PACROFI VI - Electronic Program

Determination of the Effective Composition of Single Petroleum Inclusions Using Confocal Scanning Laser Microscopy and PVT Simulation

G. Macleod1, S. R. Larter1, A. C. Aplin1, K. S. Pedersen2, T. A. Booth3

  1. Newcastle Research Group In Fossil Fuels and Environmental Geochemistry, Postgraduate Institute, Newcastle upon Tyne, Drummond Building, University of Newcastle, Newcastle upon Tyne, NE1 7RU, UK.
  2. CALSEP-A/S, Gl. Lundtoftevej 7, DK-2800 Lyngby, Denmark.
  3. Biomedical Electron Microscopy Unit, Medical School, University of Newcastle upon Tyne, Newcastle upon Tyne, NE2 4HH, UK.

It is desirable to determine an accurate composition (e.g. C1-C20+ alkanes) of petroleum in fluid inclusions within diagenetic cements from petroleum reservoirs and carrier units for two reasons. Firstly, when coupled with microthermometric data, it enables us to track the compositional evolution of petroleum during secondary migration and field filling. This information allows exploration and production geoscientists to determine the timing and direction of petroleum movement in sedimentary basins, and to determine filling histories for reservoirs. This information can be used to help locate satellite reservoirs and fill-spill sequences.econdly, an accurate compositional analysis is required to model the PVT behaviour of included petroleum. This is central to the use of coeval aqueous and petroleum inclusions in the determination of palaeopressure using the "intersecting isochores" technique (Roedder and Bodnar, 1984). Accurate palaeopressures can then be used to determine the timing of such phenomena as seal failure and the onset of overpressure. It may also be possible to use palaeobarometry to determine fluid potentials during the geological history of a have been many documented attempts to determine the composition of the 'live' petroleum trapped inside petroleum inclusions, generally by crushing a large number of inclusions into Gas Chromatographs or Gas Chromatograph Mass Spectrometers (Burruss, 1987, Macleod et al. 1993, Macleod et al, 1994, Bigge et. al., 1995). The problems with this approach are firstly that the composition is the average of the many crushed inclusions and secondly that the analysis will be biased by organic species such as methane derived from aqueous inclusions. Furthermore, the techniques are difficult and time-consuming (Bigge et. al., 1995) and the compositional data cannot be assigned to the single inclusion on which microthermometric measurements have been made. Thus we require an assessment of the composition and PVT properties of petroleum within single inclusions.

Our approach to this problem has been to exploit the ability of the Confocal Scanning Laser Microscope (CSLM) to produce a three dimensional image of the liquid and vapour within a fluid inclusion with the expected relationship between the composition of petroleum and its liquid:vapour ratio. An example of the general relationship between the L:V ratio and petroleum composition is shown in Figure 1, in which part of the calculated phase envelope for three petroleums is displayed along with an estimate of the L:V ratios at 20oC. The L:V ratio can be calculated with a PVT petroleum simulation software package using the composition of the included petroleum and the homogenisation temperature of the inclusion (Saturation Temperature) as inputs. The software also enables one to calculate the isochores displayed in Figure 1.

Fig. 1. Phase envelopes for "typical" North Sea reservoir petroleums, and the calculated percentages of liquid and vapour for each of the petroleums in a fluid inclusion at surface conditions. The inclusions are given an arbitrary homogenisation temperature of 90oC. The PVT calculations were performed using a modified Soave-Redich-Kwong Equation of State.

Analysis of the 3D images from the CSLM enables one to calculate the true volume percent of vapour and liquid inside the inclusion. These are usually very different (lower) to those estimated by 2-dimensional observation in the heating-freezing stage. By combining the measured homogenisation temperature of the inclusion being studied, with the CSLM measured volume percent of vapour and liquid at room temperature, we can use commercially available PVT software to iteratively back calculate the 'effective' C1-C20+ composition of the 'live' included petroleum. Although the calculated composition will not represent the real, detailed geochemical composition of the included petroleum, it will mimic the phase behaviour of the included petroleum and thus can be used to construct PVT diagrams for the included petroleum. It also allows isochores to be calculated and gives a representative GOR of the petroleum in the inclusion. The GOR of the included petroleum can then be used to interpret the compositional evolution of petroleum through time. This powerful new technology allows petroleum fluid inclusion micro-thermometry to be linked to petroleum composition, offering exciting applications of fluid inclusion studies in petroleum exploration.