Org. Geochem. Vol. 20, No. 7, pp. 917-926, 1993 0146-6380/93 $6.00 + 0 00 Pnnted m Great Britain. All rights reserved Copyright © 1993 Pergamon Press Ltd Thermodynamic calculations on alkylated phenanthrenes: geochemical applications to maturity and origin of hydrocarbons H. BUDZINSKI, l PH. GARRIGUES, 1 M. RADKE, 2 J. CONNAN 3 and J.-L. OUDIN 4 I URA 348 CNRS Universit6 de Bordeaux I, 33405 Talence Cedex, France, :Institute of Petroleum and Organic Geochemtstry, Research Centre Jiilich (KFA), D-52425 Jiilich, Germany, 3Elf-Aquitaine Production, CSTJF, Avenue Larribau, 64018 Pau, France and 4Total, Centre Technique, Domaine de Beauplan, 78470 Saint-Remy les Chevreuse, France (Received 11 January 1993; accepted m revised form 27 April 1993) Abstract--Enthalpies of formation of alkylated phenanthrenes have been evaluated by molecular mechanics method. These thermodynamic data have been successfully applied as predictive tools in the case of a highly complex class of compounds, the trimethylphenanthrenes, in order to restrict the synthesis work of standards to isomers that are likely to be present in natura[ samples. The changes in the distributions of isomers at different maturity stages are broadly in agreement with these theoretical calculations. Key words--thermodynamic stability, phenanthrenes, petroleum, alkylated hydrocarbons, thermal maturity, origin indicators INTRODUCTION Alkylated phenanthrenes are common constituents of petroleum and ancient sediments. They have been widely used as thermal maturity indicators of sedi- mentary organic matter (Radke, 1987). It is now well known that with increasing thermal maturity distri- bution changes are characterized by an increase in the abundance of the most stable isomers. Generally, the compounds substituted in the at position are less stable than related t-isomers (Fig. 1). This concept, which is easily applied to compounds with a low degree of substitution, is problematic if the degree of substitution is high. Therefore, it would be desirable to have access to the thermodynamic stab- ilities of the various isomers when considering large series of alkylated compounds, such as the 56 trimethylphenanthrenes, Theoretical methods of calculating thermodynami- cal data that circumvent the inherent difficulties of experimental determinations are of two kinds: quan- tum mechanical methods [mostly based on molecular orbital (MO) calculations] (Heilbronner and Bock, 1976; Dewar and Thiel, 1977), and nonquantum methods, such as molecular mechanics methods (Allinger and Sprague, 1973; Allinger, 1977) or group additivity methods (Benson, 1976; Stein et al., 1977). Excessive computation times required for MO calcu- lations are still a major practical problem when this method is applied to large molecules. The group additivity method has proved to be insufficiently accurate (Budzinski et aL, 1993a). The molecular mechanics (MM) or force field method has been shown to be a very reliable, fast and efficient way of deriving molecular structures, energies and other properties for a wide variety of molecules (Allinger, 1977; Kao and Allinger, 1977; Kao, 1987; Bowen et aL, 1988; Tai et al., 1989; de Leeuw et al., 1989; Moldowan et al., 1991) and thus has been used in this study to calculate thermodynamic data for alkylphenanthrenes. Isomer distributions of methyl- and dimethylphenanthrenes (MP, DMP) in natural, and laboratory-experiment samples were interpreted in terms of thermodynamic stability calculated by this method. Moreover, predictions about the absence or presence of certain isomers in alkylphenanthrene classes such as trimethylphenanthrenes have been based on data obtained by this modelling program. EXPERIMENTAL Molecular mechanics calculations have been car- tied out both on a Vax system (model number 11/760) and on a microcomputer 80286 Tandy (model 3000 NL) for the alkylphenanthrenes. MMP2 software (version 1982, Molecular Design Ltd, San Leandro, Calif. U.S.A.) (Allinger, 1977; Kao and Allinger, 1977) and MMX software (version 88, Serena soft- ware, Bloomington, Ind., U.S.A.) was used. 33 trimethylphenanthrenes (TMP) were available as reference compounds for this study. Five TMP were purchased from Chiron Laboratories, Trond- heim, Norway. Twenty-eight were synthesized by Radke et al. (unpublished results) at the Institute of Petroleum and Organic Geochemistry (FRG), in the same way as the DMP (Garrigues et aL, 1987; Radke et aL, 1990) through oxidative photocyclization of alkylstilbenes. The Kimmeridge clay sample comes from a well in the North Sea collected between 2306 and 2572m 917