Regional reservoir compartmentalization within offlapping, top-truncated, mixed-influenced deltas, Wall Creek Member, Frontier Formation, Powder River Basin, Wyoming.

摘要

We show that the upper Turonian Wall Creek Member of the Frontier Formation, Wyoming, previously interpreted as an offshore bar or shelf ridge, is better interpreted as a delta system formed during a lowstand, individual lobes of which have been top-truncated by marine ravinement during high frequency episodes of transgression. Basinwide correlations and mapping reveal consecutive stacks of lensiod, offlapping deltaic parasequences and parasequences sets constituting the Wall Creek Member. Analyses of stratigraphic dip azimuth values show that the mean resultant paleotransport direction of the Wall Creek deltaic system is towards the southeast with a northwest-southeast orientation of the contemporaneous shoreline, at normal to the direction of delta progradation. In the Salt Creek oilfield, the Wall Creek Member is represented by a thick, "blocky" sandbody, superficially resembling incised valley fill deposits. A closer examination, integrating physical sedimentology and ichnology demonstrates that the apparently blocky depositional unit has no genetic or scale difference with the deltaic parasequences separated by distinct prodelta shales. Amalagamtion of these parasequences may be attributed to differential accommodation caused by proto-Laramide tectonics. Sub-regionally, the Wall Creek compartmentalizes into two tiers of parasequence sets, separated by a major flooding surface reflecting two higher order lowstand systems tracts punctuated by a transgressive systems tract within an overall lowstand scenario. Individual parasequences within each set are locally farther compartmentalized into bedsets. Internal facies architecture of preserved prodelta and delta front deposits show varied influence by river, tide and wave processes. Sandbody isolith maps show strike-elongate to lobate and dip-elongate paleogeomorphology, which also reflect different degrees of reworking by waves and tides over time.