Maturity of kerogen and asphaltenes determined by partial-least-squares (PLS) calibration and target projection of diffuse reflectance Fourier transformed infrared spectra

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.