Robinson A. Quezada

Constraining the oil charge history of the South Pepper oilfield from the analysis of oil-bearing fluid inclusions

Abstract The South Pepper oilfield, located in the Barrow Sub-basin on the NW margin of the Australian continent, has experienced a multi-phase charge history. Abundant oil-bearing fluid inclusions are present in samples from within the current gas cap, suggesting that an oil column existed prior to gas. This palaeo-oil (gas-leg FI oil) has Ts/Tm and C29/C30 αβ hopane ratios of ∼1 and the C35 homohopanes are a significant proportion of the extended homohopanes. It has lower Pr/Ph and diasterane/sterane ratios than the currently reservoired live oil and contains gammacerane, a series of peaks tentatively identified as C30 to C34 17α(H)-30-norhopanes and a large amount of 2α-methylhopanes. Collectively, geochemical analysis of the gas-leg FI oil suggests that it was generated from a less mature, more calcareous source rock, deposited under more reducing conditions than the Upper Jurassic Dingo Claystone, the main source of the live oil. In addition the presence of C30 dinosteranes in the gas-leg FI oil provides a Triassic or younger age constraint. This makes Palaeozoic carbonates an unlikely source. Possible source intervals for the gas-leg FI oil are thin, Lower Jurassic limestones and marls which occur at the base of the Lower Dingo Claystone, or a thin limestone unit (the Cunaloo Member) at the base of the Triassic Locker Shale. Samples from within the present oil-leg also contain abundant oil inclusions, consistent with high oil saturations at the present day. However, these oil inclusions exhibit different fluorescence colours, suggesting they represent a second oil charge. Geochemically the oil-leg FI oil has an intermediate composition between the live oil and the gas-leg FI oil, suggesting that gas charge displaced the first oil charge, samples of which were preserved as fluid inclusions in the oil-leg sample. Biodegradation of the first oil charge, indicated by the presence of 17α(H)-25-norhopanes in the currently reservoired live oils, can be attributed to the ingress of meteoric waters, probably during sub-aerial exposure of the basin margin during Miocene wrenching. Changing environmental conditions curtailed bacterial activity and allowed unaltered oil sourced from the Dingo Claystone to accumulate below the gas cap and mix with the biodegraded residues of the first oil charge to achieve the live oil composition. The biodegradation event must have preceded the second charge as the live oil contains compounds such as n-alkanes which would have been removed had alteration occurred after the second charge. Complex charge histories are common and the analysis of palaeo-oils trapped within fluid inclusions provides the opportunity to achieve a more comprehensive assessment of hydrocarbon charge

A study of the gross compositions of oil-bearing fluid inclusions using high performance liquid chromatography

A high performance liquid chromatography (HPLC) method for assessing the compositions of crude oils and fluid inclusion oils is described. The necessary increase in instrumentation sensitivity was accomplished using a narrow bore Waters Spherisorb column with 5 μm particle diameter. This HPLC method has advantage over the latroscan TLC-FID technique previously described as the < C 16 fraction is measured by the HPLC method. However, due to uncertainties in the calibration procedure, the HPLC method is only semi-quantitative. Aliphatic hydrocarbons were calibrated taking into account the molecular weight distribution of a typical oil derived by gas chromatography. Aromatic hydrocarbons were calibrated using standards according to retention times, but the response factors span two orders of magnitude, dependent on ring size. Polar compound response factors are strongly standard dependent. The gross composition of a suite of 12 inclusion oils and 17 co-occurring crude oils from various basins in Australia and Papua New Guinea have been analysed. The inclusion oils at the Jabiru and Tirrawarra wells are enriched in polar compounds relative to the co-occurring crude oils from these wells. This is attributed to an adsorption effect during trapping. However, many inclusion oils do not appear to be polar enriched, which may relate to trapping of inclusion oils in sealed fractures as well as on overgrowths. Inclusion oils tend to be relatively depleted in aromatic hydrocarbons, which could reflect a trapping phenomenon, with preferentially less aromatic hydrocarbons and/or more waxy aliphatic hydrocarbons being trapped in fluid inclusions. Alternatively, this may be a characteristic of the different chemistry of oils that are preserved early in the charge history of a reservoir.

Chemical Composition Of Humic Acids: A Comparison With Precursor ‘light Fraction’ Litter From Different Vegetations Using Spectroscopic Techniques

Humic acids from a range of Italian soils and the residual suspended organic material (light fraction litter) associated with them have been studied by pyrolysis gas chromatography mass spectrometry, solid state nuclear magnetic resonance spectroscopy, and infra-red spectroscopy. Although the major vegetation types on the soils differ considerably, spectra of humic acids from soils with different major vegetation types were similar. Because no relationship between humic acid structure and major vegetation type was observed, gross assumptions about humic acid structure should not be drawn from a knowledge of macro-vegetation types on soils. Light fraction litter in the soils from minor vegetation may be more significant in affecting the eventual structure of the humic acids.

Organic compounds trapped in aqueous fluid inclusions

Fluid inclusion samples from several Australian oil wells have been analysed to document the prevalence and composition of volatile hydrocarbons contained within aqueous inclusions. These results clearly establish that trapped palaeo formation waters can be a source of such compounds, which are frequently predominant in samples with a low content of oil-bearing inclusions. The apparent “anomalous” hydrocarbon distributions derived from aqueous inclusions contain abundant water-soluble compounds, such as benzene and toluene, which may originate from interaction of formation waters with subsurface petroleum accumulations. Aqueous inclusions are also often enriched in alkenes and oxygenated species, such as furan, which are minor constituents of petroleum but could form via secondary processes such as anoxic microbial degradation in formation waters. The co-occurrence of aqueous-derived organic compounds within samples containing oil inclusions suggest the need for caution when interpreting volatile hydrocarbon distributions. However, the presence of these components in samples from dry wells could be used as a tool to substantiate the proximity of a petroleum accumulation in an area which would otherwise be considered to have low prospectivity.

The degradation of wood in old Indian Ocean shipwrecks

Abstract Woods from five old ships wrecked in the Indian Ocean over the period 1629–1886 have been studied by Fourier transform infra-red spectroscopy, by solid state 13 C nuclear magnetic resonance spectroscopy and by pyrolysis gas chromatography/mass spectrometry. The results from all three techniques strongly support each other. It is shown that samples of outer wood in closer proximate contact with the marine environment have lost carbohydrates, whereas the inner wood is relatively intact. There appears to be no correlation between the age of ships and degradation, however one wreck ( James Matthews ) which is located at an anaerobic site appears to have undergone little or no degradation, unlike the samples obtained from aerobic sites.

Surface changes in coal hydrogenation autoclaves that affect autoclave reactions

Abstract The morphology and chemical composition of the internal wall of a batch autoclave reaction vessel, which had been extensively used for coal hydrogenation studies, have been investigated. Pits resulting from corrosion of the vessel wall were found to contain three types of deposit. Two of the deposits are of carbonaceous and siliceous matter and can be regarded as non-catalytic. The third type of deposit consists of mixed sulphides. These are known to exert a catalytic effect in coal hydrogenation. The results show that a steel reaction vessel used for coal hydrogenation can retain ‘memory effects’ from previous experiments.

Reactivity of methane at low temperatures (∼400°C) with ‘refractory’ bonds

Abstract The thermal cracking of benzene, toluene, diphenylmethane, biphenyl, diphenyl sulfide, diphenyl ether, phenol and dinaphthol was studied in the presence of inert gases, methane, methane-hydrogen mixtures and Cu-Beta, H-Beta and Cu-ZSM-5 catalysts at various reaction temperatures (350–480°C), pressures (3.5–9 MPa) and times (1–4 h). For some systems methane assists conversion by either methylation (benzene, biphenyl) or debenzylation and methylation (diphenylmethane). In others, where methylated materials are reactants, demethylation may occur at the same time as debenzylation. The activities of the various catalysts are compared. Their effectiveness in conversion or selectivity varies with organic substrate. Two mechanisms of conversion are identified. One involves direct addition of methyl groups, the other includes a disproportionation process probably resulting in carbon deposition.

Determination of total, organic and mineral nitrogen in oil shales☆

Abstract Comparison of methods for determining the total, organic and mineral nitrogen contents of oil shales has shown that the Dumas (combustion) technique effectively measures the total nitrogen content, which is underestimated by the Kjeldahl procedure if ammonium-substituted feldspars (e.g. buddingtonite) are present. The difference corresponds to the mineral nitrogen content as determined using a hydrofluoric acid digestion/distillation technique. Low temperature ashing (plasma or conventional) is not an appropriate method for removing organic nitrogen. The ashing procedure is time-consuming, and incomplete ashing and formation of nitrates cause errors in the subsequent mineral nitrogen determination. The Dumas method and the hydrofluoric acid digestion/distillation technique are recommended for the determination of total and mineral nitrogen contents, respectively, in oil shales.

Tetraethyl lead as a coal liquefaction promoter

Abstract Tetraethyl lead has been shown to be a useful reagent for liquefying a range of bituminous Australian coals at temperatures of 375–425 °C and at low (6.9 MPa cold, charge) hydrogen pressures. A hydrogen carrier vehicle e.g. tetralin, is not necessary. For Millmerran coal, liquid yields in excess of 50 wt% coal were obtained at 425 °C after a reaction time of 1 h. Under these conditions the product was a bituminous tar-like material (termed heavy oil) of weight average molecular weight 400–600 g mol −1 as determined by gel permeation chromatography. However, at extended reaction times (4 h), small (10 wt%) yields of volatile products were produced, and when heavy oil was further upgraded with hydrogen (6.9 MPa) almost all the original coal was converted to volatile oil and gas. In contrast, conventional hydrogenation with tetralin at similar temperatures and pressures produced only a solvent-refined coal which could not be distilled. However, yields of liquid products obtained from coals and distillation residues from continuous hydrogenation in the presence ot tetraethyl lead were not as high as those produced by iodomethane under the same conditions.

Role of minerals and additives during kerogen pyrolysis

Abstract Kerogen has been pyrolysed with additives (including minerals found in shale) under various flow rates (50–250 ml/min) of helium to sweep the product oil from the kerogen and mineral residues. For the product oil, carbon preference indexes for alkanes and alkenes, relative proportions of external and internal alkenic protons and carbon aromaticities were measured. The aromaticities of the pyrolysis residues were also measured and the fraction of aliphatic carbon converted to aromatic carbon during pyrolysis was calculated. The ratio of alkenes with external protons to alkenes with internal protons was altered by some of the additives and changes in the carbon preference indexes were also observed. Only charcoal and massive (10:1 kerogen) amounts of pyrite induced aromatisation. It is suggested that alkene and alkane formation during kerogen pyrolysis is independent of aromatisation and much of the aromatisation is due to the intrinsic structure or the kerogen and not to mineral matter effects.