The Mississippian Madison limestone reservoir rock in the Beaver Lodge field, Williams County, North Dakota, was studied to determine its lithologic character and distribution of the porous zones.

The reservoir is a fine- to medium-grained fragmental limestone and dolomitic limestone. Grains include fossil fragments, crystal fragments, and fecal pellets. Originally, porosity was intergranular but later it was increased by solution and dolomitization and decreased by recrystallization and cementation.

There are three major, separate, porous zones in the reservoir; each zone contains lenticular streaks of porous rock which form an interfingering pattern like that of interbedded sandstone and shale. Porosity, controlled by original texture, has been increased through dolomitization by 3 to 5 times. Fracturing is most common in the finer parts of the rock, and the fracturing provides permeability connections between the porous lentils.

The mineralogy is similar to that found in modern deposits on the Bahama Banks; the environment of deposition was probably similar.

The porosity pattern at Beaver Lodge Mississippian limestone reservoir appears to be due partly to a sedimentary response to intermittent uplift and folding during Mississippian time.