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Deltaic Sedimentation and Stratigraphy of the Late Cretaceous Frontier Formation in the Southeast Bighorn Basin, Wyoming

Abstract

Deltaic Sedimentation and Stratigraphy of the Late Cretaceous Frontier Formation in the Southeast Bighorn Basin, Wyoming

Sheridan Mullen

Abstract

The Cretaceous Frontier Formation in the southeast Big Horn Basin is a prograding deltaic sequence in the Western Interior Cretaceous Seaway.  The mapped area includes 41 Townships near Worland and Tensleep Wyoming with over 700 wells evaluated. West to east in the map area the overall Frontier section thickens by over 400 feet. Stacking patterns of the Frontier sands show a distinct west to east progradation. Individual sands show evidence of differential compaction and delta lobe shifting. Localized paleo-structure features that include thrust structures and regional faulting has influenced geometry and location of sands in the 4th Frontier indicating that these structural features were moving at the time of deposition.  The large, down to the north, Tensleep fault channeled sand on the downthrown side and restricted sand development on the south (upthrown side) of the fault in the 1st but not the 3rd and 2nd Frontier sands. Delta development in the 3rd Frontier seems to be most influenced by differential compaction and lobe shifting. Detailing the sand deposition in the 2nd Frontier shows that younger sands were thicker to the west and a clear set of clinoforms developed filling the developing basin to the east. Sequence stratigraphic evaluation of the Frontier sandstones in this region indicate that the Frontier is composed of four parasequences capped by maximum flooding surfaces. The sands have a funnel shaped gamma ray curve typical of coarsening upward deltaic sequences. Each sequence is topped by a transgressive shale which contains the maximum flooding surface.

The current geometry of the basin was formed during the Laramide Orogeny at the end of the Cretaceous period. Uplift around the flanks of the basin caused large scale subsidence and formed the basin. Later uplift eroded sediments from the flanks of the basin exposing the Cretaceous sediments. Down warping of the basin during the Cretaceous and into the Paleogene moved the source rocks deposited in the Cretaceous into the oil window and ultimately the gas generation window in the center of the basin. Maturation profiles for the basin seem fairly consistent. In the deepest part of the basin sediments are in the oil window at depths of 2000 meters, with maximum oil generation occurring at 3000 meters.