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Hydrocarbon Source Potential Of The Upper Roper Group, Mcarthur Basin, Northern Territory.

Zaunbrecher, Michelle L.

Honours Degree, 1988

University of Adelaide


Several stratigraphic holes drilled by the Bureau of Mineral Resources between 1979 and 1985, and petroleum exploration wells drilled by Pacific Oil and Gas in 1987 and 1988, encountered `live' oil in the Proterozoic Roper Group sediments of the McArthur Basin, Northern Territory. The hydrocarbon potential of source rocks contained in the upper part of the Roper Group sequence has been assessed by Rock-Eval pyrolysis and the determination of total organic carbon. Gas chromatography and gas chromatography-mass spectrometry of oils and source rock extracts have been employed to determine oil to source correlations, and to assess any compositional changes in the oils due to migration and biodegradation.

Five potential source rock intervals have been recognised: The Kyalla Member of the McMinn Formation, the upper, middle and lower units of the Velkerri Formation and the Lansen Creek Shale. The best source interval is the middle Velkerri Formation which contains oil-prone Type II kerogen. The Kyalla Member also comprises good quality (Type II/III kerogen; oil and gas prone) source material whereas poor to fair, and occasionally good, source rock potential is evident in the Lansen Creek Shale, and upper and lower units of the Velkerri Formation.

The Kyalla Member and upper part of the upper Velkerri Formation are marginally mature within the study area. The middle Velkerri, the Lansen Creek Shale, and the remainder of the upper Velkerri Formation are all mature for hydrocarbon generation. Source rocks of the lower Velkerri Formation are overmature for oil and gas generation.

The oils of the Bessie Creek Sandstone and source extracts of the Velkerri Formation are dominated by C12-C22 saturated hydrocarbons. The presence of alkylcyclohexanes, iso- and anteiso-alkanes, and hopanes implies a prokaryotic source.

Two phases of hydrocarbon migration into the Moroak Sandstone and Bessie Creek Sandstone are apparent, the first of which preceded a major period of diagenesis involving quartz cementation of reservoir sands. Further exploration efforts should be directed towards deeper parts of the basin where the initial phase of hydrocarbon migration into reservoirs may have inhibited this diagenetic reduction of primary porosity.

Oil-to-source correlations identify the lower Velkerri Formation as the likely source of the second phase of oil reservoired in the Bessie Creek Sandstone. The elevated maturity of these oils suggests that an additional source rock deeper in the stratigraphic section, such as the Barney Creek Formation, may have contributed the first phase of hydrocarbons.

Alkane distribution profiles suggest that the Kyalla Member and the upper Velkerri Formation have sourced the hydrocarbons reservoired in the Moroak Sandstone.

Australian School of Petroleum



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