Not all Shales are Created Equal — Obscure Fine Grained Intraclasts and Lithics and Possible Consequences for Reservoir Quality in Dunvegan Mudstones

Abstract

The Cenomanian Dunvegan Formation (Western Canadian Sedimentary Basin) contains abundant silt sized fine grained aggregate grains, the origin of which is still under debate. Samples from basal allo-members of the Dunvegan are characterized by mudstones interbedded with sharp based graded very fine, silt to mud beds with rare wave and current ripples. Argon ion milled samples show a large variety of aggregate types, with widely varying proportions from sample to sample. A portion of these aggregates (variably compacted) probably represent material eroded from the seabed and then redistributed by bottom currents. Other fine grained aggregates appear to be rock fragments of volcanic origin (variably altered), low grade metamorphic particles, chert with a variable clay component, and shale lithics (fragments derived from weathered shale outcrops). In spite of these general groupings, there is a veritable “zoo” of aggregate types that may tempt one to “split” too much as well as offering opportunities for highly sophisticated interpretation of these mudstones. For example, some aggregate types (seabed derived intraclasts) may systematically change from base to top of parasequences, whereas others may be characteristic to specific sequences as new source rocks are eroded in the course of continued thrusting. Abundant silt sized fine-grained aggregates in both silty and muddy intervals suggest that in spite of “muddy” appearance, these muds were deposited as relatively high energy silt successions. Excepting diagenetic cementation, siltstones tend to have favorable porosity and permeability characteristics when compared with clay dominated mudstones. In a case where there is an abundance of both soft (water-rich intraclasts) vs hard (lithics) aggregates, the proportion of aggregate types may have a strong bearing on the amount of potentially preservable porosity. Given that lithics have very little porosity to start with, and that soft aggregates get squeezed between hard grains, one could for example imagine large pores and good permeability in an “all lithics” rock, and low porosity and permeability for a rock that consists of lithics mixed with soft aggregates. Because of the impact of poro-perm characteristics on reservoir quality, these compositional parameters are critical for any sensible reservoir assessment. However, they cannot be derived from geochemical or petrophysics data, and instead require careful petrographic study of the rocks in question.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016