The effect of minor to moderate biodegradation on C 5 to C 9 hydrocarbons in crude oils Simon C. George a, *, Christopher J. Boreham b , Sandra A. Minifie c ,StanC.Teerman d a CSIRO Petroleum, PO Box 136, North Ryde, NSW 1670, Australia b Geoscience Australia, GPO Box 378, Canberra, ACT 2601, Australia c Chevron Australia Pty Ltd, GPO Box S1580, Perth, WA 6001, Australia d Chevron, PO Box 6046, San Ramon, CA 94583-0746, USA Abstract Asuiteof18oilsfromtheBarrowIslandoilfield,Australia,andanon-biodegradedreferenceoilhavebeenanalysed compositionally in order to detail the effect of minor to moderate biodegradation on C 5 toC 9 hydrocarbons. Carbon isotopic data for individual low molecular weight hydrocarbons were also obtained for six of the oils. The Barrow Island oils came from different production wells, reservoir horizons, and compartments, but have a common source (the Upper Jurassic Dingo Claystone Formation), with some organo-facies differences. Hydrocarbon ratios based on hopanes,steranes,alkylnaphthalenesandalkylphenanthrenesindicatethermalmaturitiesofabout0.8% R c formostof the oils. The co-occurrence in all the oils of relatively high amounts of 25-norhopanes with C 5 toC 9 hydrocarbons, aromatic hydrocarbons and cyclic alkanes implies that the oils are the result of multiple charging, with a heavily bio- degraded charge being overprinted by fresher and more pristine oil. The later oil charge was itself variably biode- graded, leading to significant compositional variations across the oilfield, which help delineate compartmentalisation. Biodegradation resulted in strong depletion of n-alkanes ( > 95%) from most of the oils. Benzene and toluene were partially or completely removed from the Barrow Island oils by water washing. However, hydrocarbons with lower water solubility were either not affected by water washing, or water washing had only a minor effect. There are three main controls on the susceptibility to biodegradation of cyclic, branched and aromatic low molecular weight hydro- carbons: carbon skeleton, degree of alkylation, and position of alkylation. Firstly, ring preference ratios at C 6 andC 7 show that isoalkanes are retained preferentially relative to alkylcyclohexanes, and to some extent alkylcyclopentanes. Dimethylpentanes are substantially more resistant to biodegradation than most dimethylcyclopentanes, but methyl- hexanes are depleted faster than methylpentanes and dimethylcyclopentanes. For C 8 and C 9 hydrocarbons, alkylcy- clohexanesaremoreresistanttobiodegradationthanlinearalkanes.Secondly,thereisatrendoflowersusceptibilityto biodegradation with greater alkyl substitution for isoalkanes, alkylcyclohexanes, alkylcyclopentanes and alkylben- zenes. Thirdly, the position of alkylation has a strong control, with adjacent methyl groups reducing the susceptibility of an isomer to biodegradation. 1,2,3-Trimethylbenzene is the most resistant of the C 3 alkylbenzene isomers during moderate biodegradation. 2-Methylalkanes are the most susceptible branched alkanes to biodegradation, 3-methyl- alkanesarethemostresistantand4-methylalkaneshaveintermediateresistance.Therefore,terminalmethylgroupsare morepronetobacterialattackcomparedtomid-chainisomers,andC 3 carbonchainsaremorereadilyutilisedthanC 2 carbon chains. 1,1-Dimethylcyclopentane and 1,1-dimethylcyclohexane are the most resistant of the alkylcyclohexanes and alkylcyclopentanes to biodegradation. The straight-chained and branched C 5 –C 9 alkanes are isotopically light (depletedin 13 C)relativetocycloalkanesandaromatichydrocarbons.Theeffectsofbiodegradationconsistentlyleadto 0146-6380/02/$ - see front matter Crown Copyright # 2002 Published by Elsevier Science Ltd. All rights reserved. PII: S0146-6380(02)00117-1 Organic Geochemistry 33 (2002) 1293–1317 www.elsevier.com/locate/orggeochem * Corresponding author. Tel.: +61-2-9490-8718; fax: +61-2-9490-8921. E-mail address: Simon.George@csiro.au (S.C. George).