Pore systems in dolostone of the Upper Ordovician Red River Formation of the Williston Basin, Montana, have been analyzed by a combination of capillary-pressure histograms of pore-throat size distributions generated by mercury porosimetry and pore-size distribution histograms generated by image analysis. Pore-throat size distributions are calculated using 130^o, 140^o, and 180^o contact angles in the Washburn equation and are correlated and compared to pore-size distributions. It is assumed that planar-euhedral dolomite with unimodal pore throats represent a sheet type of geometry with no distinction between throats and pores. Calculating pore-throat diameters using 130^o or 140^o contact angles in the Washburn equation during mercury porosimetry most often generates pore-throat size distributions with pore-throat diameter modes similar to pore-size distributions and pore-diameter modes.

Intercrystalline pore systems have a unimodal pore-throat size distribution and tend to have a unimodal or bimodal pore-size distribution. The pore-throat sizes and pore sizes are well sorted with a narrow range in the pore-throat size distribution but with a slightly wider range of pore sizes. Pore-throat diameter modes, ranging from 1.05 mm to 4.59 mm, are similar to the pore-diameter modes, ranging from 2.19 mm to 4.59 mm, when using either a 130^o or a 140^o contact angle in the Washburn equation.

In moldic-intercrystalline pore systems, three different kinds of pore throats occur: 1) moldic pore to moldic pore interconnections, 2) moldic pore to intercrystalline pore connections, and 3) intercrystalline pore to intercrystalline pore connections. Pore throats have a polymodal distribution, the smaller sized modes relate to intercrystalline pore throats and the larger modes relate to throats that directly connect moldic pores. Pore-size distributions are generally bimodal. Pore-throat and pore sizes also tend to be moderately sorted with moderate ranges in size. Most pore-throat sizes are determined by crystal-boundary surfaces and by the junction of crystal boundaries to molds. A wide range of dolomite grain sizes tends to produce a wide range in pore-throat sizes. Intercrystalline porosity represents the majority of the pores, with the larger mold interconnections skewing the pore-throat size distribution to the larger values.