Natural gases from all of Australia’s major gas provinces in the Adavale, Amadeus, Bass, Bonaparte, Bowen/ Surat, Browse, Canning, Carnarvon, Cooper/Eromanga, Duntroon, Gippsland, Otway and Perth basins have been examined using molecular and carbon isotopic compositions in order to define their source, maturity and secondary alteration processes.The molecular compositions of the gaseous hydrocarbons range from highly wet to extremely dry. On average, reservoired gases predominantly derived from land plants are slightly wetter than those derived from marine sources. The non-hydrocarbon gases CO2 and N2 were sourced from both inorganic and organic materials. A mantle and/or igneous origin is likely in the majority of gases with CO2 contents >5%. For gases with lower CO2 contents, an additional organic input, associated with hydrocarbon generation, is recognised where δ13C CO2 is A strong inter-dependency between source and maturity has been recognised from the carbon isotopic composition of individual gaseous hydrocarbons. This relationship has highlighted some shortcomings of common graphical tools for interpretation of carbon isotopic data. The combination of the carbon isotopic composition of gaseous hydrocarbons and the low molecular weight nalkanes in the accompanying oil allows our knowledge of oil-source correlations and oil families to be used to correlate gases with their sources. This approach has identified source rocks for gas ranging in age from the Ordovician in the Amadeus Basin to Late Cretaceous- Early Tertiary sources in the Bass and Gippsland basins. The carbon isotopic composition of organic matter, approximated using the δ13C of iso-butane, shows a progressive enrichment in 13C with decreasing source age, together with marine source rocks for gas being isotopically lighter than those from land plant sources. The Permian was a time when organic matter was enriched in 13C and isotopically uniform on a regional scale.Secondary, in-reservoir alteration has played a major role in the modification of Australian gas accumulations. Thus, biodegradation, prominent in the Bowen/Surat, Browse, Carnarvon and Gippsland basins, is found in both hydrocarbon and non-hydrocarbon gases. This is recognised by an increase in gas dryness, elevated isoalkane to n-alkane ratio, differential increase in δ13C of the individual wet gas components, a decrease in δ13C of methane and a reduction in CO2 content concomitant with enrichment in 13C. Evidence of water-washing has been identified in accumulations in the Bonaparte and Cooper/Eromanga basins, resulting in an increase in the wet gas content. Seal integrity is also a major risk for the preservation of natural gas accumulations, although its effect on gas composition is only evident in extreme cases, such as the Amadeus Basin, where preferential leakage of methane in the Palm Valley field has resulted in the residual methane becoming enriched in 13C.The greater mobility of gas within subsurface rocks can have a detrimental effect on oil composition whereby gas-stripping of light hydrocarbons is common amongst Australian oil accumulations. Alternatively, the availability of gas, derived from a source rock common to or different from oil, was likely to have been a prime factor controlling the regional distribution of oil, whereby mixing of both results in increased oil mobility and can lead to a greater access to the number and types of traps in the subsurface.

New data, including regional high resolution aeromagnet ic coverage, acquired by the New South Wales Department of Mineral Resources (DMR) as part of its Discovery 2000 Initiative, have provided the first opportunity for a comprehensive review of the regional framework of the Darling Basin. Covering an area of 90,000 km2 in central western NSW, the Darling Basin contains over 8,000 m of mainly Palaeozoic sediments. With only 17 petroleum wells drilled in the basin, mostly during the 1960s and 1970s, and some 1,550 km of modern multifold seismic coverage, the Darling Basin represents one of the major frontier basinal regions of onshore Australia.The initial phase of petroleum exploration was discouraged by the lack of shows, the likelihood of gas-prone source rocks and presence of a thick, red-bed dominated, organically lean, Late Devonian sequence. Renewed interest in the Darling Basin's prospectivity followed from favourable, albeit superficial, comparisons between the Darling Basin and Queensland's Adavale Basin, where commercial gas is produced at the Gilmore Gas Field. Additionally, as part of some $15 million expenditure by the DMR on acquiring new and reassessing old data from the Darling Basin, new geochemical analyses of extracts collected from core holes and out-crop suggest the presence of at least one active Palaeozoic petroleum system. This system has been responsible for generating oil and possibly substantial quantities of gas found dissolved within artesian waters in the overlying shallow Mesozoic sequences.

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Proton nuclear magnetic spin−spin relaxation time distributions (T 2 distributions) for a water-saturated cylinder of sandstone (radius 6.5 cm and length 17.55 cm) have been studied as a function of drying time. By freezing the sample to −13.7 °C, a proportionality constant of 38.8 μs/nm was determined that can be used to relate pore size to T 2. The results show external water cannot enter some intermediate sized pores of 400 nm diameter; however, these pores are available to water that has been frozen and allowed to thaw. On drying from saturation, water loss commences immediately from pores of the order of 1000 nm and greater and there is a lag period of 51 h before the smaller pores begin to drain. On freezing and then thawing, intermediate sized pores are filled because the large pores are blocked by ice.

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