Microphotometry is the computerized microscopic measurement of reflectance, fluorescence, and transmittance of organic matter in sedimentary rocks. Microphotometric data are an indispensable tool in exploration of hydrocarbons.

The chromaticity of kerogen particles in fluorescence and transmitted light is derived from spectral analysis and provides new scales for thermal maturity: the Fluorescence Color Index and Transmittance Color Index. The TCI could substitute for the inaccurate TAI. Generation of crude oil within thermally mature kerogen particles shows characteristic fluorescence phenomena when microscopically observed and measured.

Spectral data and chromaticity values provide an improved method for determining the thermal maturity of petroleum source rocks, in particular when vitrinite reflectance data are unreliable or unavailable. The "oil floor" in a basin is marked by the disappearance of the fluorescence of fossil alginite, exinite, amorphous organic matter, and most bitumen. This method provides a better mapping of thermally mature petroleum source rocks.

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Other applications of microphotometry are: (1) calculation of the effective organic carbon content from the amount of fluorescent oil-prone kerogen and its fluorescence characteristics, a technique providing a better estimation of the volume of hydrocarbons that may have been generated by a source rock, and (2) "fingerprinting" of various types of kerogen, bitumen, drilling contaminants, and caved rock material, and identification of kerogen and bitumen in the mineral matrix of rock samples.

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