Stochastic evaluation of fluvial to marginal marine sealing facies

Abstract

Hydrocarbon top seal analyses are particularly difficult in fluvial and marginal marine settings. Although these settings often contain beds comprised of a low-permeability, clay-rich lithology associated with high quality seals, their presence does not guarantee an effective seal because individual beds within the stratal unit may be laterally discontinuous. Lateral facies changes, truncation of fine-grained beds, or the presence of crevasse splays may provide cross-stratal migration pathways, reducing the effective sealing capacity to a magnitude less than the closure height. To evaluate probable bed seal capacities in such a setting, we have calculated 75 statistically equivalent geologic models of a heterogeneous bed seal based upon facies interpretations derived from measured data. Measured capillary and hydraulic properties of rocks in each facies were then fit to Gaussian distributions. We populated the geologic models with flow properties by randomly selecting from these distributions. Two realizations of the rock property distributions were performed for each of the 75 object-based geologic models. Gas charge was simulated in each of the rock property model trials using a percolation-based hydrocarbon migration software program. The results of the simulations return a probability distribution of bed seal capacities, measured in equilibrium gas column heights, from which cumulative frequency diagrams and percentile rankings may be derived. Channel amalgamation by incision juxtaposing levee overbank and/or channel facies formed the most common type of cross-stratal leak point. A factor of 10 difference in the P50 seal capacity was observed by varying the net volume fraction of channels from 10% to 30%. Higher seal capacity realizations were found to have a low density of channels near the structural crest, a variable which outweighs all other stochastic parameters. These results highlight the importance of a risking-based methodology to account for variability in bed seal stratigraphy in fluvial to marginal marine settings.