Abstract

Lazy B Field, located in the geographic center of Wyoming's Powder River Basin, is a northeast-trending, narrow, sandstone reservoir that has produced more than 2.2 MMBBL of oil and 4800 MMCF of gas from the Lower Cretaceous Muddy Sandstone. At Lazy B Field the Muddy is anomalously thick (up to 154 ft) and the unconformity between the Muddy and underlying Skull Creek Shale displays local erosion of more than 40 ft. The Muddy at Lazy B Field has been previously interpreted as a fluvial deposit that correlates to productive fluvial sandstones in the lower part of the Muddy in Kitty and Recluse Fields to the northeast.

In contrast to fluvial quartz arenites at Kitty and Recluse Fields, productive sandstones at Lazy B Field are chert-rich subarkoses and arkoses. They contain a variety of sedimentary and biogenic structures characteristic of wave-dominated or storm-modified marine shoreface deposits. Lithofacies are typically arranged in coarsening- and thickening-upward sequences interpreted as progradational strandplain deposits. To the northeast these strand-plain deposits have been eroded and replaced by fluvial deposits characteristic of the Kitty-Recluse productive trend. In all other directions these strandplain deposits have been eroded by either subaerial or transgressive ravinement processes prior to the deposition of younger members of the Muddy. The resultant northeast-trending geometry is an erosional remnant of a sheet strandplain deposit that was preserved within a tectonic graben.

Lazy B Field parallels a northeast-trending lineament, which is interpreted to reflect a basement fault or shear zone between PreCambrian basement blocks. The field may overlie a slight bend along this basement fault. Strike-slip motion along this fault caused alternating periods of crowding and rising of one side of the bend and stretching and sinking of the other side. These alternating periods of uplift (erosion) and subsidence (preservation) are characteristic of terrains dominated by wrench tectonics.