ABSTRACT: Organic maturity evaluation of the Proterozoic middle Velkerri Formation, Northern Territory, Australia: Use of aromatic hydrocarbon distributions

George, Simon C.¹, and Manzur Ahmed²  
(1) CSIRO Petroleum, Sydney, Australia 
(2) CSIRO Petroleum, North Ryde, Australia

The middle Velkerri Formation is a rich Proterozoic (c. 1.4 Ga) source rock (TOC up to 6%) in northern Australia. It was deposited in a restricted marine basin, characterised by periods of fluctuating organic matter preservation that controlled TOC. A fall in hydrogen index from > 500 in shallow immature sediments to around 150 in deeper sediments has been suggested to coincide with the main oil generation window. The middle Velkerri in deeply buried sections is believed to be in the late to overmature stage of oil generation. Thermal maturity 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 seven different wells, covering a wide maturity range and forming a composite depth section. Maturity-sensitive ratios based on naphthalene, phenanthrene, biphenyl, dibenzothiophene and their alkyl-substituted isomers were calculated. The large number of parameters obtained from about 100 samples, at depths from 67 to 1,518m, enabled correlation of the molecular ratios with depth, organic maturity and tectonic/palaeogeographic history. The methylphenanthrene index and other ratios based on alkylphenanthrenes exhibit the best fit at high maturities, partly due to demethylation processes. However, other parameters derived from alkyl-substituted naphthalenes were also found to show good correlations. The applicability of the various ratios was determined and they have been cross-calibrated for use in vitrinite-lean Proterozoic source rocks.

AAPG Search and Discovery Article #90913©2000 AAPG International Conference and Exhibition, Bali, Indonesia