A Geological Model for the Origin of Fluid Compositional Gradients in a Large Saudi Arabian Oilfield: An Investigation by Two-Dimensional Gas Chromatography (GC × GC) and Asphaltene Chemistry

Abstract

The heavy oil rim of a large Saudi Arabian oilfield has been shown to be in vertical and lateral equilibrium, matching predictions of the gravity term from the Flory–Huggins–Zuo equation of state for asphaltenes in the form of 5.2 nm clusters of the Yen–Mullins model. The large (10×) vertical gradient of asphaltene concentration over a very large perimeter (≫10 km) of the oilfield provided a stringent test of this equation-of-state fit. Two-dimensional gas chromatography (GC × GC) and stable isotope analysis (δ D and δ 13C) were used to determine the consistency of the liquid-phase components with equilibration and the effects of biodegradation or thermal maturity on the observed asphaltene gradient. These analyses confirm homogeneity of equilibrated liquid-phase components of similar chemical character and equilibrated asphaltene isotopes. Biodegradation is minimal and there is no maturity variation among the samples. Thus, the large asphaltene gradient did not result from these secondary processes and is not a remnant from how the reservoir was charged with crude oil. The results are consistent with original findings that the oil column is equilibrated. Thermodynamic equilibration over such large distances (>10 km) requires convective currents and provides constraints on fluid dynamic processes in reservoirs. A simple one-dimensional (1-D) three-component single-phase model is introduced to account for asphaltene accumulation by way of convective currents established from a diffusive gas front at the top of the oil column.