Combined use of Confocal Laser Scanning Microscopy and PVT simulation for estimating the composition and physical properties of petroleum in fluid inclusions

We present a method to determine the composition and PVT properties of petr oleum in individual petroleum fluid inclusions. Confocal Laser Scanning Mic roscopy is used to generate three dimensional images of single petroleum in clusions. Because liquid petroleum fluoresces under the laser. the images r eadily distinguish the liquid and vapour within the inclusion and can be us ed to determine the inclusion's volumetric liquid:vapour ratio. Using PVT m odelling software, the liquid:vapour ratio is used along with the homogenis ation temperature to determine the bulk composition. phase envelope, isocho re and a range of physical properties of the included petroleum. This is do ne using an iterative series of PVT calculations which match two parameters : (1) the molar volume of the petroleum at room and homogenisation temperat ures; (2) the liquid:vapour ratio of the inclusion at room temperature. Key uncertainties in the method are explored, including the accuracy with whic h the liquid:vapour ratio can be determined: the composition of the titrant gas used in the iterative procedure; and the composition of the petroleum chosen to model the physical properties of the included petroleum. Data fro m coeval inclusions suggest that the saturation pressure, Gas-Oil Ratio, vi scosity, molar volume, density and surface tension of included petroleum ar e determined with a precision of a few percent. Confirmation of the accurac y of the method awaits tests using inclusions grown in the laboratory under carefully controlled PVTX conditions. However, it is likely that the physi cal properties of included petroleums are more accurately modelled by fluid s which are genetically related to them. If proven to be accurate, the meth od will provide a routine method for determining palaeopressure in petroleu m systems. Finally, we present data from a Central North Sea example which are geologically realistic and which for the first time record the evolutio n of fluid pressure and petroleum composition in a petroleum reservoir. (C) 1999 Elsevier Science Ltd. All rights reserved.