An integrated analytical approach for determining the origin of solid bitumens in the McArthur Basin, northern Australia

Abstract

An integrated analytical approach using petrological, geochemical and isotopic techniques has been used to determine the origin of solid bitumens in the Middle Proterozoic Roper Group, McArthur Basin, northern Australia. Sandstone and dolerite samples, which in hand specimen contain dark material believed to be solid bitumens, were examined from six wells. Under ultraviolet illumination, non-fluorescent solid bitumens can be distinguished from two generations of crude oil, one of lower maturity (orange fluorescence), the other of higher maturity (white fluorescence). Other petrographic observations indicate that the precursor hydrocarbons to the solid bitumens and the subsequent charges of crude oil postdate all diagenetic clay and quartz cementation. Geochemical analyses indicate that many of the sandstones contain both an aliphatic-rich oil and solid bitumen, mixed in varying proportions, thus corroborating the petrological results. On the basis of n-alkane distributions, the oil appears to have two 3ubtly-different source inputs, one of which is typified by a greater proportion of high molecular weight n-alkanes which have a slight even carbon-number predominance and an isotopically lighter aliphatic hydrocarbon fraction. One sandstone sample contains solid bitumen in a fracture which is characterized by a high abundance of asphaltenes, polar compounds and sulphur and may have formed by deasphalting of a precursor oil. The solid bitumens recovered from fractures in dolerite sills were formed by thermal processes which in one sample resulted in an unusual series of ketone isomers. However most solid bitumens, particularly where present in the coarser sandstones, are related to the generation of secondary porosity. The large proportion of unresolved complex mixture, the lack of n-alkanes and the isotopically heavier aliphatic hydrocarbon fractions of samples containing predominantly solid bitumens indicate that bitumen formation occurred mainly by biodegradation of a precursor oil.