--> Relating Deposition Process to Reservoir Behavior Using Computational Stratigraphy to Define Complex Hierarchical Scales of Reservoir Heterogeneity Within River-Dominated Delta Deposits
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AAPG ACE 2018

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Relating Deposition Process to Reservoir Behavior Using Computational Stratigraphy to Define Complex Hierarchical Scales of Reservoir Heterogeneity Within River-Dominated Delta Deposits

Abstract

It is difficult to define the complex organization of facies that can influence reservoir production behavior based on sparse subsurface data. Chevron’s Computational Stratigraphy program develops physics-based morphodynamic depositional models to predict preserved 3D facies patterns formed within different depositional settings. As an example, we examine facies patterns across a fluvial deltaic system preserved during varying rates of sea level change and different sand to mud input ratio to a basin with either forward or Previous HitbackwardNext Hit tilted subsidence profile. The influence of superimposed scales of sea level variations on depositional patterns are observed in cross section, thickness and net sand maps, and 4D Wheeler diagrams. Subsurface flow simulations predict potential impacts on reservoir production behavior. The locus of sand deposition switches with distributary avulsion even when sea level is steady, defining reservoir elements composed of clustered mouth bar deposits connected to a common terminal channel deposit. Channels extend basinward during regression and deposit a succession of rapidly-prograding elongate mouth bar deposits clustered along the path of channel mouth advance. During transgression, distal channel segments are gradually filled and distributary branching increases as sands disburse more widely across the drowned delta top. The resulting more-equant mouth bar and thin splay deposits crisscrossed by small channels comprise a broader heterolithic sheet. The apparently continuous basal erosion surface of the distributary channel deposit along the core of this element is very diachronous. Successive elements formed during continued subsidence tend to be offset laterally along strike. Stacked elements that are vertically-isolated reservoir compartments in distal areas can connect landward where distributary channels in adjacent elements converge. During reservoir production by water-flood, subsurface flows that progress up depositional dip rise through inclined beds to the top of contacted mouth bar deposits and then converge into progressively larger distributary channels, bypassing adjacent mouth bar deposits not directly on the path between an injector and producer. Flows moving down depositional dip Previous HitbranchTop into the splaying distributary channel deposits and are shunted down inclined beds through mouth bars into poorer quality facies, which leads to more stable displacement that flushes broader reservoir volumes before breakthrough.