Abstract
Detection of the chemical constituents of hydrocarbons in the hydrocarbon-bearing fluid inclusions in diagenetic mineral cements, secondary fractures and overgrowths could be a useful indicator of the nature of oil in a basin. Microscope-based Raman spectroscopy is a non-destructive, optical vibrational spectroscopic technique that can precisely isolate and analyse hydrocarbon fluid inclusions (HCFIs). The main challenge with Raman spectral studies on natural HCFIs is the common presence of fluorescence emission from minerals and aromatic compounds in HCFIs leading to the masking of Raman signals. The present study is a demonstration of how best the Raman signals from natural hydrocarbon-bearing fluid inclusions could be detected using an excitation wavelength of 785 nm with suitable optical parameters and with special wafer preparation techniques to negate the background fluorescence. Using the laser Raman technique we were able to detect peaks corresponding to cyclohexane (786 and 3245 cm −1 ), benzene and bromobenzene (606, 1010, 1310, 1486 and 1580 cm −1 ), carbon monoxide (2143 cm −1 ), nitrogen (2331 cm −1 ), ethylene (1296 cm −1 ), sulphur oxide (524 cm −1 ), carbonyl sulphide (2055 cm −1 ), hydrogen sulphide in liquid form (2580 cm −1 ) along with the presence of a broad peak of liquid water at 3100–3500 cm −1 , peaks of calcium carbonate (710, 854 cm −1 ) and calcium sulphate (1135 cm −1 ). The study samples were specially prepared with fluorescence-quenching dyes added with a resin-hardener mixture to eliminate background fluorescence. Nine fluid-inclusion assemblages in minerals like quartz, feldspar and calcite from the RV-1 well of the Ratnagiri Block, Mumbai Offshore Basin, India were investigated.
Highlights
Fluid inclusion studies can provide geological information of fundamental importance to the petroleum exploration and production industry
Selection of optimum excitation wavelength is another challenge, while many previous studies noticed the problem of fluorescence from the fluids contained in the inclusions or by the surrounding host mineral with shorter excitation wavelengths around 260 nm used for Raman analysis of Hydrocarbon fluid inclusions (HCFIs)
By analysing inclusions on a grain-by-grain basis, it may be possible to differentiate inclusions from different source rocks. Such information is of significant importance to the petroleum exploration industry, concerning the study of petroleum reservoirs, where it may be necessary to chart small changes in fluid composition (Karlsen et al 1993; Ryder et al 2004)
Summary
Fluid inclusion studies can provide geological information of fundamental importance to the petroleum exploration and production industry. The known techniques require the extraction of the liquid from individual inclusions by drilling (mechanically, laser assisted or ion etched) and subsequent analysis by gas chromatography (GC) and/or mass spectroscopy (MS) (George et al 2001; Parnell et al 2001; Blamey & Ryder 2007) This results in sample destruction and it is timeconsuming, presumably needing big fluid inclusions too. The limitations with the fluorescence technique in studying petroleum inclusions are well explained by Burruss (2003a); Blanchet et al (2003) gave a detailed analysis of fluid inclusions from the North Sea using microspectrofluorometric techniques and they observed that the fluorescence colour of inclusion oils is variable even within a single fluid inclusion assemblage This necessitates the need for non-destructive optical techniques for accurately characterizing the HCFIs. Compositional studies of individual inclusions have been conducted using vibrational spectroscopic techniques. Accurate analysis of the chemical composition of the entrapped hydrocarbons in HCFIs yields vital information about the history, evolution and migration of petroleum fluids, and is crucial data for the petroleum exploration industry (Goldstein 2001; Munz 2001; Frezzotti et al 2012)
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