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Lower Cretaceous Muddy Sandstone forms stratigraphic traps in the northeast Powder River basin. The most productive reservoirs are local concentrations of clean, quartzose sandstones deposited in linear bodies as marine bars. Sediment supply to the bars was accomplished by transverse sedimentation from associated fluviodeltaic environments.
Two distinct marine bar types may be recognized: (1) regressive, interdeltaic barrier bars, and (2) transgressive, delta-destructional bars. In regressive bars, grain size increases upward, and subenvironments which may be recognized include, in ascending order, lower shoreface, middle shoreface, upper shoreface-beach, and eolian. In transgressive bars, the vertical sequence of grain size and subenvironments is reversed with grain size decreasing upward, and subenvironments generally commencing with middle shoreface and succeeded upward by lower shoreface. Both bar varieties have prominent lagoonal or tidal-flat deposits as lateral equivalents. These fine-grained, back-bar sediments act as effective barriers to updip petroleum migration.
Transgressive bars are associated closely with typical fluviodeltaic deposits which they may succeed, laterally or vertically, in close succession. Therefore, they are interpreted as delta-destructional bars. Regressive bars, however, are laterally and vertically removed from the main sites of delta construction, and are interpreted to be typical coastal, interdeltaic deposits.
Although fluviodeltaic sandstones are local sources for bar sands, they are mineralogically quite distinct. Significant quantities of rock fragments and clay-rich matrix reduce the relative quartz content. Therefore, porosity and permeability values are low and, despite the fact that the fluviodeltaic sandstones are locally thick, they form less productive reservoirs.
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