Abstract: Entry Pore-Throat Size of Nonsmectite Shales

Jim Krushin

Petrophysical measurements confirm that large hydrocarbon columns may be limited by the sealing shale entry pore-throat size. Capillary, or membrane, sealing ability is determined from both the pore-throat size distribution, interfacial tension, contact angle, and hydrodynamic conditions if present. Interpretation of high pressure mercury-injection data (to 30,000 psi) defines the pore-throat size distribution of nonsmectite shale. The nature of interlayer water associated with smectites (i.e. these pores only exist because of the presence of water), preclude mercury injection as an appropriate porosimetry method due to the prerequisite drying.

"Shales" in the title refers to a broad class of predominately terrigenous, fine-grained, clastic sedimentary rock with no regard to fissility or laminations. The thirteen well-indurated shales studied range in age from Precambrian to Jurassic, and vary in mineralogy, porosity, permeability, cation exchange capacity, organic content, and stratification. Based on x-ray diffraction and visual estimation of thin sections, samples are primarily classified according to silt-clay ratio and presence of laminations. Massive samples containing less than one-third, and between one-third to two-thirds, silt of the total silt + clay matrix are claystones and mudstones, respectively. Corresponding laminated versions are clayshale and mudshale. Modifying terms such as sandy, organic, or calcareous re generally greater than 10% nonclay and nonsilt, and further classifies the samples as sandy mudstone, feldspathic mudstone, sandy dolomitic mudshale, calcareous claystone, calcareous lithic mudstone, and organic clayshale.

High-pressure mercury injection determined the entry or synonymous displacement pressure required to begin saturating the seal with the nonwetting phase. Any further generated hydrocarbons would then enter the seal due to buoyancy and not extend the reservoir's hydrocarbon column.

Nonorganic shale samples with less than 10% porosity have entry pore throats radii less than 12 nm, and are considered to be excellent capillary seals. The three nonorganic shales with the largest entry pore throat radii, ranging from 24 to 53 nm are sandy mudstones which contain silt-supported, contorted laminations. These may limit the size of large hydrocarbon columns. Three organic clayshales have entry pore throat radii ranging from 11 to 53 nm. Due to the lack of knowledge of the wettability of organic shales, no attempt is made to relate the entry pore-throat size to hydrocarbon columns.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994