ABSTRACT: Carbonate Reservoir Rocks in the Mid-Proterozoic McArthur Group, Northern Territory

KARAJAS, JOHN, Mataranka Oil N.L., Western Australia, Australia

The McArthur Group has an age of approximately 1,600-1,700 million yr. It contains a number of black shale sequences which are alginite rich and which are good to excellent oil-prone source rocks. Maturities of these shales range from marginally mature to overmature for oil generation. Direct hydrocarbon indications in McArthur Group rocks include numerous examples of live oil bleeding out of core in exploration drillholes, abundant bitumen occurrences, and, in addition, an extended gas blowout from a mineral exploration drillhole.

Preliminary study has revealed that the McArthur Group contains at least three valid carbonate reservoir objectives which are as follows: (1) open fracture sets with, in places, associated vuggy porosity, (2) preserved sucrosic, fenestral/vuggy, and/or intraparticle porosity in paleoshoals, and (3) dolomitized turbiditic carbonate beds in the Barney Creek Formation.

Open fractures appear to be more abundant in carbonate beds which display additional types of porosity. In places, different generations of fracturing can be demonstrated, with older, healed fractures occurring adjacent to younger, open fractures.

The relationship between preserved primary porosity and paleoshoals/paleohighs is excellently displayed in the Glyde River No 9 mineral exploration drillhole which yielded the extended gas blowout. This drillhole intersected an areally distinct carbonate bed, approximately 11 m thick, near the base of the Barney Creek Formation. Approximately 50% of this bed carries porosities averaging about 10%.

Carbonate reservoir rocks proximal to oil-prone source rocks are likely to occur in a variety of structural settings in the McArthur River area.

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