Using petroleum inclusions to trace petroleum systems - A review

Petroleum-bearing fluid inclusions are small encapsulations of oil and gas that offer an invaluable opportunity to better constrain the evolution of petroleum systems. Insights into palaeo fluid compositions complement observations on present day fluid compositions, which represent only the end-point of complex cumulative processes throughout basin history. In this contribution, we review a wide range of approaches used to extract geochemical information from petroleum inclusions, and how these can be used to better constrain petroleum systems. These techniques can be grouped into optical, spectrographic and thermometric non-destructive methods, or destructive chemical analyses of bulk samples or individual inclusions. Typically optical methods documenting the distribution and visual properties of petroleum inclusions are used to provide petrographic context for subsequent specialised geochemical analyses of petroleum inclusions. Additional non-destructive techniques such as Raman spectroscopy can then be applied to provide some further insights into the composition of the trapped fluids, although the complex nature of petroleum generally requires direct access to the fluid for a more complete understanding of geochemical aspects. A variety of destructive techniques have been developed, initially to analyse bulk samples released by mechanical crushing and more recently through ablation type techniques that allow the composition of individual inclusions to be characterised. Screening geochemical techniques that utilise mechanical crushing of bulk samples to analyse petroleum inclusions using mass spectrometry without prior chromatographic separation have become routine analyses. Other geochemical techniques more geared towards detailed molecular information such as biomarkers utilise chromatographic separation prior to mass spectrometry. Evaluation of the isotopic composition of petroleum inclusions is also possible for both bulk samples and compound specific analyses. The use of lasers to open individual inclusions allows the released contents to be analysed by thermal extraction-gas chromatography-mass spectrometry (GC-MS), or mass spectrometric mapping of minerals using Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS), a surface-sensitive analytical method that uses ion beams to ablate into minerals. The continued evolution of techniques to analyse the incredibly small volume of hydrocarbons trapped within fluid inclusions has progressed to a point where there is little that can be done to evaluate a live oil or gas sample that cannot be achieved for a fluid inclusion sample. The full power for tracing petroleum systems is, however, only realised where there is an effective integration of fluid inclusion data with a more conventional approach to petroleum systems analysis. (C) 2019 Elsevier Ltd. All rights reserved.