Quantitative spore colour measurement using colour image analysis

Abstract

The colour changes that spores exhibit, with increasing depth of burial, are used as a thermal maturity indicator for evaluating hydrocarbon source rocks. Spore colour is determined visually leading to an intrinsic difficulty of unequivocally assessing and recording their colour. Quantitative scales using photometers are available, but are not widely applied. Colour Image Analysis (CIA) is presented here as an alternative method for the quantification of spore colour, through the use of Image Analysis software common in many laboratories. Using the RGB (red, green, blue) colour format, the colour of spores was determined. The material included a set of artificially heated Lycopodium spores and geological samples from a range of ages and geographical areas covering the entire maturity range. These data were then compared to samples from a well section of Carboniferous age which were `fully mature' to `post mature'. With increasing maturity the colour changes in spores and pollen define a consistent and reproducible trend on the red vs. green (pixel intensity) graph. Significantly all samples suites (experimental and natural) exhibit the same behaviour. Previous research [ Milton, (1993)Ph.D. thesis; Gutjahr, 1966 Leidse Geologische Medelingen, 38, 1–30] show a strong morphological control on colour, based on differences in wall thickness and original chemistry. However, if spores and pollen are chosen carefully i.e. avoiding taxon with cracks, folds, apertures and ornamentation, as well as those with exceptionally thin (bag-type) or thick exines, a good sample average can be made by measurement of 40–50 similar spore/pollen types. Specific points on the red vs. green colour graph coincide with significant chemical transformations and these coincide with the thermal cracking of the spore wall (spore oil window).