Publication Name: Eromanga Basin Symposium, Summary Papers
Authors: C. SPRIGG
Publication Volume: Adelaide, 9 - 11 November 1982
Date Published: December 1982
Number of Pages: 31
Reference Type: Conference Proceedings
Abstract:
The Great Artesian Basin is devoid of obvious hydrocarbon seepages. Most ofits deeper sediments are non-marine. None of the widespread thousands of Artesian
water bores produced a single flow of oil or gas. However, small volumes of methane were known to escape with artesian water from the Coononanna and
Yandama bores lying north-east to Lake Frome and elsewhere.
Early geologists regarded the Artesian Basin as a structurally stable, Mesozoic
development; a simple saucer-like depression interrupted only by
buried basement ridges and local warpings. Dr. R. L. Jack in 1924~first
mapped definite fold structure at Haddon Downs in the NE corner of the State
in the course of underground water investigations. His factual geological
cross-sections disclosed 200 metres of structural turn-over in Tertiary outcrop,
overlying with strong erosional unconformity a still steeper,limbed late
Cretaceous anticline. The significance of this Mesozoic-Tertiary growth structure
went almost unrecognised for another 30 years. Dr. Woolnough visiting
the area in 1927, missed this and, at Innamincka, reported only flat-lying
Tertiary mesas surrounding a central core of "gneissic granite". This was presumably
the giant Innamincka dome development in thick Mesozoic and Palaeozoic
sediments.
The first Eromanga crude oil "discovery" came fortuitously in 1925 in drilling
the Longreach Town water bore. Lander Oil Co. moved in and, two years later,
confirmed the discovery by producing several barrels of waxy crude ojl from
Lower Mesozoic sands overlying granite basement.
1927 saw growing eastern States' interest in the conversion of coal to oil.
This led to authoritative pronouncements that Australia's extensive coal basins
may well source signigicant petroleum, a fact which most geologists seemed unable
to accept for ~nother 30 years or more. This was also despite Cosmo Newberry's
discovery in 1890 of traces of free oil in the Leigh Creek, Triassic
coal measures, and the discovery of petroliferous gas at Roma in 1900.
Iriomo Oil Ltd. subsequently took over regional exploration about Longreach,
and then, late in the 1930's, Shell Development (Queensland) extended geological
exploration in this area and carried gavjty-magnetic surveys far across S.W.
Queensland. In the mid 1940's, Zinc Corporation reported favourably on the natural
gas potential of the Frome Embayment and centred their initial activities
on the Joulnie anticline. Frome Broken Hill Co. then expanded gravity-magnetic
surveys across Innamincka Dome ( then unrecognised) to Haddon Downs and to the
Flinders Ranges. Interpretation of up to 3,-00 metres of supra-basement sediments
was noted south of Innamincka. Seven wells were drilled by the early
1950's without encouragement. In the late 1940's, Dr. Ivan Tschebotarev of the
S.A. Mines Department reviewed hydro-chemcial aspects of artesian basin waters
and concluded on the basis of his Russion oil-field experience that the best
place to seek "Jurassic" oil in S.A. was in the north-east.
In 1952, W. D. Mott summarised oil and gas showings in artesian wells to be
widespread in Queensland, thereby providing new encouragement. Still it was the
1954 discovery of Mesozoic flow oil in Rough Range W.A. that was to provide real
exploration impetus. John Bonython then formed Santos Ltd. for whom Reg Sprigg's Geosurveys of Aust.Ltd. operated and progressively took up most of the centralwestern
basin as major companies lost interest. An anticline was mapped and
drilled near Oodnadatta to disclose free oil traces in the Jurassic and Lower
Cretaceous. Interest was then transferred to Haddon Downs and Innamincka where
groups of anticlines were shown to extend far back into Queensland (Sprigg 1958).
The major drainage occupied the broader synclines. BMR seismic then confirmed
the Oodnadatta and Haddon anticlines in depth, after which Santos farmed out a
half interest in all its acreage to Delhi Taylor Corp. of Dallas. The drilling
of Innamincka and Beetoota anticlines thereafter revealed oil showings at a
number of levels within the Jurassic as well as gas showings in Innamincka
Permian. Low oil prices and poor test results accorded the Jurassic shows little
interest. Five wells later, gas discovery in the underlying Permian deflected
interest almost completely away from the Jura-Cretaceous for the next decade or
so.
Commonwealth Government exploration subsidies in the late 1950's greatly encouraged
more sophisticated oil exploration technilogy. This culminated in widespread
hydrocarbons discoveries across Australia, as well as grossly improving
basin understanding generally.
By 1970, Brooks of C.S.I.R.O. and others were clearly regarding Australia's
sedimentary coaly detritus as potential sourcing material for petroleum generation.
Coal hydrocarbons were seen to change abruptly in the sub-bituminous coal
range to produce petroleum-type hydrocarbons from components of waxy leaf
cuticles, pollen and spore casings. They noted further that oil occurs in the
Cooper infra-basin where the coals are of lower rank (80-85% Carbon). The
central deeper Eromanga Basin also proved to fit this category. Thomas (1982)
concluded further that land plants contributiori to oil formation related particularly
to ac~umulation in the paralic-deltaic or marginal marine environment to
which a number of siltstone-shale sequences of the Eromanga Jurassic related.
It was concluded that generation and migration of petroleum fluids was
primarily a function of time, temperature and depth of burial. Furthermore,
Thomas (1982) noted that pre-Jurassic coal measures are deficient in exinite
materials and are therefore mainly gas-prone. In contrast, Jurassic to Tertiary
coal-rich sediments often contain abundant exinite and may have substantial
potential to generate oil in commercial quantities. This is the result of the
dominance of conifers in the swamp floras of those periods, together with the
evolution of flowering plants in the late Cretaceous.
Pooloowanna Eromanga oil to the surface was discovered in the Simpson Desert by
the Santos-Delhi-WMC partners in 1977. Still it was not until 1978 that the
first commercial oil discovery was made in Dullingari No.8 well with a flow of
2,810 b.p.d. from the Murta Formation, soon after which Strzelecki No. 4 flowed
3,500 b.p.d. from the Hutton. By now, sands all the way from the basal Jurassic
up into the Cretaceous Coorikiana and Toolebuc Formations were recording oil
flows or encouraging showings. A broad Jura-Cretaceous "oil window" appears now
well established in relation to temperature/depth parameters.
Timing of structuring in relation to migration is undoubtedly important. Undoubtedly,
the thicker, more rapid deposition of sediments climaxed in late Cretaceous
to earliest Tertiary time during which burial would have been sufficiently deep
to generate hydrocarbons and activate their expulsion. Too, the full significance
of hydrodynamic flushing in stratigraphic entrapment or oil escape has
still to be assessed but the discovery of fossil oil seepages within the Lake
Eyre mound springs system by the S.A. Mines Department indicates that it could
be a significant factor.
Import parity pricing of crude oil introduced by the Commonwealth Government in
the mid-1970's has acted to greatly stimulate ongoing oil search. Continued
sound pricing will undoubtedly result in far more widespread discovery.