Pore networks in continental and marine mudstones: Characteristics and controls on sealing behavior

Abstract

Mudstone pore networks are strong modifi ers of sedimentary basin fl uid dynamics and have a critical role in the distribution of hydrocarbons and containment of injected fl uids. Using core samples from continental and marine mudstones, we investigate properties of pore types and networks from a variety of geologic environments, together with estimates of capillary breakthrough pressures by mercury intrusion porosimetry. Analysis and interpretation of quantitative and qualitative three-dimensional (3D) observations, obtained by dual focused ion beamscanning electron microscopy, suggest seven dominant mudstone pore types distinguished by geometry and connectivity. A dominant planar pore type occurs in all investigated mudstones and generally has high coordination numbers (i.e., number of neighboring connected pores). Connected networks of pores of this type contribute to high mercury capillary pressures due to small pore throats at the junctions of connected pores and likely control most matrix transport in these mudstones . Other pore types are related to authigenic (e.g., replacement or pore-lining precipitation) clay minerals and pyrite nodules; pores in clay packets adjacent to larger, more competent clastic grains; pores in organic phases; and stylolitic and microfracture-related pores. Pores within regions of authigenic clay minerals often form small isolated networks (<3 μm). Pores in stringers of organic phases occur as tubular pores or slit-and/or sheet-like pores. These form short, connected lengths in 3D reconstructions, but appear to form networks no larger than a few microns in size. Sealing effi ciency of the studied mudstones increases with greater distal depositional environments and greater maximum depth of burial.

Deformation in cemented mudrock (Callovo–Oxfordian Clay) by microcracking, granular flow and phyllosilicate plasticity: insights from triaxial deformation, broad ion beam polishing and scanning electron microscopy

“…FIB and BIB), which allow imaging of mineral fabrics and porosity down to the nanometre scale in very high-quality cross sections with SEM and TEM (Lee et al, 2003;Desbois et al, 2009Desbois et al, , 2011Desbois et al, , 2013Desbois et al, , 2016Loucks et al, 2009;Curtis et al, 2010;Heath et al, 2011;Klaver et al, 2012;Keller et al, 2011Keller et al, , 2013Houben et al, 2013Houben et al, , 2014Hemes et al, 2013Hemes et al, , 2015Laurich et al, 2014;Warr et al, 2014;Song et al, 2016). Serial sectioning allows the reconstruction of microstructure in 3-D (Keller et al, 2011(Keller et al, , 2013Milliken et al, 2013;Hemes et al, 2015), and cryogenic techniques can image the pore fluid in the samples and avoid artefacts produced by drying Schmatz et al, 2015).…”

Observations of pore systems of natural siliciclastic mudstones

“…The idea that pores in clay-rich mudstones are relatively flat, thus reflecting the shape of clay grains, is intuitively reasonable and has, to some extent, been validated by recent 3D images from the FIB-SEM (e.g. Desbois et al, 2009;Heath et al, 2011).…”

“…Desbois et al, 2009;Heath et al, 2011;Klaver et al, 2012;Clarkson et al, 2012Clarkson et al, , 2013Ruppert et al, 2013;Yang et al, 2014), e.g. in pressure shadows or at the interfaces of clay aggregates.…”

Impact of Effective Pressure on Threshold Pressure of Kazusa Group Mudstones for CO₂ Geological Sequestration

“…In studies associated with the pore structure of sedimentary rocks, Heath et al 28,29) investigated the characteristics of pore networks for mudstones using dual-beam focused ion beam scanning electron microscopy (FIB/SEM) to provide an insight into the mechanism of fluid flow into rock media. They identified seven types of pores in the mudstone specimens used in their studies, and they observed that these pores had a significant proportion of microscale crack-like structures.…”