Abstract: Mudstone Porosity: Quantity and Connectivity

Andrew C. Aplin, Yunlai Yang, David Dewhurst, Sven Hansen

As reservoir seals, source rocks and, more generally, the least permeable and most abundant lithology in sedimentary basins, mudstones exert a critical influence on mass and heat transfer on both the basin and reservoir scale. However, little information exists on the true porosity of mudstones and, critically, its connectivity.

We report here some results of a study of almost 100 mudstone samples from three wells on the Norwegian Margin, ranging in depth from 500-3500 m. Grain densities determined by He porosimetry for whole cuttings are lower than those (i.e. true densities) for disaggregated samples, probably reflecting the presence of unconnected pores in the bulk samples. These data suggest that at depths greater than 3 km much mudstone porosity may be unconnected. We have also inferred the relative proportions of effective and unconnected porosity by comparing the specific surface areas (SSA) of bulk and disaggregated samples. The former estimates the SSA of connected porosity whilst the second gives the grain SSA. Greater differences between the two measurements imply a greater proportion of unconnecte porosity. For organic-lean mudstones buried to <3 km, the two measures generally agree to within 30%. However, with increasing organic richness, the SSA measured on bulk samples declines relative to that of the disaggregated material, implying that organic matter is blocking pore throats. This observation could explain why pressure kicks are quite commonly observed across organic rich mudstones in petroleum provinces. Together, the data suggest that a significant fraction of mudstone porosity is unconnected at > 3km burial and, in organic-rich mudstones, at in most depths. The data also have profound implications for the estimation of permeability from porosity.

AAPG Search and Discovery Article #90956©1995 AAPG International Convention and Exposition Meeting, Nice, France