Geochimica et Cosmochimica zyxwvutsrqponmlkjihgfedcbaZY Acta,Vol. 58, No. 18, pp. 3803-3822, 1994 Pergamon Copyright 0 1994 Elsevier science Ltd Printed in the zyxwvutsrqponmlkjih USA. All rights reserved COl6-7037/94 $6.00+ .OO zyxwvu 0016-7037(94)00176-6 Hydrocarbon biomarkers, thermal maturity, and depositional setting of tasmanite oil shales from Tasmania, Australia A. T. REVILL,’ J. K. VOLKMAN,’ T. O’LEARY,’ R. E. SUMMON&~ C. J. BOREHAM,’ M. R. BANK&~ and K. DENWER~‘* ‘CSIRO Division of Oceanography, GPO Box 1538, Hobart, Tasmania 7001, Australia *Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT 2601, Australia ‘Geology Department, University of Tasmania, GPO Box 252C, Hobart, Tasmania 7001, Australia zyxwvutsrqponmlkjihg (Received June 9, 1993; accepted in revised form March 18, 1994) Abstract-This study represents the first geological and organic geochemical investigation of samples of tasmanite oil shale representing different thermal maturities from three separate locations in Tasmania, Australia. The most abundant aliphatic hydrocarbon in the immature oil shale from Latrobe is a C,v tricyclic alkane, whereas in the more mature samples from Oonah and Douglas River low molecular weight n-alkanes dominate the extractable hydrocarbon distribution. The aromatic hydrocarbons are predominantly derivatives of tricyclic compounds, with 1,2,8_trimethylphenanthrene increasing in relative abundance with increasing maturity. Geological and geochemical evidence suggests that the sediments were deposited in a marine environment of high latitude with associated cold waters and seasonal sea- ice. It is proposed that the organism contributing the bulk of the kerogen, Tasmanites, occupied an environmental niche similar to that of modem sea-ice diatoms and that bloom conditions coupled with physical isolation from atmospheric COz led to the distinctive “isotopically heavy” b13C values (- 13.5%~ to -11.7%~) for the kerogen. 613C data from modern sea-ice diatoms (-7%0) supports this hypothesis. Isotopic analysis of n-alkanes in the bitumen (- 13.5 to -3 1’%0) suggest a multiple source from bacteria and algae. On the other hand, the n-alkanes generated from closed-system pyrolysis of the kerogen (- 15%) are mainly derived from the preserved Tasmanites biopolymer algaenan. The tricyclic compounds (mean -8%~) both in the bitumen and pyrolysate, have a common precursor. They are consistently enriched in 13C compared with the kerogen and probably have a different source from the n-alkanes. The identification of a location where the maturity of the tasmanite oil shale approaches the “oil window” raises the possibility that it may be a viable petroleum source rock. INTRODUCTION THE OIL PROSPECTIVITY of onshore Tasmania has long been problematical. Interest in the possibility of finding oil has been stimulated by repeated reports of bitumen strandings on western and southern beaches since the late 19th century (TWELVETREES, 19 17). This interest has continued, despite the fact that these coastal bitumens are now thought to arise from Mesozoic or Cainozoic offshore sediments that are poorly represented onshore (VOLKMAN et al., 1992). There have been, however, numerous reports over the last century of oil-seeps onshore (BENDALL et al., 1991), suggesting the possibility that older onshore rocks may also be a source of petroleum. Central to much of this interest has been the or- ganic-rich tasmanite oil shale (subsequently referred to simply as “tasmanite” or “oil shale”) which occurs particularly in the north-west of the state (Fig. 1). JAMES et al. (1932) reported that the oil shale was retorted to liberate hydrocarbons as early as 19 10, and this carried on until the 193Os, producing about I. 13 megalitres of shale oil. The tasmanite occurs as a distinctive band low in the Quamby Mudstone. The stratigraphy of Late Palaeozoic sed- iments in Tasmania has been the centre of much research interest (see CLARKE and FARMER, 1976; CLARKE, 1989) due to the difficulty of applying the (warm water based) in- * Present address: RGC Exploration, PO Box 1166, Milton, Queensland 4064, Australia. ternationally accepted biostratigraphic divisions to the cold water environment of Tasmania at this time. Because of this difficulty, the more appropriate Rekunian Series has been proposed (CLARKE and BANKS, 1975; CLARKE and FARMER, 1976) with a subdivision, the Tamarian stage, within which is the Quamby Mudstone (Fig. 2). That part of the Quamby Mudstone containing the oil shale has consistently yielded stage 2 microfloras (TRUSWELL, 1978) and a Fauna1 Zone 1 macrofauna (Fig. 2; CLARKE and BANKS, 1975). The age has been given as either Early Permian (FOSTER and WATER: HOUSE, 1988) or Late Carboniferous (CLARKE, 1992; i.e., a little older or a little younger than 290 my BP, taken as the age of the beginning of the Permian by GARLAND et al., 1990). The only lithological distinction between the oil shale and surrounding mudstone is that the former contains abundant algal remains. These are dominated by the unicellular alga Tasmanitespunctatus [NEWTON (1875)] whose biological af- finities have been suggested to lie with the extant green alga Pachy sphaera pelagica [OSTENFELD (1899)] (WALL, 1962). Initially, the tasmanite was thought to have been deposited in an extensive lake (MILLIGAN, I852), but the discovery of marine fossils (GOULD, 1861) precluded this. Recent work has suggested a nearshore marine origin (BANKS, 1962; CAL- VER et al., 1984) with the oil shale representing a period of algal blooms (CALVER et al., 1984; CLARKE, 1989). This hy- pothesis is further supported by comparison of the known occurrence of tasmanite with the inferred palaeogeography of Tasmania during the early Tamarian (BANKS and CLARKE, 1987; Fig. 3). 3803