Mudrock Pore Networks: Lithological Control and Implications for Prediction of Flow Properties

Andrew C. Aplin and Julian K. S. Moore
University of Newcastle, Newcastle upon Tyne, United Kingdom

Flow properties of mudrocks are controlled by pore networks, which can be measured but not easily predicted. Our thesis is that mudrock pore networks, or pore size distributions, are controlled by grain size distributions, and thus lithology. In turn, mudrock lithology may be predictable from stratigraphic or depositional models.

We have analysed the grain size distributions of around 150 ancient mudrocks from a wide range of depositional environments. The distributions are remarkably consistent and can be understood in terms of well constrained physical processes. In essence, mudrocks comprise two components: flocs (mainly aggregates of clay minerals with a diameter less than 10 microns), and single grains (mainly non clay minerals with diameters greater than 10 microns). Mudrock lithology can then be described by the relative contributions of the two components.

Integration of grain size data, pore size data and petrographic observations suggests a critical division between (a) floc-dominated mudrocks whose structure is supported by the clay matrix and (b) silt-rich mudrocks whose structure is supported by a silt/fine sand framework. Floc-dominated mudrocks with clay matrix support develop low permeabilities and become very good capillary seals at relatively shallow depths. In contrast, silt-rich mudrocks with framework support only become low permeability units and very good capillary seals at much greater levels of compaction. The framework proposed here can form the basis of predictive flow and seal capacity models for mudrocks.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005