John C. McCaslin
Exploration Editor
Exploration in the Otway basin of southern Australia is at a moderately mature stage, according to S. Laing, C.N. Dee, and P.W. Best of the Claremont Group of Companies in Sydney.
Well density in the basin is one per 1,500 kM2, including offshore sections. The basin covers an area of about 150,000 kM2 both onshore and offshore southwestern Victoria and southeastern South Australia. This interesting basin is nearly 500 km long, trending west-northwest from the Mornington Peninsula in the east to Cape Jaffa on the west, containing over 10,000 m of Jurassic-to-Tertiary sediments.
PROVEN GAS PROVINCE
The Otway basin is a proven gas exploratory theater, with commercial gas production at Caroline 1 (carbon dioxide) and North Paaratte.
Although no commerical oil production has yet been found in the basin, oil has been recovered at Port Campbell 4, Lindon 1, and Windermere 1. The presence of excellent reservoir rocks in the basin, mature source rocks, and adequate seals, plus a number of untested play types and favorable economics, foretell significant wildcat prospects later.
The search for hydrocarbons in the Otway basin began way back in 1892 at Bore A near Kingston, S.A. Commercial production did not begin in the basin until 1986 when first gas sales were made at North Paaratte field, found in 1979. To date, 100 deep wells have been drilled in the basin, both onshore and offshore. Most of these wells were drilled on the basis of surface geology, gravity, or sparse seismic. The Australian geologists note in their APEA Journal paper, 1989, that the first deep well drilled in the basin was at Nelson Bore (Glenelg 1), a strat test that went to 2,226m.
The first offshore wells were drilled in 1967 by Shell and Esso. Results were nil at all of these marine wells. Onshore, North Paaratte was opened for 9.6 MMcfd of gas in the Waarre fm in 1979, followed by Wallaby Creek and Grumby gas pools in 1981. The gas accumulations at North Paaratte, Wallaby Creek, Grumby, Iona, and Caroline are all within the Waarre fm sandstone in traps formed by faulted anticlines. These fields contain up to 20 bcf of gas reserves each.
WITH PROMISING RESERVOIRS
The oldest and best developed unit in the basin is the fluvial Pretty Hill sandstone with a porosity of 25% and a permeability of over 1 d.
The Heathfield ss is a secondary reservoir in the basin, where present. The basal unit of the Sherbrook gp, the Waarre fm, is a more predictable pay unit, traditionally representing the primary reservoir of the Otway basin. Numerous gas shows have been reported in the formation, both onshore and offshore. In some cases, oil fluorescence has also been seen. The proximal deltaic deposits of the Late Cretaceous Paaratte fm consist of quartzose ss with generally excellent reservoir properties.
The Eocene Dilwyn fm is the youngest potential reservoir in the Otway basin. These sands generally have good reservoir qualities with porosities up to 30% and permeability of more than 1 d.
Copyright 1990 Oil & Gas Journal. All Rights Reserved.