Study of Kerogen Maturity using Transmission Fourier Transform Infrared Spectroscopy (FTIR)

Abstract:

Maturity of kerogen in shale governs the productivity and generation hydrocarbon type. There are generally two accepted methods to measure kerogen maturity; one is the measurement of vitrinite reflectance, %R_o, and another is the measurement of T_max through pyrolysis. However, each of these techniques has its own limits; vitrinite reflectance measurement cannot be applied to marine shale and pre-Silurian shales, which lack plant materials. Furthermore, %Ro, requires the isolation and identification of vitrinite macerals and statistical measurements of at least 50 macerals. T_max measurement is questionable for mature and post-mature samples. In addition, there are questions involving the effects of solvents on T_max determinations. Fourier Transmission Infrared Spectroscopy, FTIR, can be applied for both qualitative and quantitative assessment on organics maturity in shale. The technique does not require separating organic matter or identifying macerals. A CH2/CH3 index, R_CH, calculated from FTIR spectra is more objective than other measurements. The index increases with maturity (both natural maturation and synthetic maturation through hydrous and dry pyrolysis). The new maturity index R_CH can be calibrated to vitrinite reflectance which allows the definition of the following values for levels of maturity: 1) immature—R_CH > 1.6±0.2; 2) oil window-- 1.6±0.2 < R_CH > 1.3±0.3; 3) wet gas window--1.3±0.3 < R_CH> 1.13±0.05; and 4) dry gas window R_CH < 1.13±0.05.