Year

1989

Degree Name

Doctor of Philosophy

Department

Department of Geology

Abstract

Organic matter in middle Proterozoic sedimentary rocks of the McArthur Basin occurs predominately within fine-grained clastic sediments and is made up of the petrographically discernable entities, lamalginite, bitumen, thucholite and oncalginite. With increasing maturation, fluorescence in lamalginite diminishes from bright yellow to dull brownish-orange and eventually ceases. Disaggregation of nonfluorescent lamalginite may have led to the formation of some types of bitumen. The lamalginite probably formed from a variety of micro-organisms but few are preserved in the lamalginite.

Reflectances of discrete maceral types generally show systematic increases with increasing depth, but discontinuities occur where fluorescent lamalginite gives way to nonfluorescent lamalginite and bitumen. The relationships of reflectance and Tmax indices to the oil window, and of observed reflectances to calculated vitrinite reflectances, derived from methylphenanthrene indices, are established.

Maturation levels in the McArthur Basin range from marginally mature to overmature. Contact metamorphism in the Roper Group and hydrothermal activity, associated with base metal mineralisation in places, in the McArthur Group, have locally increased maturation levels. Maturation levels in the Roper Group that are unaffected by contact metamorphism, display a pattern consistent with the concept that the Roper Group formed during the sag phase of the basin's development and that there has been subsequently a relatively uniform geological history. Maturation levels in the McArthur Group that are unaffected by hydrothermal activity, show a different pattern resulting from variable subsidence rates and geothermal gradients during the final phase of extension in the basin which preceded the sag phase.

Comparisons between modelled vitrinite reflectances and observed reflectances, recalculated to equivalent vitrinite reflectances, suggest that the Roper Group was up to about 1000 m thicker than its observed maximum thickness and that sedimentary cover overlying the McArthur Group was less than 2000 m in places.

Hydrocarbon generation occurred from organic-rich units within the Roper and McArthur Groups, particularly from the Velkerri and Barney Creek Formations which contain up to 7% total organic carbon. Oil generation from these formations commenced before 1000 Ma but low rates of oil generation may have continued, at low temperatures, for a considerable period of time extending well into the Phanerozoic.

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