Date of Graduation

2000

Document Type

Thesis

Degree Type

MS

Committee Chair

Richard A. Smosna

Abstract

Recent exploration in the Powder River basin has targeted shallow, coal-bed methane, and basin reserves may be as high as 39 trillion cubic feet. Both coal and sandstone beds serve as target methane reservoirs; however, the thick, persistent nature of the upper Tongue River Member coals makes them easier to correlate and map than any sandstone body. Correlating these coals and considering the differential compaction model for shallow structural development may help shed light on recognizable patterns in clastic-sediment and coal distribution. This study had a threefold purpose: (1) establish a regional stratigraphic framework for the upper Tongue River Member of the Fort Union Formation including major coals beds and interbedded sandstones; (2) evaluate stratigraphic evidence as it relates to either the fluvial or deltaic model for the deposition of coal in the Tongue River Member; and (3) determine the controlling factors for the development of shallow folds and potential gas reservoirs. Nearly 1,700 well logs from an area located along the northern Wyoming border were used in this study. Twelve regional stratigraphic cross sections were constructed to provide a framework for a detailed correlation of the upper Tongue River coals. Upon completion of the correlation, the picks were entered into a spatial database. This database was used to create coal-structure, coal-isolith, coal-isopacb, and clastic-sediment isopach maps. Information collected from these four general types of maps was used to create paleogeographic maps. Detailed correlation of the well logs proved to be difficult even in areas of considerable well control. The complexity of the stratigraphy can be attributed to localized and regional southwestern splitting o f coals. The coal and clastic sediment also thicken and thin over both long and short distances. Two lithofacies were identified on the paleogeographic maps: the swamp and thriving-swamp lithofacies. These swamp environments shared a close relationship with channels, although coal and clastic sediment deposition simultaneously occurred at different locations. The channels also typically had a north-south to northwest-southeast orientation. These patterns were not consistent with the deltaic model, but supported the fluvial model for coal deposition. The shallow folds found throughout the study area shared a direct relationship with coal thickness. Thick coal deposits that were stratigraphically adjacent to a particular coal-structure map typically coincided with structural lows, whereas thin coal deposits usually coincided with structural highs. Thick clastic sediment may have occupied the same approximate location and shared similar trends as the axes of many structural highs, but they usually did not correlate very well. Differential compaction most likely created the structural anomalies within the study area.

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