Compaction and fluid migration in cretaceous shales of western Canada
Authors: Magara K 
Published: March 2017

Part I: Shale porosity distributions within Cretaceous and Tertiary rocks of the Alberta and Saskatchewan subsurface have been determined by the use of sonic and formation-density logs, and the examination of cores and surface rock samples. At shallow depths,
shale porosity appears to be exponentially related to depth. Porosity at depth in Cretaceous shales, especially in the western part of the area studied, tends to be greater than the porosity-depth trend established at shallower depths would suggest, and is related to anomalously high fluidpressure
conditions. Thus, abnormally high fluid pressures may be expected where shales are incompletely compacted.
Part II: Fluid-pressure gradients in shales can be determined by the porosity distribution, as derived from sonic logs, of incompletely compacted shales. Differing permeabilities in shales may be estimated through use of the fluid-pressure gradient and Darcy's law. Calculated shale perrneabilities
and porosity values can be integrated to establish a relationship between shale porosity and permeability in the subsurface. This method of analysis applied to Cretaceous shales in the subsurface
of Alberta and Saskatchewan reveals that in shale the permeability increases less with increase in porosity than the amount given by Archie's relation, which is based on sandstones and carbonate rocks. This calculated porosity-permeability relationship for shales has been verified in numerous other
studies by laboratory-measured porosity and permeability data.
Part III: Anomalously high pressures in the deep subsurface can be explained by fluid-release mechanisms related to compaction of shales.The volume of fluids which should be expelled from shales in unit time to reach compaction equilibrium may be determined for several different rates of
sedimentation, based on the shale-porosity data in western Canada. For each rate of sedimentation or subsidence there is a minimum permeability for reaching compaction equilibrium, which may be calculated according to Darcy's law. Comparison of this calculated minimum permeability with actual shale
permeabilities, determined by laboratory measurements , suggests that at relatively shallow depths shale should usually be permeable enough to permit compaction equilibrium to be attained,
and to maintain normal hydrostatic pressure. At depth, however, actual permeabilities are less than the calculated minimum necessary for compaction equilibrium, so that abnormal pressures may occur at these greater depths. The incidence of such abnormal pressures should increase with increases in
the rate of sedimentation and in the total thickness of the sequence.
Part IV: Shale-porosity distribution in incompletely compacted shale zones may also be affected by the permeability of adjacent sandstone or carbonate rock bodies. Sharp decrease of porosity in shales close to such rock bodies would suggest that relatively large volumes of fluids have been expelled
from the shales into the adjacent sandstones or carbonates. If this expelled fluid volume is large, the possibility of hydrocarbon accumulation in such sandstone or carbonate rocks is considered to be strong. To illustrate such a study, Cretaceous shales associated with hydrocarbon reservoirs in the
subsurface of northwestern Alberta and northeastern British Columbia have been examined. Most Mesozoic oil and gas pools in this area are concentrated where a greater volume of fluids is considered to have been expelled downward from the overlying shales.
Part V: In order to obtain laboratory data on compaction and fluidexpulsion relationships, a number of experiments were conducted on several kinds of clays. The results yielded porosity distributions similar to those of Cretaceous shales in the subsurface of western Canada.
Part VI: In order to understand the fluid-migration history in a given area, porosity patterns of incompletely compacted shales can be constructed for several stages of compaction. In part VI, these relationships are investigated and some suggestions made for future work.