Microspectrofluorimetric and microthermometric evidence for variability in hydrocarbon fluid inclusions in quartz overgrowths: implications for inclusion trapping in the Alwyn North field, North Sea

Hydrocarbon inclusions trapped in quartz cement from Alwyn North field sandstones of well 3/9a-6 (North Sea) have been analyzed using microthermometric and microspectrofluorimetric methods. They are two-phased, with consistent gas–oil ratios, and mainly occur along the detrital grain–overgrowth boundary. Their homogenization temperatures vary from 60 to 129 °C with a great variability within a single fluid inclusion assemblage (up to 40 °C). The fluorescence colour of the hydrocarbon inclusions has been characterized using a microspectrofluorimeter which allows the analysis of very small inclusions (>1.7 μm) with good reproducibility. Hydrocarbon inclusions display large variations in their fluorescence colour, converted in chromaticity coordinates, even within a single fluid inclusion assemblage. Moreover, they are concentrically distributed within the detrital grain–overgrowth interface. According to the relationship between API gravity and fluorescence colour, established with reference to crude oils from Alwyn North field, this observation is interpreted as representing compositional variations. Such compositional variability in a single fluid inclusion assemblage could result from either the progressive entrapment of oils in which API gravity evolves through time, and/or the fractionation of hydrocarbons at the time of trapping.