Innovative fluorescence spectroscopic techniques for rapidly characterising oil inclusions

Abstract

Two fluorescence techniques for rapidly screening oil inclusion abundances and compositions are presented. The techniques measure emission of trace fluorescence from reservoir grains and their solvent extracts using a spectrophotometer. The first one, called Quantitative Grain Fluorescence plus (QGF+), directly measures dry, disaggregated reservoir grains after a robust cleaning procedure involving solvent, hydrogen peroxide and aqua regia or dilute hydrochloric acid (HCl). The second technique measures crushed Inclusion solvent extracts from the grains cleaned for QGF+ using the Total Scanning Fluorescence method (iTSF).Fluorescence spectra of 43 oil inclusion-bearing samples from seven basins in Australia, Papua New Guinea, SE Asia and China were obtained using QGF+ and iTSF. The fluorescence intensities (QGF+ Index and iTSF Intensity) correlate moderately well to the yields of the total n-alkanes obtained by the molecular compositions of oil inclusions (MCI) method, with linear correlation coefficients (R2) of>0.6. The QGF+ spectral peaks (λmax) and the iTSF spectral ratio R1 broadly reflect the thermal maturity or API gravities of the inclusion oils. The fluorescence spectra of the samples display a variety of signatures that can be generally correlated with the n-alkane profiles of the inclusions. The QGF+ spectral peak (λmax) and the iTSF R1 parameter also correlate with thermal maturities derived from biomarkers. Condensates have n-alkane maxima at n-C12 to n-C14 and corresponding spectral peaks<420nm and R1 values<2.0. Most of the normal to light oils have a uni-modal n-alkane distribution with maxima at around n-C16 to n-C23 and corresponding fluorescence spectral peaks around 420–450nm and R1 values of 2.0–3.0. Medium to heavy oils have n-alkane maxima>n-C24 and corresponding spectral peaks>450nm and R1 values of>3.0.This work demonstrates that the bulk fluorescence spectroscopy methods can be cost effective rapid screening alternatives to the conventional petrographic (point counting) or offline crushing extract molecular compositional of oil inclusion (MCI) methods for estimating the abundance and compositions of oil inclusions.