Following growth of the Canadian Cordillera during the Mesozoic, the southern Cordillera was subject to extension during the Paleocene and Eocene that correlated with widespread volcanic activity in south-central British Columbia, including across much of the Nechako-Chilcotin plateau. In 2008, Geoscience BC acquired 330 km of deep vibroseis reflection profiles on the plateau, mostly over the Stikinia arc terrane, but also over its eastern contact with the oceanic Cache Creek terrane. All seven seismic reflection lines reveal a strongly reflective lower crust that extends from 7 to 9 s down to the Moho, which is defined by the downward termination of reflectivity at 11-12 s. In the uppermost crust, extension occurred by block faulting with faults soling into subhorizontal to shallowly dipping detachments above 10 km depth. Extension in the deeper upper and middle crust, which was partly controlled by antiforms likely related to earlier shortening, was accommodated on a network of anastomosing shear zones that sole out into the top of the reflective lower crust. The lower crustal reflections correlate with seismic P-wave velocities of 6.45-6.98 km/s, indicating that the reflective lower crust has a more mafic composition than the middle crust. As in other extensional settings, we suggest that this pervasive fabric of reflectors arises from the intrusion of mantle-derived basaltic magma into zones of ductile shearing, and that differentiation of these melts resulted in the widespread Paleocene to Eocene volcanism. Reflector dips indicate that extension was approximately east-west, consistent with north-northwest-trending horsts separated by basins filled with Paleocene to Eocene volcanic and volcaniclastic rocks.Résumé : Après la croissance de la Cordillère canadienne au Mésozoïque, la Cordillère sud a subi une extension durant le Paléocène et l'Éocène; cette extension était corrélée avec une activité volcanique étendue dans le centre-sud de la ColombieBritannique, incluant à travers une grande partie du plateau Nechako-Chilcotin. En 2008, Geoscience BC a acquis 330 km de profiles vibrosismiques en profondeur sur le plateau, surtout au-dessus du terrane d'arc de Stikinia mais aussi par-dessus son contact est avec le terrane océanique de Cache Creek. Les sept lignes de sismique réflexion révèlent toutes une croûte inférieure fortement réflective qui s'étend depuis 7-9 s jusqu'au Moho, ce qui est défini par la terminaison vers le bas de la réflectivité à 11-12 s. Dans la croûte sommitale, l'extension s'est produite par le morcellement en bloc par des failles; les failles formant une semelle dans les détachements subhorizontaux ou avec un léger pendage, au-dessus de la profondeur de 10 km. L'extension dans les croûtes supérieure et médiane, plus profondes, partiellement contrôlée par des antiformes fort probablement reliés à un rétrécissement antérieur, était facilitée par un réseau de zones de cisaillement anastomosées qui forment une semelle sur le dessus de la couche inférieure réfléchissante. Les réflexions de la ...
Across the Nechako-Chilcotin plateau of British Columbia, the distribution of Cretaceous sedimentary rocks, which are considered prospective for hydrocarbon exploration, is poorly known due to the surface cover of glacial deposits and Tertiary volcanic rocks. To constrain the subsurface distribution of these Cretaceous rocks, in 2008 Geoscience BC acquired seven long, up to 14.4 km, offset vibroseis seismic reflection lines across a north-northwest-trending belt of exhumed sedimentary rocks inferred to be part of the Taylor Creek Group. P-wave velocity models, which are consistent with sonic logs from nearby wells, have been estimated using three-dimensional first-arrival tomography to depths ranging from 1 to 4 km. Igneous basement can be identified on most lines using the 5.5 km/s isovelocity contour, which locates the top of the basement to an accuracy of ϳ400 m where its depth is known in exploration wells. There is no general distinction on the basis of seismic velocity between Cretaceous sedimentary and PaleoceneEocene volcanic-volcaniclastic rocks, both of which appear to be characterized in the tomographic models by velocities of 3.0-5.0 km/s. The geometry of the igneous basement inferred from the velocity models identifies north-trending basins and ridges, which correlate with exposed rocks of the Jurassic Hazelton Group. Identified Cretaceous sedimentary rocks occur beneath less negative Bouguer gravity anomalies, but the original distribution of these rocks has been disrupted by later Tertiary extension that created north-trending basins associated with the most negative gravity anomalies. We suggest that Cretaceous sedimentary rocks, if deposited, could be preserved within these basins if the rocks had not been eroded prior to Tertiary extension.Résumé : À travers le plateau de Nechako-Chilcotin de la Colombie-Britannique, la distribution des roches sédimentaires du Crétacé, lesquelles sont considérées prometteuses pour l'exploration des hydrocarbures, est mal connue en raison des dépôts glaciaires et des roches volcaniques du Tertiaire qui recouvrent la surface. Afin de délimiter la distribution sous la surface de ces roches du Crétacé, Geoscience BC a acquis, en 2008, sept longues lignes décalées, de réflexion vibrosismique, atteignant 14,4 km, à travers une ceinture à tendance nord-nord-ouest de roches sédimentaires exhumées, qui feraient partie du Groupe de Taylor Creek. Les modèles de vitesse de l'onde P, concordant avec les diagraphies acoustiques provenant de puits avoisinants, ont été estimés au moyen de la tomographie tridimensionnelle des premières ondes arrivées à des profondeurs variant de 1 à 4 km. Le socle igné peut être identifié sur la plupart des lignes en utilisant le contour d'isovitesse de 5,5 km/s, ce qui situe le sommet du socle avec une précision de~400 m, où sa profondeur est connue dans les puits d'exploration. En se basant sur les vitesses sismiques, il n'y a pas de distinction générale entre les roches sédimentaires du Crétacé et les roches volcaniques-volcanoclastiques du P...
One of the challenges encountered during the life cycle of an oil-sand thermal-production reservoir is the prediction of the formation pore pressure and in situ stress regime during the assessment phase of the reservoir development and, more importantly, during the development phase. We have investigated the state of formation pore pressure and stress in the overburden — represented by the Clearwater Formation, Grand Rapids Formation, and Colorado Group — of a preproduction oil-sands reservoir situated in the Athabasca Basin of Alberta, Canada. Our methodology integrates pressure data from piezometers, stress data from mini-frac (MF), dipole sonic logs, and elastic properties obtained from multicomponent 3D seismic inversion data. It combines the Terzaghi effective stresses with the Schoenberg and Sayers elastic stiffness matrix for horizontal transversely isotropic fractured materials. The total principal stresses (vertical, minimum, and maximum horizontal stresses) are expressed as functions of the normal fracture weakness (anisotropic correction factor), formation pore pressure, seismic data (Lamé constants), and the Biot-Willis coefficient. The effective principal stresses are estimated from the equivalent total principal stresses and the formation pore pressure multiplied by the Biot-Willis coefficient. On all three overburden intervals analysed, the relations between principal stresses indicate a normal stress regime. The estimated total minimum horizontal stress matches the MF values within 10%. The formation pore pressure, along with the 3D seismically derived estimates of the total and effective principal stresses, allows for better assessment of the caprock integrity and for operational savings based on a reduced number of MF tests. It can also be used for stress estimation within the formations hosting aquifers, which is so important for thermal production. Understanding the subsurface on the reservoir area is important for efficient production, but knowing the subsurface of the overburden is equally important for reducing potential issues due to production.
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