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AAPG Bulletin

Abstract


Volume: 31 (1947)

Issue: 8. (August)

First Page: 1341

Last Page: 1371

Title: Geology of Roma District, Queensland, Australia

Author(s): Frank Reeves (2)

Abstract:

Since the discovery of gas in 1900 in drilling for water at Roma, southeastern Queensland, 40 wells have been drilled in the area. A few of these wells encountered some gas and showings of oil. Most of the wells were drilled in areas where the outcropping Cretaceous and Jurassic strata show no evidence of structure except for a slight southward tilt toward the center of the Great Artesian Basin. Consequently, most of the drilling was a hit-or-miss affair. About all that was known about the subsurface geology was that basement rocks in the vicinity of Roma underlay the surface at depths of 2,000-4,100 feet, and that oil and gas showings were found slightly above the basement rocks in non-marine formations thought to be Jurassic in age.

In 1933 Oil Search Ltd. of Sydney initiated a program of geological work and exploratory drilling in the region. During 1934 and 1935 a large area was mapped north of Roma, and two or three promising structures were outlined. During 1934-1939 Oil Search drilled three deep tests in the area, two of which were located on pronounced anticlines located 60-85 miles north of Roma, the other being a test of a minor structure a few miles east of Roma. Gas was struck in two of the wells, but only slight showings of oil were encountered. The main facts established by the geological investigations and exploratory drilling are as follows.

1. Well defined anticlinal folds are present in outcropping Permian strata 80-120 miles north of Roma. Although a slight unconformity exists between the Permian and Triassic, and a marked unconformity occurs at the base of the Upper Triassic, the folds in the Permian can be traced southwest in the mild folds of the Triassic, but disappear in the gently southerly tilt of the Jurassic strata.

2. The combined thickness of the Jurassic and Triassic is only 3,500-5,500 feet, and not 12,000 feet as formerly estimated. The Permian strata at their outcrop are only 5,100-7,000 feet thick, or about half as thick as previously reported.

3. The principal oil and gas showings at Roma were found in sandstones and grits of Triassic, not Jurassic age.

End_Page 1341------------------------------

4. None of the forty wells drilled in the region, except Oil Search's Hutton and Arcadia tests, was located on an appreciable fold. All earlier wells, except the one at Wallumbilla, were situated on a southeasterly trending basement ridge, the crest of which is transgressed by Triassic strata.

5. The gas encountered in the Triassic near Roma and in the Permian at Arcadia probably originates from Permian carbonaceous strata. The oil encountered at Roma may have had its source in the few hundred feet of marine strata that occur in the middle and lower Bowen series of Permian age. All other strata below the thin mantle of Cretaceous in the vicinity of Roma are non-marine in origin.

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