Laser ablation GC–MS analysis of oil-bearing fluid inclusions in petroleum reservoir rocks

Abstract

The hydrocarbon composition of co-occurring blue and yellow fluorescing fluid inclusions (FIs) in Ordovician reservoirs of the Tarim Basin, NW China have been separately analyzed by laser microprobe gas chromatography–mass spectrometry (GC–MS) using a 193 nm excimer laser to drill through the host quartz. A newly designed GC inlet system enabled detection of C 4 C 30 hydrocarbons, extending the molecular weight significantly beyond the C 21 limit of previous online laser-based hydrocarbon analysis of oil inclusions. The blue fluorescing inclusions contain notably higher proportions of aliphatic hydrocarbons (e.g., n-alkanes, cycloalkanes) and gaseous range products, whereas the yellow fluorescing inclusions contain relatively more aromatic hydrocarbons (e.g., alkylnaphthalenes). Hydrocarbon-based parameters also indicate that the blue fluorescing inclusions are of higher thermal maturity than the yellow fluorescing inclusions, consistent with maturity estimates based on fluorescence colors. The compositional differences of the co-occurring inclusions are indicative of different oil types and charge events. This highlights the capacity of the new laser ablation technology to separately analyse specific inclusions amongst a mixture of inclusions of different fluorescing colors. In the future, integration of this new compositional detail with other inclusion and reservoir data will help reconstruct the oil charge history of the Tarim Basin.