ABSTRACT

The Lower Cretaceous rocks produce oil from stratigraphic traps in the northeast Powder River basin. Density of subsurface data allows reconstruction of paleodepositional environments. Through detailed subsurface studies it is possible to relate time-equivalent sandstone bodies to depositional patterns. Structure has little or no bearing on petroleum accumulation in this area.

Identification of paleodepositional environments by means of subsurface studies depends on precise electric-log correlations and a geomorphic interpretation.

The depositional trends of the Lower Cretaceous Fall River sandstone bodies and the so-called “Muddy” sandstone beds are determined readily by selective isopachous mapping. Care must be exercised to recognize unconformities, disconformities, or other breaks in an apparently continuous cycle of deposition. Isopachous maps are constructed by using a lithologic time marker as the upper boundary and either a lithologic time marker or an unconformity as the lower boundary. These continuous cycles of deposition indicate a genetically related depositional environment.

The Fall River contains three sandstone bodies that are mappable as separate units in a complex marginal-marine shoreline environment. These sandstone beds thicken locally and, in places, merge into a single unit. The Fall River does not have channel characteristics in this area.

The “Muddy” contains two sandstone bodies previously named Newcastle and Dynneson. These sandstone units have distinct depositional patterns and the Newcastle shows a pronounced dendritic drainage pattern.

Oil production is from the eastern updip edges of maximum sandstone development in both the Fall River and “Muddy” sandstone units. By detailed subsurface mapping it is possible to relate time-equivalent sandstone bodies to the depositional environments which control oil accumulation.

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