Application of diamondoids to distinguish source rock facies Linda K. Schulz *, Arnd Wilhelms, Elin Rein, Arne S. Steen Norsk Hydro Research Centre, 5020 Bergen, Norway Received 4 September 2000; accepted 9 January 2001 (returned to author for revision 11 November 2000) Abstract Diamondoid distributions in solvent extracts from marine siliciclastic shales, marine carbonates, and coals of a wide variety of maturities were determined by GC±MSD. For this sample set, published diamondoid maturity para- meters did not change systematically throughout the oil window. It was found that several novel diamondoid ratios were indicative of source. It was possible to distinguish between terrigenous, marine carbonate and marine sili- ciclastic facies. The proposed dimethyl diamantane facies ratios are virtually unaected by maturity, while the ethyl adamantane ratio appears to be aected by maturity in the late gas-window range. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: Diamondoid hydrocarbons; Source rocks; Facies; Maturity; Oil correlation 1. Introduction Diamondoid hydrocarbons in petroleum have been identi®ed in a number of crude oils (Petrov et al., 1974; Williams et al., 1986; Wingert, 1992 and references therein) and are more stable than most other hydro- carbons. It has been speculated that diamondoids in petroleum form by rearrangements of polycyclic hydro- carbons under thermal stress with a strong Lewis acid acting as catalyst (Petrov et al., 1974). Several studies have focused on the application of diamondoid hydrocarbons in petroleum characteriza- tion. Chen et al. (1996) identi®ed diamondoids in oils and source rock samples from several Chinese basins. Based on their ®ndings, two diamondoid maturity parameters were introduced and used to determine the thermal maturity of thermogenic gas and condensates, which may be dicult to assess using routine geochem- ical techniques. Dahl et al.(1999) used the relative abundance of diamondoids to identify and estimate the extent of oil cracking. They were also able to identify oils consisting of mixtures of high- and low-maturity components. Because diamondoids appear to be resis- tant to biodegradation, their application for character- ising heavily biodegraded oils has been suggested (Williams et al., 1986; Wingert, 1992). This has recently been demonstrated by Grice et al. (2000) who used dia- mondoids in a set of Australian oils to extend the bio- degradation scale of Peters and Moldowan (1993). So far, little focus has been given to the facies dependency of diamondoid distributions. In the present study, the diamondoid distributions in solvent extracts from mar- ine siliciclastic shales, marine carbonates, and coals of a wide variety of maturities have been investigated. The published maturity parameters have been tested and our work clearly shows that diamondoid ratios are related to source as well as thermal maturity. 2. Samples and experimental Source rock maturity sequences from dierent organic facies covering the entire oil window (Table 1) and 64 oil/condensate samples from oshore Norway, were selected for this study. Table 2 shows some geo- chemical background data for the sample sets. Rock-Eval and TOC analysis were performed as described by Espitalie et al. (1977). Crushed sediment 0146-6380/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S0146-6380(01)00003-1 Organic Geochemistry 32 (2001) 365±375 www.elsevier.nl/locate/orggeochem * Corresponding author. E-mail address: linda.kristin.schulz@hydro.com(L.K.Schulz).