Sedimentology, micropaleontology, geochemistry, and hydrocarbon potential of shale from the Cretaceous lower Colorado Group in western Canada

Shale of the Cretaceous lower Colorado Group extends across the subsurface of the Western Canada Sedimentary Basin (WCSB) and along the length of the Rocky Mountain Foothills. Four regionally mappable shale units are recognized by their distinctive geochemical, mineralogical, biofacies and sedimentological characteristics: the Late Albian Westgate, the Early Cenomanian Fish Scales, the Middle to Late Cenomanian Belle Fourche, and the latest Cenomanian to Middle Turonian Second White Specks Formation. These units were deposited in a foreland basin during a period of overall sea-level rise punctuated by periods of tectonically induced, relative sea-level falls. The Westgate Formation is a progradational siltstone to mudstone having a low total organic carbon (TOC) content (< 2 wt %) of dominantly Type III organic matter (OM). The Westgate was deposited above storm wave-base in the Mowry Sea. The inner to middle neritic foraminiferal assemblage (Milliammina manitobensis Zone) and oxygen isotope composition of early diagenetic carbonate concretions indicate a cool, low-salinity water mass of Boreal affinity. Compacted sedimentation rates ranged from 1 to 6 cm/10 3 yrs. The Fish Scales Formation is a transgressive mudstone to claystone with TOC contents of up to 8 wt % of mixed types II and III OM. The basal conglomeratic layer indicates a drastic environmental change at the end of the Albian. The water column was stratified with widespread anoxic bottom waters, as is indicated by well-laminated sediments and the absence of benthic foraminifera. Compacted sedimentation rates ranged from 1 to 2 cm/10 3 yrs. The contact between between the Fish Scales and the Belle Fourche formations is gradational, signifying a gradual improvement of benthic conditions. OM changes to Type III and the abundance is generally less than 2 wt%. By this time, the seaway connecting Boreal and Tethyan water masses was established but waters of Boreal affinity were dominant in the WCSB. The northwestern part of the basin was influenced by the progradation of deltaic sediments of the Dunvegan Formation. Compacted sedimentation rates ranged from 0.7 to 5.0 cm/10 3 yrs. The laminated marlstones to limestones of the Second White Specks Formation were deposited under dominantly anoxic conditions beneath an increased water column that fostered fully marine conditions. Increased water depths allowed Tethyan planktonic foraminifera and nannofossils to migrate into the WCSB. These conditions resulted in increased TOC contents of up to 12 wt% of Type II OM. Bottom currents reworked bioclastic sediments deposited below storm wave-base. Compacted sedimentation rates ranged from 1.3 to 2.5 cm/10 3 yrs. The early diagenesis of lower Colorado Group shales is dominated by sulfate reduction and methanogenesis. These microbially mediated processes resulted in the formation of pyrite, carbonate concretions and some clay mineral neoformation, primarily kaolinite. Carbonate concretions in the Westgate, Fish Scales and Belle Fourche formations are dominantly siderite, whereas those of the Second White Specks Formaton are calcite. Alteration of volcanic ash and hydrolysis of K-aluminosilicates resulted in smectite, mixed-layer illite/smectite and kaolinite authigenesis. Rock-Eval pyrolysis data indicate that, west of approximately 114° longitude, the lower Colorado Group shales are mature source rocks. The Second White Specks Formation is an effective source rock and apparently there is active localized hydrocarbon migration from the Fold and Thrust Belt. It is inferred that hydrocarbon migration and production from the Second White Specks Formation is controlled largely by fractures.