|Titre||Compaction and fluid migration in cretaceous shales of western Canada|
|Licence||Veuillez noter que la Licence du gouvernement
ouvert - Canada remplace toutes les licences antérieures.|
|Source||Commission géologique du Canada, Dossier public 49, 1971, 105p., https://doi.org/10.4095/119781 Accès ouvert|
|Media||papier; en ligne; numérique|
|Référence reliée||Cette publication est remplacée par Compaction
and fluid migration in cretaceous shales of western Canada |
|Province||Colombie-Britannique; Alberta; Saskatchewan; Manitoba; Ontario; Québec; Nouveau-Brunswick; Nouvelle-Écosse; Île-du-Prince-Édouard; Terre-Neuve-et-Labrador; Territoires du Nord-Ouest; Yukon; Nunavut;
|SNRC||1; 2; 3; 10; 11; 12; 13; 14; 15; 16; 20; 21; 22; 23; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33; 34; 35; 36; 37; 38; 39; 40; 41; 42; 43; 44; 45; 46; 47; 48; 49; 52; 53; 54; 55; 56; 57; 58; 59; 62; 63; 64; 65;
66; 67; 68; 69; 72; 73; 74; 75; 76; 77; 78; 79; 82; 83; 84; 85; 86; 87; 88; 89; 92; 93; 94; 95; 96; 97; 98; 99; 102; 103; 104; 105; 106; 107; 114O; 114P; 115; 116; 117; 120; 340; 560|
|Sujets||schistes; migration des fluides; porosité; perméabilité; solution pressurisante; densités; Groupe de Banff ; Groupe de Bullhead ; Groupe de Rundle ; Formation de Schooler Creek ; Groupe de Wabamun ;
géologie de l'ingénieur; sédimentologie; géologie structurale; Crétacé; Tertiaire|
|Diffusé||1971 01 01; 2017 11 22|
|Résumé||(disponible en anglais seulement)|
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
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.