Stratigraphy and Oil Resource Potential of the Mississippian Heath Formation, Central Montana, USA

Richard Bottjer
Cirque Resources LP

The Heath Formation of central Montana is an emerging tight oil play. Historically studied for oil shale mining potential, these organic-rich beds are the primary source rocks for more than 100 MMBO of oil produced from the Tyler Sandstone and other reservoirs in central Montana. The late Mississippian Heath Formation is the highest stratigraphic unit in the Big Snowy Group and is unconformably overlain by the Pennsylvanian (and/or latest Mississippian) Tyler Formation. The Heath exceeds 450 feet in thickness in areas of little basal Tyler erosion. Internally the Heath can be informally subdivided into, in ascending order, a lower 'shale', the Van Dusen Zone, Potter Creek Coal, Forest Grove Limestone, Cox Ranch Oil Shale, Heath Limestone, and an upper 'shale'. Oil shows and saturations are present throughout the entire Heath Formation. The Potter Creek and several other thin coals indicate humid conditions were present in near shore areas in older parts of the Heath. Intertidal to supratidal dolomites with up to 18 percent porosity interbedded with nodular anhydrites in the Heath Limestone indicate hot, arid conditions dominated near shore areas in the middle part of the Heath. Oil production from the Heath Formation was discovered in 1919 at Devil's Basin Anticline. Recent horizontal and vertical completions in the Heath Formation have initial potentials up to 447 BOPD and demonstrate the economic potential of the Heath. With Oil-In-Place estimates ranging from 6.6 to 22.4 MMBO/section and total Oil-In-Place of more than 14.0 BBO there is sufficient oil potential to warrant additional drilling and testing.

AAPG Search and Discovery Article #90169©2013 AAPG Rocky Mountain Section 62^nd Annual Meeting, Salt Lake City, Utah, September 22-24, 2013