Org. Geochem. Vol. 17, No. 1, pp. 63-74, 1991 0146-6380/91 $3.00 + 0.00 Printed in Great Britain. All rights reserved Copyright © 1991 Pergamon Press plc Maturity of kerogen and asphaltenes determined by partial-least-squares (PLS) calibration and target projection of diffuse reflectance Fourier transformed infrared spectra ALFRED A. CHRISTY 1, OLAV M. KVALHEIM 1, KJELL ~YGARD 2, BIRGERDAHL 3 and TERJE V. KARSTANG 1 Department of Chemistry, University of Bergen, 5007 Bergen, Norway 2Geolab, Statoil, P.O. Box 300, 4001 Stavanger, Norway 3Norsk Hydro Research Centre, P.O. Box 4313, 5001 Bergen, Norway (Received 27 November 1989; accepted in revised form 14 June 1990) Abstract--Maturation of sedimentary organic matter expressed as: (1) vitrinite reflectance; (2) depth within the same geological formation; and (3) pyrolysis temperature in hydrous pyrolysis experiments is assessed by diffuse reflectance i.r. spectroscopy either using an index defined from intensities of specific peaks or through a multivariate calibration of the spectral profiles with their respective maturation indicators as dependent variables. The samples were of three kinds: (a) kerogen samples (46) of varying maturity from the North Sea; (b) asphaltene samples (108) prepared from sediments obtained from two different wells and a reservoir from the North Sea; and (c) asphaltene samples prepared from hydrous pyrolysate of a sediment containing type II kerogen. The spectra show that the following chemical processes take place during maturation: (i) elimination of carbonyl groups; (ii) depletion of aliphatic chains; and (iii) increase in aromatic content. The univariate index, although sensitive to changes in organic facies, seems applicable within the oil window with the best results obtained for type III kerogen. Multivariate calibration of kerogen samples reveals the capability of the PLS method for predicting vitrinite reflectance. Target projections of the PLS components provide validated spectral profiles, related to the collective changes taking place during both natural and simulated evolution. Key words--diffuse reflectance, i.r. spectroscopy, multivariate calibration, target projection, maturity factor, kerogen, asphaltenes INTRODUCTION Determination of the type and maturity of kerogens is important in petroleum exploration. Typing is done by, inter alia, determining the elemental com- position of kerogen, and the maturity level is gener- ally determined by means of vitrinite reflectance measurements. Maturity determination by vitrinite reflectance measurements has its origin in rank deter- mination of coals, where vitrinite is generally abun- dant, thus leading to reliable results. For sedimentary organic matter, where the total organic content of the sediment is low, vitrinite particles are scarce. Further- more, the organic matter is generally finely dispersed in a mineral matrix which affects the vitrinite reflec- tance measurements in a negative way. Demineraliz- ation of the rock to release the organic matter requires treatment with concentrated hydrofluoric and hydrochloric acids. Low temperatures are necess- ary, otherwise changes in the kerogen structure are possible making vitrinite reflectance measurements less reliable (Beny-Bassez and Rouzaud, 1985). Many other maturity parameters have been proposed to complement the vitrinite reflectance measurements, for instance, biomarker analysis (McKenzie et al., 1984; Philp, 1985), and indices derived from aromatic hydrocarbon distributions (Radke et al., 1986; Kvalheim et al., 1987a). The use of spectroscopic techniques to study coals and kerogen is not new. However, with the invention of the Fourier-transform i.r. spectrometer, and the development of new sampling techniques, the number of applications has increased dramatically (Painter et al., 1981, 1983; Tooke et al., 1983; Fuller et al., 1982). Data handling by the use of computers and the availability of multivariate software packages (Kvalheim and Karstang, 1987b), have made it poss- ible to analyse spectroscopic data for correlations and trends. Thus, Christy et al. (1987) calibrated diffuse reflectance Fourier transform i.r. profiles against vitrinite reflectance measurements, of well-character- ized whole coal samples, and demonstrated the ability of the resulting multivariate calibration model for predicting vitrinite reflectance. In this paper, a similar approach is used to obtain a regression model for predicting the vitrinite reflectance of kerogen samples. The structural similarity between kerogens, and their associated asphaltenes, has been established by several authors (Pfeiffer and Van Doormaal, 1936; 63