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A Petrographic And Geochemical Study Of Gippsland Basin Volcanics.

McPhail, Andrew

Honours Degree, 2000

University of Adelaide

Abstract

The Gippsland Basin has been a major producer of oil and gas for over 25 years. Exploration has now entered a mature phase. Intra-Latrobe Group and sub-volcanic Golden Beach Group reservoirs are now major targets. The project aims to investigate the nature of the volcanic seal of the major gas and oil accumulation at Kipper, and small, uneconomic accumulations at Manta, Gummy, Remora, Sunfish, Moonfish and Wirrah Fields. Volcanics of these northern margin fields were extruded during the Late Cretaceous Tasman Sea rifting.

Volcanics from the northern margin are described as extrusive olivine alkali basalts. They are highly altered and weathered rocks. These are compared with the less altered and weathered localised volcanics from Bream, described as alkali basalt. Initial alteration of basalt results in chlorite replacement of pyroxene. Iddingsite and orthoclase also replaced altered olivine crystals. Chemical weathering of alkali basalt altered plagioclase and pyroxene to clay minerals (particularly kaolinite, illite and smectite), and to zeolite and leucoxene. Alteration and weathering of basalt appears to have no effect on the sealing potential of basalt. Fractures within Kipper 2 basalts indicate that complete cementation of fractures occurred after chlorite alteration. Reactivation on major faults has been determined to be a primary factor in formation of fractures. Accumulation of hydrocarbons beneath fractured volcanics post-dates healing of fractures.

Geochemical data of northern margin and Bream volcanics exhibit a “within-plate” signature, typical of a continental - intercontinental rift tectonics. Altered samples from the northern margin and Bream can be discriminated by immobile trace elements, particularly Zr and Nb. Ni and Cr are used to differentiate of basalt from olivine-rich basalt to basalts lacking olivine. Major element analysis is used to describe weathering and carbonate alteration of the basalts.

Volcanism ceased relatively suddenly with active rifting continuing north along the eastern margin of Australia. The Gippsland Basin at the southern end is a failed rift or aulocogen. Emplacement of volcanic seals in the Golden Beach Group is diachronous with its reservoir facies. A fortunate tectonic framework allows the Gippsland Basin to host an unusual hydrocarbon play.

Australian School of Petroleum
THE UNIVERSITY OF ADELAIDE

SA 5005 AUSTRALIA

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