ABSTRACT

Natural gas was first discovered in the Blackleaf Canyon area of northern Montana in 1958 but low gas prices and the lack of an accessible market delayed commercial production until 1982. From 1982 to late 1991 four wells produced approximately 6.9 billion cubic feet of natural gas from the dolomitized upper portion of the Mississippian Madison Group in two fault-bounded anticlinal traps. Productive structures are the result of east-directed Late Cretaceous-to-Paleocene thrusting that created an imbricate stack of Paleozoic and Mesozoic rocks.

The upper part of the Madison Group is exposed west of the producing fields, and a measured surface section has been projected into the subsurface utilizing both electric log correlations and sample examination. Rocks at the surface equivalent to those of the productive intervals in the adjacent fields include mudstone, wackestone, and packstone deposited in a shallow water and tidal flat environment, crinoidal grainstone and packstone deposited in a marginal sand or shoal facies, and packstone and wackestone of a restricted circulation platform or lagoon environment. Gas production is not confined to specific depositional environments and initial completions in some newer wells appear designed to take advantage of intervals with fracturing in the lower portion of the dolomitized section. Fracturing in the outcrop is found to be most intense in dolomitized mudstone and wackestone intervals that correlate with the productive interval in the discovery well for the Knowlton structure.

Correlation with the Dupuyer Creek principal reference section for the Castle Reef Formation that is located approximately 6 miles (10 km) north of Blackleaf Canyon indicates that productive intervals in the Blackleaf area wells are mostly within that formation. Facies relationships between Blackleaf Canyon and surface sections to the north are consistent with Mississippian structural development in the area of the South Arch of the Sweetgrass Arch and Pendroy Fault Zone. Higher energy grainstone and packstone deposits at the base of the Castle Reef Formation to the north may be the time equivalent of lower energy restricted platform and tidal flat deposits in the Blackleaf Canyon measured section. These lateral facies variations may make the contact between the Castle Reef Formation and underlying Allan Mountain Formation less distinct.