Observations of pore systems of natural siliciclastic mudstones

Abstract

Grains within siliciclastic muds are deposited either as flocs, in which grains are generally ,10 mm, or as single grains: “sortable silt,” generally .10 mm. When clay-size (,2 mm diameter) particles form .30% of mudstones, pore-size distributions are controlled mainly by the interaction of phyllosilicates; these materials are ‘matrix-supported.’ Pores associated with clay-size particles are typically ,20 nm, even at shallow burial. When clay-size particles comprise ,30% of the grain-size distribution, a second, much larger pore system is observed, controlled by the amount and size of sortable silt; these mudstones are ‘framework-supported.’ Compaction of these silt-rich materials occurs mainly by the loss of the largest pores, but large pores still exist up to high effective stresses in the absence of chemical compaction. Mercury injection porosimetry (MIP) gives information about pore-throat size and pore connectivity and thus provides useful data with which to estimate permeability. Models based on generally flat pore shapes can estimate the permeability of homogenous mudstones to+a factor of 3 of the true value, but cannot be used for heterogeneous, laminated mudstones, which exhibit highly anisotropic permeabilities. As MIP gives information about pore throats and microscopy gives information about pore bodies, the two techniques generate different results. Both are required, along with other techniques such as small-angle neutron scattering and low-pressure gas sorption, in order to fully appreciate the complexity of mudstone pore systems.