Biomarker maturity parameters: the role of generation and thermal degradation PAUL FARRIMOND 1 *, ALEX TAYLOR 1 and NILS TELNáS 2 1 Fossil Fuels and Environmental Geochemistry (NRG), Drummond Building, University of Newcastle upon Tyne, Newcastle upon Tyne, NE1 7RU, U.K. and 2 Norsk Hydro E and P Research Centre, Bergen, Norway AbstractÐThe eects of burial maturation upon seven molecular maturity parameters, and the concen- trations of individual biomarkers employed in these parameters, have been studied in a well in the Barents Sea. The depth interval between 1400 and 3000 m comprises homogenous Eocene claystones with eectively uniform organic carbon content (0.4±0.6%) and kerogen type (HI of 100±200 mg HC/g TOC). The quantitative biomarker data thus contain minimal interference from organic facies, hydro- carbon expulsion eects and the presence of migrated hydrocarbons. Bulk geochemical data de®ne the hydrocarbon generation threshold (or ``oil window'') at around 2300 m. Molecular maturity parameters proceed to equilibrium, and in some cases demonstrate inversion at high maturity. These molecular changes are associated with major changes in biomarker concentrations. The %bb hopane, %22S C 31 ab hopane and %20S C 29 aaa sterane parameters proceed as a result of the balance between the relative rates of generation and thermal degradation of the dierent isomers. Increased concentrations of the in- dividual isomers are interpreted to be the result of biomarker generation from the kerogen (mainly) and other macromolecular/polar fractions. The %abb C 29 sterane parameter and the three parameters invol- ving rearranged hopanes (%Ts, %29Ts and %diahopane) operate at higher maturity levels, and pro- ceed as a result of the relative rates of thermal degradation of the compound pairs. Isomerisation in the bitumen may also in¯uence the operation of the parameters involving stereoisomers, but must be a rela- tively minor contribution. # 1998 Elsevier Science Ltd. All rights reserved Key wordsÐisomerisation, maturity parameters, maturation, kerogen, generation, cracking, T max INTRODUCTION Quantitative studies of maturity-driven biomarker transitions as a result of progressive burial are rare, primarily because of the scarcity of suitable sample suites. However, such studies are essential to estab- lish how biomarker concentrations control the mol- ecular maturity parameters which are routinely applied in petroleum geochemistry. Molecular maturity parameters have been widely employed since the recognition of systematic changes in biomarker composition with increasing depth (and thus thermal maturation) in natural bur- ial sequences (e.g. Seifert and Moldowan, 1980; Mackenzie et al., 1980). Most of these maturity par- ameters are based on the relative abundances of two stereoisomers, and involve a relative increase of the more thermally stable (non-biological) isomer compared to the isomer with the original biologi- cally-inherited stereochemistry. Originally, this pro- cess was widely assumed to comprise direct isomerisation or epimerisation of the free bio- markers in the bitumen (e.g. Seifert and Moldowan, 1980). However, studies providing quantitative data have shown that the mechanisms behind the matur- ity parameters are more complicated; relative rates of release of kerogen-bound biomarkers (``gener- ation''), and their thermal stabilities (``cracking)'', are also important (e.g. Lu et al., 1989; Abbott et al., 1990; Peters et al., 1990; Bishop and Abbott, 1993; Requejo, 1994; Farrimond et al., 1996). The uncertainty regarding the processes responsible for the observed changes in isomeric composition was recently highlighted by van Duin et al. (1997) who presented a kinetic model for hopanoid isomerisa- tion which involved a full isomerisation scheme rather than one direct chiral isomerisation step. Despite observing generation of abundant hopanes from the kerogen, and their subsequent thermal destruction at high temperatures (processes which could not be included in the model), their computer simulations using an isomerisation scheme was able to reproduce the isomer compositions in a suite of arti®cially matured samples. This was interpreted as indicating the importance of isomerisation as a pro- cess controlling the relative concentrations of bio- markers (and thus molecular maturity parameters). Org. Geochem. Vol. 29, No. 5±7, pp. 1181±1197, 1998 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0146-6380/98 $ - see front matter PII: S0146-6380(98)00079-5 *To whom correspondence should be addressed. Tel.: +44-191-222-6852; Fax: +44-191-222-5431; E-mail: paul.farrimond@ncl.ac.uk 1181