ABSTRACT: Assessing The Hydrocarbon Potential of Precambrian and Cambrian Petroleum Source Rocks in Australian Sedimentary Basins

GLIKSON, Miryam, Department of Earth Sciences, The University of Queensland, Australia, DENNIS TAYLOR, CRA-Pacific Oil & Gas, Box Hill, Victoria, Australia, BRUCE McCONACHE,* Comalco Aluminum Ltd, Brisbane, Australia

Organic-rich samples derived from a Middle Cambrian Formation in the Georgina basin, from the middle Proterozoic of the McArthur and South Nicholson basins, and the lower-middle Proterozoic of the Lawn Hill Platform in northern and central Australia, yielded alginite ranging from immature oil shale material to over mature residue. A maturation scale has been developed based on the thermal evolution of alginite as determined from reflectance and fluorescence. The coalification path of alginite is marked by jumps in contrast to the linear path of wood-derived vitrinite. Six zones have been recognized, ranging from undermature (zone 1), through the mature (zones II/III), followed by a stable stage of no change (zone IV) to the overmature (zones V and VI): The onset of oil generation in a ginite as evident from the present study is at 0.3% Ro Alg., and is expressed in a change of fluorescence from yellow to brown, and a coalification jump from 0.3 to 0.6% Ro of Alg. In many boreholes zone III can be distinguished between 0.6-0.8% Ro of Alg. where subsequent oil generation occurs. Zones II and III represent the oil window.

A zone of little or no change designated zone IV, at 0.9/1.0% Ro of alginite follows zones IVIII. A marked coalification jump characterizes zone V, where a pronounced change in reflectance occurs to > 1.0% Ro Alg., signifying peak gas generation. The border of oil preservation lies at the transition of zone V and VI, at 1.5% Ro Alg. In zone VI gas generation only occurs.

Comparison of reflectance results with experimental and geochemical pyrolysis data supports high activation energies for hydrocarbon generation from alginite, and therefore a later onset of oil generation (0.3 Ro Alg. = 0.6 Ro V) than other liptinite macerals (i.e., cutinite, exinite, resinite) as well as a narrow oil window.

Transmission electron microscopy (TEM) confirms that alginite does not go through a distinct intermediate stage but that the percentage of unreacted organic matter decreases as maturation proceeds. A clear distinction can be made in TEM between immature alginite, alginite after oil generation, and alginite residue following gas generation. Alginite beyond 1.6% Ro acquires very high densities and the appearance of inertinite in TEM.

Bitumens/pyrobitumens, the residue of oil generated from alginite make a pronounced contribution to the organic matter throughout the basins and have been shown to effect pyrolysis results by suppressing Tmax. The bitumens/pyrobitumens have been divided into three groups, based on their reflectance and morphology, which in turn appears to be an expression of their genetic history. Their significance is in aiding the understanding of the basins' thermal history, the timing of oil and gas generation, and their movement through the sequence.

AAPG Search and Discovery Article #91015©1992 AAPG International Conference, Sydney, N.S.W., Australia, August 2-5, 1992 (2009)