Fluid inclusion oils

Off-Line Crashing to Produce Fluid Inclusion Oils... 

GC-MS of the fluid inclusion oils can be performed on benchtop quadrupole, triple quadrupole, or sector instruments. The main criterion is sensitivity because such low amounts of oil are recovered from fluid inclusions, sometime lower-sensitivity instruments such as benchtop quadrupole instruments do not give sufficient quality data to be reliably interpreted. Further subordinate criteria are mass resolution and resolving power it helps to separate certain compounds using better than unit mass resolution and/or tandem mass spectrometry... 

Usually, fluid inclusion oils are measured by GC-MS as whole oils (i.e., without prior fractionation), because extraction yields are usually very low (too low for gravimetric analysis). However, fluid inclusion oils can be fractionated into total hydrocarbon fractions and nitrogen-sulfur-oxygen (NSO) fractions to avoid unwanted interference of NSO compound peaks with aliphatic and aromatic hydrocarbon peaks. Furthermore, samples that have significant n-alkane interference, in particular in their biomarker distributions, can be further separated into n-alkanes and branched and cyclic hydrocarbon fractions by using molecular sieves (e.g., ZSM-5 sieve such as silicalite, a zeolitlic form of silica). 

The branched and cyclic hydrocarbon fraction is isolated from the fluid inclusion oil by trapping n-alkanes in the molecular sieve. The fractionation is conducted in a Pasteur pipette plugged with glass wool and dry-packed with 5 cm of silicalite (about 650 mg). To ensure adequate packing, the pipette is gently tapped while loading, since too rapid elution may result in incomplete removal of C15+ n-alkanes. The fluid inclusion oil sample is then introduced onto the column in a minimum amount of solvent (n-pentane), and is allowed to adsorb onto the molecular sieve before the addition of solvent, dropwise at first. The branched and cyclic hydrocarbon fraction is then slowly eluted with 4 mL of n-pentane. If required, the n-alkanes may be retrieved from the molecular sieves by hydrofluoric acid digestion. 

This technique is widely used in petroleum geochemistry to correlate crude oils and source rocks [72,73]. More recently, a wider range of isotopes (H, N, and O) and applications have been developed (for a current literature snapshot, see Reference [74] and other papers in that special issue). However, CSIA has had rather limited application for analysis of fluid inclusion oils. This is because at least 0.5 nmol C entering the mass spectrometer (as CO2) is required to get a reasonably precise isotopic measurement [70], and this is roughly equivalent to a small size peak on a GC-FID. As fluid inclusion oils are commonly not detectable by GC-FID, but only by more sensitive GC-MS, this means that many cannot be isotopically analyzed. However, some success with richer fluid inclusion oils has been reported [75,76]. If possible, it is better to fractionate fluid inclusion oils prior to CSIA so as to obtain a cleaner n-alkane fraction, as this enables better and cleaner resolution of these compounds [76]. Purified n-alkane fractions can be obtained by adduction onto silicalite packed in a Pasteur pipette, followed by digestion of the silicalite sieve in hydrofluoric acid [77,78]. 

Unusually high abundances of water-soluble compounds such as benzene, toluene, xylene, and furan have been reported in some inclusion oil samples, particularly those lean in petroleum inclusions [81]. It has been well established that formation waters in oil fields contain water-soluble organic compounds due to partitioning between the oil and water phases [92-94]. One explanation for water-soluble compounds in fluid inclusions is that they derive from aqueous inclusions that typically co-occur with oil inclusions in petroleum reservoirs thus, the chemical composition of the inclusions partially reflects the composition of the water phase in the reservoir [81]. Fluid inclusion oils that are anoma-... 

FIGURE 30.9 Partial m/z 191 and 205 mass chromatograms for fluid inclusions oils and crude oil, South Pepper-1, showing the distribution of hopanes and methylhopanes. Peak assignments define the stereochemistry at C-22 (S and R) a 3 and 3a denote 17a(H),2ip(H) and 17 3(H),21a(H)-hopanes, respectively.Ts = C2718a(H)-22,29,30-trisnorneohopane Tm = C2717a(H)-22,29,30-trisnorhopane = diahopane and 2a(Me) = methylhopane. Numbers in italics refer to the relative abundance of chromatograms (modified from Reference [34]). 

Section 30.3.5 provides details on methodologies for isotopic measurements on fluid inclusion oils. In this section, two examples are provided to show the application. At... 

FIGURE 30.10 Carbon isotopic compositions of /i-alkanes in the Jabiru fluid inclusion oil, production oil, and urea nonadducted fraction of the production oil (modified from Reference [75]). 

A common observation concerning oil inclusion abundance is that paleo-oil columns are common not only below current oil columns, but also in wells that are completely dry at present. For example, in the Timor Sea region of northern Australia, there was an extensive period of fault-seal breach of oil reservoirs during Late Miocene/Early Pliocene fault reactivation, and this left many paleo-oil columns in the presently water-filled reservoir sections [28,128]. In dry wells, the analysis of fluid inclusion oils offers the possibility of understanding a petroleum system without having access to current fluids in the reservoir [49,55,129], and potentially then being able to predict where the oil may have leaked to, or where it may be trapped in nonbreached structures. 

FIGURE 30.11 Partial added m/z 128.1 + 142.1 + 156.1 + 170.1 + 184.1 mass chromatogram of the fluid inclusion oil from Octavius-2 (3200 m) in the Vulcan Sub-basin, northern Australia, showing the distribution of alkyhiaphthalenes.The enlargements show details of the partial m/z 170.1 and 184.1 mass chromatograms over the indicated retention times. EN, ethylnaphthalene... 

FIGURE 30.12 Partial m/z 85 mass chromatogram of the Ludmilla-1 fluid inclusion oil, showing the distribution of n-alkanes, monomethylalkanes, and isoprenoids. Compound abbreviations n-alkane /-Cxx, isoprenoid MUD, methylundecanes MDD,... 

Fluid Inclusion Oils in Igneous Rocks and Ore Deposits... 

Second, it is known that fluid inclusion oils are sometimes enriched in polar compounds compared with crude oils from the same reservoir [46,141], although this is not always the case [142]. This is likely due to fractionation of the oil in the reservoir, with preferential adsorption of polar compounds onto mineral surfaces [143-146]. The possibility of carryover of polar compounds highly adsorbed onto grain surfaces into inclusion oils is limited by the extensive cleanup steps advocated and can be checked by outside rinse blanks (see Section 30.3.1) and/or by the use of surrogate extraction standards [50],... 

Contaminants of unknown origin sometimes appear in fluid inclusion oils (e.g., C27 cholestane aaa 20R is... 

Volk, H., Boreham, C., Kempton, R.H., George, S.C. (2004) Geochemical and compound specific carbon isotopic characterisation of fluid inclusion oils from the offshore Perth Basin (Western Australia) implications for recognising effective oil source rocks. The Australian Petroleum Production and Exploration Association Journal, 44(1), 223-239. 

Volk, H., George, S.C., Killops, S.D., Lisk, M, Ahmed, M., Quezada, R.A. (2002) The use of fluid inclusion oils to reconstruct the charge history of petroleum reservoirs— an example from the Taranaki Basin. In Proceedings of the 2002 New Zealand Petroleum Conference, Crown Minerals, Auckland, pp. 221-233. 

Volk, H., George, S.C., Dutkiewicz, A., Ridley, J. (2005) Characterisation of fluid inclusion oil in a Mid-Proterozoic sandstone and dolerite (Roper Superbasin, Austraha). Chemical Geology, 223,109-135. 

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