Reservoir Development in the Tensleep Formation, Pryor and Bighorn Mountains, South-Central Montana

Lopez, David A. ¹, Steven W. VanDelinder² (1) Montana Bureau of Mines and Geology, Billings, MT (2) Ballard Petroleum Holdings, Billings, MT

The Permo-Pennsylvanian section in the Big Horn and Powder River Basins is the most prolific oil producing system in the central Rocky Mountains, having produced 2+ BBO in the Bighorn Basin and 525 MMBO in the Powder River Basin. Dramatic stratigraphic changes occur within this section across south-central Montana.

To better understand these stratigraphic changes, 13 surface sections were measured and described in the Pryor and Bighorn Mountains. The results are summarized in the following conclusions: 1) The Upper Tensleep contact is unconformable, showing topographic relief of as much as 50 feet due to a combination of erosion and the presence or absence of dunes. 2) In the west, both Upper and Lower Tensleep are present. The Lower Tensleep is characterized by repeated cycles of marine sandstone, tens of feet thick, capped by very limy to dolomitic sandstone beds 1 to 5 feet thick. The Upper Tensleep is characterized by cycles of eolian dune sandstone capped by marine limy to dolomitic sandstone. Dune sandstones can be as thick as 60 feet. The total Tensleep thickness in the western part of the area can be as much as 250 feet. 3) In the east and central parts of the area the Upper Tensleep is absent and dune sandstones develop in the lower Tensleep. The vertical cycles are similar to the ones in the west. On the east side of the Bighorn Mountains the Tensleep thins to as little as 35 feet.

AAPG Search and Discovery Article #90055©2006 AAPG Rocky Mountain Section Meeting, Billings, Montana