PORE-SIZE AND PORE-THROAT SIZE DISTRIBUTIONS IN DOLOSTONE RESERVOIRS OF THE ORDOVICIAN RED RIVER FORMATION, WILLISTON BASIN, MONTANA

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.