- pounds were previously reported to be ubiquitous in oils and bitumens from carbonate source rocks in the Aquitaine Basin (Poinsot et al., 1995) and in some biodegraded oils of the Spanish foreland of the western Pyrenees (Dorronsoro et al., 1994). Our investigations of aromatic hydrocarbon frac- tions obtained from Ostracode Zone extracts reveal that they are dominated by compounds that are structurally related to the unusual polycyclic alkanes reported earlier (Li et al., 1996). These com- pounds range from monoaromatic to at least tetra- aromatic hydrocarbons, of which only a few monoaromatic components have been reported pre- viously from the Eocene Messel shale in Germany (Schaeer et al., 1994; Poinsot et al., 1995). The purpose of this paper is to report the occurrence of several novel series of these unusual polycyclic aro- matic hydrocarbons (PAHs). A tentative diagenetic pathway of progressive aromatisation is postulated to account for the origin of these unusual com- pounds. EXPERIMENTAL Sample description Forty Ostracode Zone sediments were selected for solvent extraction after an initial screening of a much larger sample set using Rock±Eval/TOC analysis. These extracts and 20 crude oil samples from the Provost area in southeastern Alberta were used for this study. The basic geological and geo- chemical data for most of the samples have been reported previously in Riediger et al. (1993, 1995a,b). The reference Ostracode Zone source rock sample used in this report was taken from a core at the lo- cation of 8-29-30-23W4 (depth 1512.5 m). It con- tains 7.04% TOC, with a Rock±Eval hydrogen index of 611 mg hydrocarbons g À1 TOC. The mol- Org. Geochem. Vol. 27, No. 7/8, pp. 439±448, 1997 # 1997 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0146-6380/97 $17.00 + 0.00 PII: S0146-6380(97)00026-0 *Geological Survey of Canada Contribution No. 1996492 439