Authors / CoAuthors
Boreham, C.J. | Hope, J. | Jackson, P. | Uwins, P.
Abstract
This work is a baseline study used to underpin the role of bacteria in the alteration and mineralisation of CO2 during geological storage following its injection into depleted natural gas reservoirs. In doing so it is paramount to first understand and characterise natural deep-earth biological systems. Here we report the molecular and isotopic signatures of gas, oil and formation waters from the biodegraded Tubridgi gas field. The onshore Tubridgi gas field is thought to lie at the end of a fill-spill chain from the offshore major oil and gas accumulations in the southern Barrow Sub-basin. An initial oil column at Tubridgi has been subsequently displaced by later gas charges. The Tubridgi gas is very dry (%methane/%ethane ~ 1000). Methane is isotopically light (delta13C = -49.2) and is depleted in 13C by ~10 compared to non-biodegraded gases from the Barrow Sub-basin. This, together with an isotopically heavy CO2 (delat13C = +1.8; ~6 enriched in 13C compared to non-biodegraded gas), suggests a major biogenic methane input derived from anaerobic methanogenic bacteria. The carbon isotopic composition of ethane (delta13C = -27) is only slightly enriched in 13C compared to non-biodegraded gas. Much larger enrichments occur in the hydrogen isotopes of ethane (deltaD = +42; ~180 enriched in D compared to non-biodegraded gas), suggesting anaerobic biodegradation has completely removed the higher (C3-C5) wet gases. This is supported by the less severely biodegraded Barrow Sub-basin natural gases, which can show up to 17 and 225 enrichment in 13C and D of propane, respectively, compared to non-biodegraded Barrow gas. Interestingly, the strong biogenic methane input seen in the carbon isotopes is not expressed in the hydrogen isotopes of methane (deltaD = -177 ), which is similar to the non-biodegraded gas. The Tubridgi-2 residual biodegraded heavy oil has a low API gravity of 23.5o and is the most sulphur-rich oil (S= 1.14 %) of all Australian oils. The gas-chromatogram displays an unresolved complex mixture with no n-alkanes. The level of biodegradation is heavy with 25-norhopane being present but no alteration of the sterane distributions are observed. The biomarker distribution of the Tubridgi-2 oil implies derivation from Late Triassic Middle Jurassic calcareous-influenced source rock deposited in a sub-oxic marine environment. Organic material extracted from the Tubridgi formation waters associated with the biodegraded gases mainly reflect the biodegraded oil input since very little low molecular weight `organics' was detected. Thus, the neutral organic compounds extracted at pH 7 are dominated by a homologous series of C19-C30 n-alkanes, while organic compounds extracted from acidified (pH 1) waters include a homologous series of C8-C18 n-alkylmonocarboxylic acids. The mutual exclusion between carbon numbers of the n-alkanes and n-alkylcarboxylic acids suggests a precursor-product relationship mediated by bacteria. However, the major organic components in the "acid" fraction are unidentified N and O containing compounds, most likely metabolic by-products of the biological activity. Cell counting is in progress, which will give an independent measure of the diversity and activity of the biological community within the reservoir.
Product Type
nonGeographicDataset
eCat Id
63520
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Cnr Jerrabomberra Ave and Hindmarsh Dr GPO Box 378
Canberra
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Australia
Keywords
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- External PublicationAbstract
- ( Theme )
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- geological storage of CO2
- ( Theme )
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- hydrocarbons
- ( Theme )
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- natural gas
- ( Theme )
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- organic geochemistry
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- AU-WA
- Australian and New Zealand Standard Research Classification (ANZSRC)
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- Earth Sciences
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- Published_Internal
Publication Date
2005-01-01T00:00:00
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