Enormous volumes of gas (.30 Tcf ) are contained within the deepest portions of the Western Canada Foreland Basin, where tight gas-saturated Cretaceous sandstones grade updip into porous water-saturated sandstones. Production has occurred from coarse-grained shoreline sands both near the updip gas-water interface, such as those found in the Elmworth Field, and from low-porosity-permeability reservoirs found deeper in the basin. These basin-centred gas (BCG) reservoirs are characterized by regionally pervasive gas-saturated lithologies, abnormal pressures and no downdip water contact, and occur in low-permeability reservoirs. The keys to Shell's exploration success were an understanding of the stratigraphy, sedimentology and rock properties of the basin, the development of structural, petrophysical and geomechanical models, development of an understanding of the desiccation or dewatering process, the distribution of water within the basin and how the pressure regime evolved, interpretation of 3D seismic, and an aggressive land strategy. The evaluation of structural leads was aided when seismic and geomechanical modelling were combined, thereby aiding in the prediction of zones with a higher probability of encountering favourable reservoir producibility characteristics, that is, areas where a well developed, well connected open fracture network is expected. This multidisciplinary approach has resulted in economic success in regions once thought to be non-productive, and where it was once said, 'People go broke chasing the Nikanassin'.
Rocks of the eastern Bella Coola (NTS 93 D) map area record several distinct deformational events representing episodic extension, contraction, and transpression in the region. Extension may have been coeval with the generation of volcanic sequences of the Jurassic Hazelton Group and Lower Cretaceous Monarch sequence. Lower Cretaceous Monarch sequence rocks exhibit tight, northeast-vergent folds and minor thrust faults. This contractional belt is cut by ductile protomylonitic to mylonitic transpressional shear zones (10-1000 m wide) that affect most rocks in the western portion of the mapped area, and are themselves cut by various granite and tonalite bodies. Brittle faults cut all rocks of the area and form prominent, north-south-trending topographic linear features.
The abundant plutonic rocks of the eastern Bella Coola map area are mostly of intermediate composition and are interpreted as Jurassic to Eocene. They are subdivided into five intrusive suites, one intrusive complex, and several undifferentiated plutons. The Salloomt suite ((?)Jurassic) is dominated by medium-grained, hornblende-rich diorite. The Kalone suite of biotite-hornblende tonalite is involved in ductile deformation within shear zones adjacent to Jump Across Creek. The Talcheazoone suite consists of uniformly foliated quartz diorite. The Firvale suite comprises altered Early Cretaceous biotite granodiorite. The Four Mile suite comprises pristine biotite-muscovite granite; these plutons truncate a regional pattern of the northwest-trending folds and shear zones. The Crag Creek intrusive complex is diorite and gabbro cut by numerous locally sheeted dykes of granodiorite, rhyolite, basalt, and andesite. Some lens-like sills between screens of country rock may have formed along pre-existing, subvertical structures.
The Endako map area (93K/3) is underlain by two main bedrock assemblages translated and tilted by Tertiary strike-slip and extension faulting. A Mesozoic complex, consisting of various Cretaceous and Jurassic granitic to dioritic phases, has been subdivided, from oldest to youngest, into the Boer, Stag Lake, and Francois Lake suites. The youngest suite, which hosts the Endako molybdenum mine, consists of granodioritic to granitic units that grade into aplitic phases containing miarolitic cavities. A Tertiary complex consists of Eocene Ootsa Lake Group dacite, rhyolite, andesite, and basalt, and Eocene and Oligocene Endako Group basalt. The Ootsa Lake Group is dominated by intermediate crystal tuffs and includes interlayered mafic and felsic volcanic rocks.
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