The non-contact, non-destructive assessment of fluids entrapped within hydrocarbon bearing fluid inclusions (HCFI) can be achieved, using time resolved fluorescence (TRF) methods. These measurements are usually independent of factors such as excitation source intensity variation, fluorophore concentration, photobleaching, geometry, opacity, and scattering effects that can adversely affect steady-state based measurements. A diode laser based fluorescence lifetime microscope (DLFLM) assembled in-house was used to make fluorescence lifetime, time-resolved emission spectroscopy (TRES), and steady-state measurements on microscopic HCFI and bulk crude petroleum oils (CPO).
Calibration models for composition (concentration of aromatics, polars, and alkanes) and API gravity of HCFI were developed by a study of the fluorescence behaviour of a 23 bulk CPO’s. The average fluorescence lifetimes of these CPO samples were measured over the 460-700 nm range and the correlation between lifetime and chemical/physical parameters investigated. The best correlations with lifetime were found between the polar and corrected alkane concentrations. Other parameters such as API gravity, sulphur, aromatic, and asphaltenes do not correlate well with lifetime. 6 HCFI from 3 different locations (South Wales, Illinois, and Argentina) were analysed using the DLFM and the data correlated with the alkane and polar models derived from the CPO data. Based on this, oils entrapped within the HCFI can be classified as either light-medium or heavy, and can also distinguish between individual oils.