Mudstone Pore Systems: Controls and Implications for Fluid Flow

Andrew C. Aplin¹, Kuncho D. Kurtev¹ and Julian K.S. Moore^1,2
1School of Civil Engineering and Geosciences, Newcastle University, Newcastle upon Tyne, NE1 7RU, UK
²Present Address: BP Exploration and Production, Chertsey Road, Sunbury on Thames, Middlesex, TW16 7LN, UK

The sizes and shapes of pores in sedimentary rocks are largely dictated by the sizes, shapes - and arrangement - of grains. This simple concept allows us to consider how pore systems are created as a result of depositional processes and how they evolve as sediments compact. It also forces us to consider how the physical segregation of grains in the aqueous environment results in sedimentological heterogeneity, and to consider the implications of heterogeneity for one and two phase fluid flow. For fine-grained sediments such as mudstones, grains are deposited by two processes: grains smaller than ~ 10 microns (largely phyllosilicates) are deposited largely as unsorted flocs whilst larger grains (largely quartz, mica and feldspars) are deposited as single particles according to Stokes Law. Grain size spectra allow one to subdivide mudstones according to the relative proportions of the two components. In a general sense, measured pore size distributions reflect grain size distributions; at a given porosity, finer grained mudstones have smaller pores and thus lower permeabilities. Measurements on well characterised mudstones indicate that lithology strongly influences porosity – permeability relationships. By regressing a unique dataset, we have generated a function which relates permeability to porosity and clay fraction. This can be used within basin models if (a) clay fraction can be estimated (for example from wireline log data) and (b) upscaling issues are resolved.

More detailed analysis of grain size and pore size data suggests that there is an important distinction between matrix-supported and framework-supported mudstones. Matrix-supported mudstones are dominated by plate-like phyllosilicates, with the sub-spherical silt grains floating in the phyllosilicate matrix. In mudstones with a substantial coarse silt component, the silt grains interact and provide framework support. Data for clay-silt pastes, shallow-buried and deeply-buried mudstones show that matrix-supported mudstones generally have unimodal pore size distributions and become low permeability, high capillary entry pressure units within the first kilometre of burial. In contrast, framework-supported mudstones are characterized by bimodal pore size distributions and retain a relatively high permeability and low CEP even when highly compacted. Depending on the sedimentary environment, matrix-supported and framework-supported mudstones may coexist, and a key challenge for the future is to understand and incorporate mudstone heterogeneity into flow models and seal evaluation. We will need to consider our approaches carefully because the critical depositional heterogeneities are often below the resolution of the geophysical tools typically deployed by the industry.

 

AAPG Search and Discover Article #90066©2007 AAPG Hedberg Conference, The Hague, The Netherlands

 

AAPG Search and Discover Article #90066©2007 AAPG Hedberg Conference, The Hague, The Netherlands