Abstract

Gas-oil ratio (GOR) and asphaltene gradients are routinely measured by downhole fluid analysis (DFA) and analyzed by the cubic equation of state (EoS) and the Flory-Huggins-Zuo EoS, respectively. These results are then placed within a reservoir perspective; for example, equilibrated asphaltenes are known to indicate reservoir connectivity. Therefore, understanding the origins of these gradients is important. The variations of different crude-oil components measured in present day are a function of charge history as well as in-reservoir fluid processes that can cause fluid components to equilibrate thermodynamically. Here, petroleum system modeling is used to create different initial reservoir fluid distributions immediately after trap filling, where layer-cake "Stainforth" density-stacking trap filling is presumed. Charges that are relatively homogeneous versus those of greatly differing thermal maturities are considered. Subsequent to trap filling, diffusion is forward modeled. For a charge of greatly differing thermal maturity, the initial (post-trap filling) versus final equilibrated gradients in GOR and asphaltenes are seen to be similar, equilibrium is attained relatively quickly. In contrast, for this case, the large initial gradient in liquid fingerprints and biomarkers is very dissimilar to the final homogenous equilibrium distribution of these components, thus, longer times are required for equilibrium. Consequently, a single oil column can exhibit equilibrium distributions of GOR and asphaltenes with disequilibrium in oil fingerprints and biomarkers. In contrast, a homogenous charge can yield the opposite, equilibrium of fingerprints and biomarkers, and disequilibrium of GOR and asphaltenes. The ‘thermodynamic distance’ from the initial condition (at end of trap filling) to equilibrium determines, to a large degree, gives the diffusive time required to achieve equilibrium.

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