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Pore-throat sizes in sandstones, tight sandstones, and shales

¹ U.S. Geological Survey, Box 25046, Federal Center, Denver, Colorado 80225-0046; pnelson{at}usgs.gov

Phil Nelson is a member of the Central Energy Resources Team of the U.S. Geological Survey, which provides assessments of undiscovered oil and gas. He held research positions in mineral exploration with Kennecott Exploration Services, radioactive waste storage with Lawrence Berkeley Laboratory, and petroleum production with Sohio Petroleum Company. His current interests are in the characteristics of tight-gas resources and the pressure and temperature regimes of sedimentary basins.

Pore-throat sizes in siliciclastic rocks form a continuum from the submillimeter to the nanometer scale. That continuum is documented in this article using previously published data on the pore and pore-throat sizes of conventional reservoir rocks, tight-gas sandstones, and shales. For measures of central tendency (mean, mode, median), pore-throat sizes (diameters) are generally greater than 2 µm in conventional reservoir rocks, range from about 2 to 0.03 µm in tight-gas sandstones, and range from 0.1 to 0.005 µm in shales. Hydrocarbon molecules, asphaltenes, ring structures, paraffins, and methane, form another continuum, ranging from 100 Å (0.01 µm) for asphaltenes to 3.8 Å (0.00038 µm) for methane. The pore-throat size continuum provides a useful perspective for considering (1) the emplacement of petroleum in consolidated siliciclastics and (2) fluid flow through fine-grained source rocks now being exploited as reservoirs.