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Keywords:

  • petroleum inclusion;
  • saturation pressure;
  • thermodynamic

Abstract

Microthermometry and volumetric analysis are now widely used techniques in petroleum geology, which allow the reconstitution of the composition and P-T trapping conditions of petroleum inclusions. However, these methods require efficient thermodynamic modeling tools of the PVT properties, coupled with compositional models for petroleum. Two methods presently available are reviewed here. The first one, called the α–β method, uses a compositional model describing the full spectrum (C1–C500) of natural hydrocarbons with two adjustable parameters α and β. The second technique is based on a so-called fluid model, i.e., a priori knowledge of the composition and the physical properties of the C7+ cut of the trapped petroleum. Up to now, no critical appraisal has been undertaken for these two methods. From an extensive compilation of literature data (201 points), we here show that both techniques are able to describe the composition of crude oils and to predict satisfactorily their saturation pressures. However, the predictive capabilities of fluid models are highly dependent on the correlations used for describing heavy cuts, and we indicate those which give the best results. Finally, we emphasize the key control of the bulk methane mole fraction (x1) on the saturation pressure: this result allows us to build simplified versions of α–β and fluid models, applicable to the majority of crude oils, and whose formulation depends advantageously on x1 only.