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HYDROCARBON FLUID INCLUSIONS, API GRAVITY OF OIL, SIGNATURE FLUORESCENCE EMISSIONS AND EMISSION RATIOS: AN EXAMPLE FROM MUMBAI OFFSHORE INDIA V. Nandakumar, and J. L. Jayanthi Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.5b02952 • Publication Date (Web): 11 Apr 2016 Downloaded from http://pubs.acs.org on April 11, 2016
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Energy & Fuels
HYDROCARBON FLUID INCLUSIONS, API GRAVITY OF OIL, SIGNATURE FLUORESCENCE EMISSIONS AND EMISSION RATIOS: AN EXAMPLE FROM MUMBAI OFFSHORE INDIA
V. Nandakumar* & J. L. Jayanthi National Facility for Geofluids Research & Raman Analysis ESSO-National Centre for Earth Science Studies Ministry of Earth Sciences, Government of India, Akkulam, Thiruvananthapuram-695 011, Kerala, India. * Corresponding author:
[email protected] Short title: API gravity of oil in HCFIs
Abstract
Hydrocarbon fluid inclusions (HCFIs) in mineral grains of source, reservoir or carrier rocks are of particular interest to the petroleum industry. The minute size of HCFIs warrants a combination of microscopy and spectroscopy for identification and characterisation. This article presents fluorescence emission data of pure petroleum oils of known American Petroleum Institute's gravities (APIG) and proposes an empirical tool for predicting the APIG of oils in micron sized HCFIs through a non-invasive, non-destructive procedure using microscopy based fluorescence estimates. It also documents how this empirical tool could be used as a way of inferring the APIG in HCFIs by considering the samples from RV-1 well (Mumbai offshore basin, India) as an example. RV-1 is a non-producing well from the Mumbai offshore basin, India with proven commercial productivity.
Fluorescence emission of 13 crude oil samples with known API gravities were recorded using a diode laser excited at 405nm in order to estimate the API gravity values of oils trapped as HCFIs. The gathered fluorescence data were fitted in a Gaussian distribution to identify unique emission peaks, i.e., around 500, 560 and 620 nm. Bivariate scatterogram with a smooth line fit using spectral ratio at F620/F560 vs. API gravity of crude oil provided a classificatory scheme or an API gravity predictor of oils in hydrocarbon fluid inclusions. Fluorescence emission of oil in HCFIs were recorded in the region 406-720 nm using the microscopy based fluorescence technique and the calculated spectral emission ratios at F620/F560 are given in the scatter plot of API gravity known oils. The APIG of unknown samples (whether they be HCFI’s or otherwise) can be inferred from the algebraic expression
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linking emission spectra to APIG for known crude oil samples. The methodology developed is reliable in deriving an accurate API (by reference to the calibration of crude oils) so as to estimate the API gravities of minute sized oil samples in HCFIs and therefore could prove to be a useful tool in the petroleum exploration and industry.
Keywords: hydrocarbon fluid inclusions, fluorescence, API gravity, emission intensity ratio
1. Introduction
Rather a quick handle on API gravity (APIG) is unquestionably an important step in managing the crude oil extraction in any oil field. We report a methodology to deduce the APIG of oil right from HCFIs in samples (single mineral grain) of well cuttings from lithologies (source, reservoir or carrier beds) encountered during drilling and having the probability to act as oil producing zones. The approach is a non-destructive and non-invasive procedure capturing the fluorescence emission spectra of HCFls trapped in the specially prepared sample ‘wafers’. The ratio of emission spectra is used to interpolate the API gravity from a “standard” line fitted scatterogram of known API values vs. emission spectral ratios. Fluid inclusions (FI) trapped and sealed in mineral cavities, are micro - samples of fluids trapped in minerals during phases of mineral growth and/ or as in subsequent modifications. FI technology is greatly diverse in its application in oil exploration and invaluable in the reconstruction of fluid flow histories in reservoir rocks1,2, indirectly yielding information about the temperature, salinity, and composition of fluids that migrated through the basin in the geological past. Hence, FIs represent samples of reservoir fluids of certain geologic time bands. In petroliferous basins, Fls, mostly in the size range of 5-20 microns, are either polyphased or biphased (containing oil, gas, soild or water, or any combination) and the oil bearing ones typify micro-samples of petroleum that once migrated through the rocks prior to trapping3. Oil inclusions are easily identified in thin sections under ultraviolet illumination fluorescence due to presence of aromatic and polar fractions in oil4. Goldstein5 concluded that Fls, one of the most sensitive and critical indicators of diagenetic environments, are an important toolset in oil exploration and development. American Petroleum Institute's (API) scale of 'lightness' or 'heaviness' of crude oil and other liquid hydrocarbons, dependent on the density of a petroleum based liquid versus that of water, is factored into the market value of crude oil. The scale of APIG indicates crude’s
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Energy & Fuels
relative density but inversely, i.e., lighter the crude, higher the APIG, and vice versa. In this scheme, oil with API value of > 300 is termed light; between 220 and 300 is medium;
