Feasibility of a 785 nm diode laser in Raman spectroscopy for characterizing hydrocarbon-bearing fluid inclusions in Mumbai Offshore Basin, India

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. Microscopebased 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 fromminerals 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 nmwith suitable optical parameters and with special wafer preparation techniques to negate the background fluorescence. Using the laser Raman techniquewewere 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 fluidinclusion assemblages in minerals like quartz, feldspar and calcite from the RV-1well of the Ratnagiri Block,Mumbai Offshore Basin, India were investigated. Received 29 April 2016; revised 23 September 2106; accepted 18 November 2016 Fluid inclusion studies can provide geological information of fundamental importance to the petroleum exploration and production industry. Organic matter present in sediments is transformed into organic liquids and gases when such sediments undergo diagenesis. The composition of petroleum fluids is complex and its species-wise characterization is fundamental in the geological interpretation of the source and maturity of oils (Goldstein 2001; Munz 2001). The organic fluids present in rock include hydrocarbons ranging from methane to high molecular weight hydrocarbons and these fluids can undergo complex chemical changes depending on the conditions present during their migration and pooling. Hydrocarbon fluid inclusions (HCFIs) can provide information on the composition, as well as the conditions under which the fluids were emplaced (Roedder 1984). Hydrocarbon inclusions are therefore of interest because their compositions are sensitive to the complex interplay between source, transport and deposition, while the variation in composition between present-day crude oils and inclusions can provide information on the genesis of the fluids (Orange et al. 1996). The rapid expansion of research on the petroleum inclusions is due to the simple fact that petroleum inclusions are ‘hidden petroleum shows’ (Lisk et al. 2002). Analysis of petroleum inclusions and the application to exploration and reservoir appraisal have experienced rapid development during the last decade. Primary and secondary petroleum inclusions in fractures within cements may give evidence for the presence of petroleum during migration and subsequent cement formation (Karlsen et al. 1993, 2004; Munz et al. 1998; Munz 2001; Karlsen & Skeie 2006). Petroleum migration can be inferred from compositional differences between inclusions in successive growth zones of fracture cement samples (Bodnar 1990) or from the presence of different populations of inclusions in different generations of cements (Burruss 2003b). HCFI analysis can be performed either by crushing bulk rock samples and extracting the trapped fluid for analysis or by the analysis of single fluid inclusions (Kihle 1995; George et al. 2001; Anderson et al. 1998; Parnell et al. 2001). The oil composition data obtained from bulk fluid inclusion analysis (by crushing) suffers from a variety of problems, including sample destruction, mixing of fluids from multiple fluid inclusion populations and contamination from materials within the rock sample itself. The mixing of fluids is especially disadvantageous because the most volumetrically abundant type of inclusion will bias the results, and the constituents of aqueous fluid inclusions could contaminate the extracted fluids. Analysis of fluids entrapped in a single HCFI is potentially much better since the system is a hermetically sealed one, but there are few quantitative techniques. 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). Unfortunately, this results in sample destruction and it is timeconsuming, presumably needing big fluid inclusions too. The nondestructive quantitative analysis of HCFIs is therefore a desirable goal for understanding the fluids involved in petroleummigration and is best achieved via optical methods (Guilhaumou et al. 1990; Piranon & Pradier 1992). Fluorescence-based methods are widely used for studying HCFIs and the most common identificationmethod is by observing their fluorescence under UV excitation. The use of © 2017 The Author(s). This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/ licenses/by/3.0/). Published by The Geological Society of London for GSL and EAGE. Publishing disclaimer: www.geolsoc.org.uk/pub_ethics Research article Petroleum Geoscience Published Online First https://doi.org/10.1144/petgeo2016-071 visually determined fluorescence colour is widely used as a qualitative guide for assessing the density and, potentially, also the maturity of oil trapped in fluid inclusions. However, the use of fluorescence colour is intrinsically prone to error and does not yield quantitative results, and the method is subjective. The limitations with the fluorescence technique in studying petroleum inclusions are well explained by Burruss (2003a); Blanchet et al. (2003) also 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 also been conducted using vibrational spectroscopic techniques. Infrared spectroscopic techniques have been used for analysing hydrocarbon inclusions (Barres et al. 1987; O’Grady et al. 1989; Guilhaumou et al. 1990; Piranon& Barres 1990), yet such measurements are limited by diffraction effects to inclusions larger than 15 μm in diameter, and a number of hydrocarbon vibrations are weak or inactive in the infrared (Dollish et al. 1974; Wopenka et al. 1990). Raman spectroscopy, being one method of getting detailed compositional data, has been widely applied successfully to aqueous and dry gas inclusions. This is because the fluids are not intrinsically fluorescent, unlike most oils, condensates and some wet gases. Burruss (2003a, b) detailed the importance of petroleum inclusion studies in petroleum exploration and development, and suggested that there are opportunities to apply Raman spectroscopy to petroleum inclusions if suitable methods to eliminate fluorescence interference can be employed (Kihle et al. 2012). Major attention has to be given to eliminate the fluorescence influence while studying Raman peaks in petroleum inclusions. Selection of optimum excitation wavelength is another challenge, while many previous studies noticed the problem of fluorescence from the fluids contained in the inclusions (hydrocarbons) or by the surrounding host mineral with shorter excitation wavelengths around 260 nm used for Raman analysis of HCFIs. Increasing the wavelength of the excitation source – red or near-infrared lasers – is theway to overcome fluorescence, since longer wavelengths excite less fluorescence from fluorophores that have visible wavelength excitation bands (Ahmadjian & Brown 1976; Orange et al. 1996; McCreery 2000; Burke 2001; Bourdet et al. 2011; Frezzotti et al. 2012). Oil constituents may differ depending on the source rocks, maturity, organic matter trapped and the pressure–temperature conditions of entrapping. In petroleum geology, fluid inclusions have proven to be useful indicators of migration pathways of hydrocarbons, they can delineate the evolution of the chemistry of hydrocarbons, and they remain important for understanding the thermal history of basins and related fluid migration events in the evolution of reservoir systems. HCFIs generally contain complex mixtures of organic compounds that depend on their sources. 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 thus crucial data for the petroleum exploration industry (Goldstein 2001; Munz 2001; Frezzotti et al. 2012). The present work is an attempt to characterize petroleum fluid inclusions from Mumbai offshore, India, using a laser exciting at a wavelength of 785 nm, pioneering work in this field in India. The present study highlights the utility of Raman spectroscopy as a powerful tool for characterizing hydrocarbons and for identifying the major functional groups in HCFI samples. The choice of excitation wavelength, laser power and the fluorescence-quenching techniques employed during wafer preparation yielded the best conducive conditions for obtaining adequate Raman signals from the HCFIs studied. Materials and methods