Relating depositional processes of river-dominated deltas to reservoir behavior using computational stratigraphy

ABSTRACT Changes in depositional patterns of a river-dominated delta that occur during multiple scales of sea-level change are linked to variations in reservoir quality using a physics-based flow and sediment-transport model. Results are compared for a fore-tilted and back-tilted basin with the same sediment supply and sea-level history. The model predicts proximal to distal fining of the depositional system as distributary channels splay into a delta lobe element and deposit channel-mouth-bar storys. Delta lobes compensationally stack at multiple scales, making reservoir correlations across the models difficult. Lateral autogeneic shifts in deposition result in local progradational–retrogradational units that are out of sync with sea-level variations. Lobe elements deposited on a low-gradient drowned delta top formed during early highstand regression or during later stages of transgression contain thin mouth-bar deposits dissected by many small distributary channels. Wells in these elements have a reasonably high production rate where they intersect amalgamated channels but tend to have low recovery. Wells intersecting an entrenched lowstand channel generally have a very high production rate but very low recovery, because they fail to contact facies outside of the channel. Better wells in the deltaic deposits (high production rate and recovery) are at the transition between a channel that bypasses sediment directly to the delta front edge and a large birdsfoot cluster of delta-front elements, because in such locations coarser backfilled channel sands extend continuously into larger vertically aggraded mouth-bar sands. Wells in compensationally stacked distal mouth-bar deposits basinward of any channel sands tend to have an uneconomically slow production rate. Line-drive flood displacement is used to measure the impact of depositional architecture on production. Subsurface flow down a delta lobe broadens as the distributary channels splay into lower-quality mouth-bar facies. Subsurface flow up dip pipes along the first channel contacted, and thus tends to bypass most of the adjacent mouth-bar deposits. Performance of individual wells depend on the size, number, and connectivity of channels in the neighborhood and the depositional setting of the channel-mouth-bar stories contacted. Contrasts in reservoir character between low-gradient delta-top deposits and those formed during progradation of the delta front onto a higher-gradient basin floor are greater than those between regressive and transgressive successions.