Abstract

A sequence of dolomitized crinoidal grainstone locally forms the upper part of the Mississippian Madison Group that unconformably underlies Jurassic strata in the Disturbed Belt, northwestern Montana. These grainstones on surface exposures have a significant vuggy, intergranular or intercrystalline porosity (4-12 percent) and are permeable (6-12 md). Many of the pores are filled with dead oil.

The porous crinoidal grainstone unit has undergone eogenetic secondary dolomitization, probably in Late Mississippian time and, hence, prior to any significant erosional events. Porosity most likely resulted from solution effects during erosion and subaerial weathering before deposition of the Jurassic strata. Phreatic calcite cement, which developed after migration of liquid hydrocarbons into the grainstone unit, partially occludes some of the pore space.

Variations in thickness of the grainstone unit are mainly the result of pre-Jurassic erosion. In places the grainstones have been completely eroded beneath the Jurassic rocks; but, where present, they thicken to more than 100 m as observed in a north-south direction along the strike of imbricate thrust slices of the pre-Tertiary section. These thickness changes resulted either from broad warping of the Mississipian strata followed by planar erosional truncation, from erosional relief on the Jurassic erosion surface carved into unfolded Mississippian strata, or from some combination of these two effects which may have different geographic trends.