Use of aromatic compound distributions to evaluate organic maturity of the Proterozoic middle Velkerri Formation, McArthur Basin, Australia

The middle Velkerri Member is a rich Proterozoic source rock in the McArthur Basin. A decrease in hydrogen index from > 500 mg pyrolysate/g TOC in shallow immature sediments to around 150 mg/g in deeper sediments has been suggested to coincide with the main oil generation window. Thermal maturity in Proterozoic rocks can not be determined by conventional vitrinite reflectance, because these ancient sediments predate the evolution of the land plants. The uniform nature of the organic matter type in these rocks suggests that molecular maturity parameters may be useful. Aromatic hydrocarbon fractions were isolated from middle Velkerri rocks in three McArthur Basin wells (Walton 2, Shea 1 and McManus 1), covering a wide maturity range and forming a composite depth section. Maturity-sensitive ratios based on alkylnaphthalenes, alkylphenanthrenes, alkylbiphenyls, and alkyldibenzothiophenes have been calculated. The aromatic ratios best suited for examining variations throughout the oil window in the Mesoproterozoic sediments of the McArthur Basin are the alkylphenanthrene ratios, in particular the methylphenanthrene index and the methylphenanthrene distribution fraction, which are sensitive to maturity variations at least from the initial phase of oil generation to the late oil window. Some trimethylnaphthalene and tetramethylnaphthalene ratios are sensitive to maturity variations in the early oil window, but then reach equilibrium, whereas alkylbiphenyl ratios are sensitive to maturity variations in the peak to late part of the oil window but show little change at lower maturities. Application of the liquid reaction environment concept (van Aarssen et al., 2000) suggests that the middle Velkerri Member in Shea 1 and McManus 1 has or had oil generation potential, consistent with the aromatic hydrocarbon ratios that suggest maturities in the oil window or above. The middle Velkerri Member in Walton 2 has a less well developed liquid reaction environment, consistent with the lower maturities in this well. An igneous intrusion in the upper Velkerri Member that has now been eroded is inferred to have locally raised the maturity of the upper part of the middle Velkerri Member in Walton 2.