HENC background___________________________________________
The Mesozoic Eromanga Basin, a subdivision of the larger Great Australian Basin, has been relatively unexplored in New South Wales but is a major petroleum producing area in the adjacent states of Queensland and South Australia.
The sediments of the Eromanga Basin represent the shallower shelfal areas of the main basin, located in Queensland and South Australia. The New South Wales components are considered distal from the recognised basin centres and main source kitchen areas associated with those depocentres.
The Eromanga Basin in New South Wales runs along the Queensland border from South Australia to about longitude 147E where easterly thickening of sediments across the Cunnamulla Shelf / Nebine Ridge marks a transition into the Surat Basin.
Due to the lack of seismic exploration data and the widespread regolith cover, in 1994 the Department engaged Encom Technology to integrate and interpret existing geophysical data sets covering the Eromanga and Surat Basins. The objective of the study was to investigate the geological structure of the region with particular reference to the implications for hydrocarbon potential. In the absence of significant seismic coverage regional gravity and aeromagnetic data constituted the principal data sets, with additional control from water bores, surface mapping and satellite imagery.
Magnetic & Gravity Data
The magnetic character of the Eromanga basement is seen as broad semi-circular shaped magnetic anomalies reflecting the presence of meta-volcanic units of Lower Devonian age, or older, and possibly similar to rocks occurring in the Cobar region. High amplitude magnetic anomalies in the north-eastern and central eastern portion of the basin in New South Wales are believed to be associated with Lower Palaeozoic volcanics, similar to those found at the base of the Bancannia Trough near Broken Hill. High amplitude trends on the western margin are associated with Precambrian rocks of the Wonominta Block.
The interpretation has largely focussed on depth to basement determinations, basement being buried beneath an almost ubiquitous blanket cover of Eromanga sediments. The most reliable depth estimates have been derived from analysis of high resolution magnetic field data, whereas regional gravity coverage mostly shows large scale structures. The interpretation reveals the presence of several previously unrecognised basement deeps. These are probably infilled with Devonian age sediments, and include the Urisino Trough, Enngonia Embayment and Brewarrina Trough. The deepest and most conspicuous trough, the Paka Tank Trough, is interpreted to be similar to the Bancannia Trough, which extends beneath Eromanga sediments near the South Australian border. Similarly the Morden Trough is an extension of the Darling Basin under the Eromanga Basin. The fault-related Caryapundy Trough in the north west of the area may contain Permian or thickened Mesozoic sediments. The fault bounding the Caryapundy Trough is also parallel to the eastern edge of a contemporaneous swampy area known as the Bulloo River Overflow. The lineament and the swampy area suggest the area may still be undergoing active subsidence.
Some gravity lows are interpreted to reflect the presence of older basins or troughs beneath the Eromanga Basin. Analysis of basement depth from gravity data alone proved unreliable because there are also gravity lows due to intrabasement granites. Discriminating intrabasement granites from basins containing thickened Mesozoic or Palaeozoic sediments has not proved possible with the existing gravity dataset. Most of the pre-Eromanga sediments are believed to be of Devonian age, similar to those of the Darling Basin to the south and the Adavale Basin to the north. However, the sharpness of the gravity anomalies suggests that in some cases there is also a contribution from a younger section which could be either thickened Eromanga section or an intermediate Permian section. The coincidence of thickened Mesozoic or Permian above preserved Devonian infra-basins is quite widely observed throughout Eastern Australia and presumably reflects differential compaction.
Bore Water Geochemistry
As a result of images of a land owner lighting gas flowing from an artesian waterbore (as a party trick for visitors to his farm stay business) on a television program, in 1995 the Department began a project to sample water and/or gas from artesian water bores in the Great Australian (Artesian) Basin (GAB) for analysis of desolved gas quantity and composition. This was considered as a relatively inexpensive potential petroleum exploration method.
A surprisingly large number of water bores in the Eromanga Basin and adjacent Surat Basin were drilled to basement. These bores, combined with bores reaching only shallower aquifers, provide the opportunity for investigation of not only the stratigraphy, but also the type, source and quantity of hydrocarbons in artesian waters. The bores draw water from the Wyandra and Hooray Sandstones in the Eromanga Basin.
Water samples are collected directly from the flumes of flowing bores, from pressure plugs in the headworks of bores, or from pumped subartesian bores. Several samples of free flowing gas have also been collected. CSIRO Petroleum uses gas chromatography to analyse quantity and composition of dissolved hydrocarbons and incorporates mass spectrometry to measure C13 isotope composition of methane and carbon dioxide.
Samples taken from the Surat Basin are dominantly dry in composition with C13 isotope ratios indicating a biogenically affected or coal seam derivation. John Smith from the CSIRO has described a process whereby after catagenesis, methane and ethane may be converted to carbon dioxide, which may be subsequently reduced back to methane in an environment, such as an aquifer, leaving dry gas only. The C13 composition of methane is consequently altered to isotopically light signatures (depleted in C13), mimicking those of biogenic gas. This mechanism can be used to explain the origins of gases observed in bore waters as coal seams may exist in concealed Permian-Triassic troughs.
Many samples taken from the Eromanga Basin also show dry gas composition but several samples show wet gas compositions and, in one sample (Fort Grey), ethane plus butane exceed methane. The isotopic composition of the Eromanga methane is more enriched in C13 and closer to that of thermogenically derived gases. This complements earlier work by Kells Investments which inferred a thermally mature source for solvent extractable hydrocarbons in bore waters from the southern Eromanga Basin. Interestingly, the CH4 C13 composition from the Hollywood bore in the Surat Basin is similar to that of Eromanga Basin gases.
While the sampling methodology used does not provide 'hard numbers' on quantities of hydrocarbon generated, the results appear to suggest that the method can be used to identify potential exploration targets. For example there is a good correlation between anomalously high hydrocarbon content and the faulted northern boundary of the Paka Tank Trough.
Petroleum Potential
The New South Wales portion of the Eromanga Basin remains under-explored. Large distances from recognised source kitchens and the perceived lack of in-situ mature petroleum source rocks has deterred exploration efforts.
New reprocessing of geophysical data and newly acquired seismic data have shown the existence of previously unrecognised troughs beneath the Eromanga Basin. The pre-Mesozoic fill of these structures is considered to be mainly Devonian aged, although possibilities for Permian-fill have been identified. The basement deeps may also contain thickened Mesozoic strata, however no stratigraphic data exists for these areas. Excellent porosities and permeabilities in the Hooray Sandstone have indicated the existence of reservoir rocks in the Mesozoic section. Geochemical analysis of the artesian waters attests to the presence and movement of hydrocarbons in regionally extensive aquifers in the Eromanga Basin.
A seismic survey carried out by the Department during 1996 across the Paka Tank Trough north of Bourke confirmed the presence of a previously unproved trough underlying Mesozoic sediments. Although there was no direct stratigraphic tie and control, the bulk of the trough shows low frequency reflectors interpreted as Devonian sediments, the gross seismic character being similar to that of Devonian sediments filling the discrete grabens and troughs within the Darling Basin. The seismic sections within the Eromanga also show onlapping sediments, drape anticlines of Mesozoic sediment over basement highs and the existence of Late Cretaceous -Tertiary structuring resulting in compressional folding and faulting. This age of structuring is the same age as trap formation in existing oilfields such as Tintaburra, which is located immediately to the north in Queensland.
In late 1998 the Department again confirmed the presence of the Paka Tank Trough (this time intersecting the trough further to the northeast, north of Brewarrina) with seismic line DMR98-02 (see figure). Both the 1996 and 1998 seismic lines suggest that the northern boundary of Paka Tank Trough is defined by the Thomson Fold Belt overthrusting the Lachlan Fold Belt.
Brewarrina 1 drilled by the Department in 1999/2000 penetrated the Paka Tank Trough south of Weilmoringle. The well intersected and confirmed Devonian sediments beneath an Eromanga Basin sequence.
Recognition of the troughs underlying the Eromanga Basin provides new possibilities for the presence of thickened sections containing mature source rocks adequate for local hydrocarbon generation. A possibility highlighted by wet gases in the artesian waters of the basin. Identification and delineation of basin deeps beneath the Eromanga Basin is essential to formulating a regional framework and to understanding basin geometry and structural evolution in New South Wales.
The Department recognises that much more work is required to understand the distribution of sediments and hydrocarbons within the Eromanga Basin within New South Wales. Recent studies demonstrate that there is sufficient evidence of the petroleum potential in the basin to warrant greater exploration activity than is currently occurring.
