Adapting palynological preparation methods in subbituminous and bituminous coals from Colombia to improve palynofacies and hydrocarbon source rock evaluations

摘要

Apart from their potential as fossil fuel, bituminous and subbituminous coals in Colombia may prove to be potential source rocks for the generation of hydrocarbons. The assessment of such properties requires the combination of petrographic and palynofacies studies. The latter correspond to the microscopic investigation in transmitted white and reflected UV lights of sedimentary organic matter insoluble in organic and inorganic solvents. To date, mineral acid and Schulze methods have failed to produce good samples of palynofacies constituents from Cenozoic coals in Colombia, so that the latter method had to be modified. The main modifications consist in adapting the KClO_3 to HNO_3 ratio in the Schulze mixture and introducing two extra phases of oxidation with NaOH and NH_4OH, which permit the washing off of humic substances. This method yields the full spectrum of palynofacies constituents, which are not destroyed or damaged by the alkali treatment. Neither is their fluorescence colour affected by the initial acid treatment. On the contrary, the particle fluorescence intensity tends to increase because of the dissolution of humic acids, making their identification easier. The achievements of the modified method are threefold: 1) it permits the correlation between macerals in petrographic polished sections and their equivalent in palynofacies slides, making the two methods complementary; 2) palynofacies provides a better identification of sporomorphs than petrography, resulting in better paleoenvironmental interpretations; 3) palynofacies tend to give a more precise quantitative evaluation of the liptinitic constituents, and consequently, of the coal hydrocarbon generation potential.