Reflected light microscopy and micro-FTIR of Upper Ordovician Gloeocapsomorpha prisca alginite in relation to paleoenvironment and petroleum generation, Saskatchewan, Canada

Abstract

Gloeocapsomorpha prisca alginites from Upper Ordovician Type I kerogen in hydrocarbon source rocks (kukersites), Yeoman Formation, Saskatchewan, Canada, have been studied using incident light microscopy and transmission micro-Fourier transform infrared spectroscopy. Gloeocapsomorpha prisca coccoidal alginite are classified as three maceral varieties: (i) small agglomerations of thin-walled disseminated A, (ii) larger agglomerations of thick-walled disseminated B; and (iii) a stromatolitic variety. Each alginite maceral variety is interpreted as a stage in the life cycle of the algal precursor resulting from progressive normal growth controlled by chemical and physical changes in the paleowater column. Thickened cell walls of disseminated B and stromatolitic G. prisca are characterized by strong i.r. absorbances in the OH region and a higher aromatic C—H absorption compared to the early growth stage of the disseminated A alginite at the same level of thermal maturity. This characteristic is attributed to the presence of highly aliphatic and resistant outer cell wall biopolymers formed by the algal precursors in response to a variation in either oxygen supply or fluctuating salinity levels within the paleoenvironment and is outlined in a proposed paleoenvironmental model. The rate of reflectance increase in oil and the degree of fluorescence red shift (lambda max) with increasing depth of burial and thermal maturation is greater for G. prisca disseminated varieties compared to the stromatolitic maceral variety. Aromatic C—H absorption (3050 cm −1) per unit area for disseminated A and B alginite increases at approximately the same rate with increasing thermal maturity. In contrast the aliphatic C absorption bands (2920 and 2850 cm −1) decrease with increasing maturity in G. prisca disseminated A alginite whereas aliphatic absorption per unit area increases in disseminated B alginites with increasing thermal maturation. The aliphatic absorption bands also increase with increasing thermal maturation for stromatolitic G. prisca, but in a pattern unlike disseminated B. A hydrocarbon generation model for G. prisca alginite proposes that the molecular structure of the disseminated B variety undergoes a transformation from an alkyl long chain structure into a more cyclic bitumen-like maceral prior to peak generation. In contrast, the smaller, thin-walled, disseminated A alginite,representing the early growth stage, probably generates free bitumen from diagenesis through to catagenesis. This is supported by a reduction in aliphatics with increasing thermal maturity and by the formation of oily-bitumen that evolves from the alginite during petrographic analysis. The stromatolitic G. prisca alginite remains morphologically very rigid throughout the diagenetic range of thermal maturities.