User:Geologyiscool123/Norman Wells Oil Field

The Norman Wells oil field (native Slavey language: Tłegǫ́hłı̨ [Thleh-go-lee] or "where there is oil") is Canada's longest operating conventional onshore oil field located in the city of Norman Wells in the Northwest portion of Northwest Territories, Canada.

Discovery
In 1789, explorer Alexander Mackenzie set out on the present-day Mackenzie River in hopes of finding a water passage through the northwest region of North America. He later wrote about a discovery he made: "Bituminous fountains; into which a pole of twenty feet long may be inserted without the least resistance. The bitumen is in a fluid state, and when mixed with gum or the resinous substance collected from the Spruce Fir, serves to gum the canoes.”

In 1911, prospector J.K. Cornwall was sent to the area by the Canadian government because oil exploration as being widely encouraged. Cornwall collected samples from small pools of oil along the Mackenzie River. Imperial Oil purchased the land, and had geologist Dr. Ted Link lead exploration with help of eight men, a drilling rig, and an ox. After a 1900 km hike, they landed at their destination in 1919. One year later, in August 1920 a rig struck oil resulting in a 25 meter “gusher”, which is now known as the site of Norman Wells.

Regional Setting
Norman wells is located on the eastern side of the foreland belt of the Pacific Cordillera. The Mackenzie mountains, which are situated about just west of Norman Wells oil field, were created by the collision of the Pacific and North American tectonic plates. The collision of the two plates causes Earth's crust to buckle, which can be seen as the mountain range. The Norman wells oil field sits within the Cordilleran foreland basin. Strata began experiencing deformation about 170 Ma, during the Jurassic, but experienced most of the deformation due to the Laramide orogeny in the Late Cretaceous.

The western North American foreland basin was an elongate trough that developed between the eastern flanks of the ancestral Rocky Mountains and the stable interior platform represented by the North American craton. At its maximum extent, this foreland basin was more than 6000 km long, stretching from the Arctic Ocean to the Gulf of Mexico and up to 1600 km wide, extending from westernmost Ontario to central British Columbia. In excess of 6 km of sediment was deposited along the western margin of the basin during its evolution from the Middle Jurassic to the middle Tertiary.

The stratigraphy that makes up the Norman Wells oil field was depositing during the Devonian. The location existed in the northern extension of a regional, intra-cratonic seaway that existed in the Middle to Late-Devonian. During this time interval sea level was high world-wide due to the absence of glaciers and much of the North American continent was covered by shallow seas. This reef complex was able to receive sufficient nutrients from paleocurrents and trade winds. The paleogeography at the time can provide an explanation for some preferential reef growth, with steep flanks and highly developed porosity and permeability on the north side.

Structural Setting
The Norman Wells oil field is extremely shallow with subsurface depths ranging from 350 to 650 meters. Canol Formation rocks were intially at a depth of at least 1.5 km, but underwent thrust-driven exhumation (by translation of the hanging wall rocks over a thrust ramp). At some critical depth below 650 m, and specific to the location and rheology, the rocks crossed a ductile–brittle transition zone and dip-oriented fractures formed sympathetic to the thrust fault. The combination of pore overpressure and new dip-directed sub vertical fractures liberated oil from the Canol Formation and allowed for up-dip oil migration. Natural fractures developed in response to deformation associated with the Late Cretaceous to Early Tertiary Laramide Orogeny. It is postulated that this fracturing process liberated oil within the Canol Formation and led to updip oil migration. Another factor related to the exhumation process is the decrease in litho-static pressure and concomitant relative increase in pore pressure, and this overpressure of fluids in the Canol Formation would have driven oil migration once the fractures had formed.

Although there is some debate, it is inferred that the Norman Range Thrust Fault, the fault that cross cuts all the pertinent strata at Norman Wells oil field, is a low angle detachment fault. This type of thrust faulting is derived from one of the smaller foreland mountain belts.

Depositional History
The Horn River Group of the Mackenzie Plain was deposited during the Middle Devonian and Upper Devonian (about 385 million years ago). This group contains the Hare Indian Formation, the Ramparts Limestone, and the Canol Shale. The Horn River group is overlain by the Imperial Sandstone.

Imperial Sandstone
Siliceous, black and green shales, calcareous shales, and siltstones of the Imperial were deposited during the maximum transgression and highstand of second-order sequence, following the terminal drowning of Norman Wells carbonate bank. The Imperial Formation is composed of prograding shale lobes that downlap onto the drowning unconformity.

The Canol Shale
The Canol Formation extends from the Franklin Mountain s east of Norman Wells to the Richardson Trough of the Yukon in the west. At this location its thickness varies from a few meters to greater than 100 m. The Canol Formation consists largely of dark grey to black, silica-rich shale with some pyrite and interbeds of siltstone and mudstone. The black mudstone can be cliff-forming because it is more resistant to weathering than other Devonian shales.

Rampart Limestone
At this location, the Ramparts Formation contains reefs of the Kee Scarp member at its top. The reef complex began to develop during Upper Middle Devonian time on shale banks of the Hare Indian formation. The Kee Scarp Formation consists of 10 carbonate megacycles of reef growth that are formed by major episodic increases in the rate of sea level rise. Each of these major cycles has minor increments within.

Hare Indian Shale
Greenish shale that has an interval of radioactive organic-rich black mudstone at its base, called the Bluefish Member. The Bluefish Member is important because it acts as a regional stratigraphic marker. The Bluefish Member is highly fissile, contains small calcareous beds, and distinctive thin tentaculid fossil bed; while the rest of the Hare Indian Formation is primarily made up of calcareous mudstone.

Source
Geochemical correlations indicate that the Canol Formation is the source rock for the oil. The Canol Shale has a high total porosity (8% and higher) and low water contents, which allows adequate storage capacity for oil or gas. It also contains high amounts of organic carbon for a shale (6% and greater), low clay contents, and high silica contents, which would make them brittle and prone to fracturing if hydraulic fracturing was applied. Hanging wall source rock passed through the ductile-brittle transition zone and fractures formed that were sympathetic to the regional thrust fault, allowing oil to pool in the reservoir. Measurements of thermal maturity indicate that the shale was heated into the oil window making them prospective for oil. Devonian to Cretaceous loading would have been insufficient to have resulted in the generation of significant amounts of hydrocarbons, but sediments loading from the west in late Mesozoic-Cenozoic (Cretaceous to Tertiary) was enough and resulted in the presently observed maturation pattern. The Canol Formation is a bituminous oil shale with type II kerogen and 2-7% total organic carbon.

Reservoir
The reef is 25 km by 8 km and is situated on a southwesterly dipping thrust plane. The oil reservoir contains approximately 108 million cubic metres (680 million barrels) of original oil in place. The reservoir is naturally fractured with low matrix permeability (avg. 2 to 4 millidarcies). The top of the reservoir reef limestone is encountered in the wells at depths from 1,050 to 1,950 feet, depending on their position on the structure. The Kee Scarp reefs have produced 274 million barrels of oil as of 2015.

Seal
Imperial Formation sandstones and mudstones form the top seal of the reservoir and are exposed at the surface directly above the reservoir at Norman Wells. Fractured reservoirs have mainly focused on how natural fractures formed during exhumation have resulted in fluid flow through fine-grained rocks, leading to a leaky seal. Canol Formation is fractured and has produced oil, which is why the argillaceous mudstones of the Imperial Formation are the effective seal. Reef growth of the Kee Scarp Formation was terminated by the advancing clastic sediments of the Canol and Imperial formations, which serve as the top seal.

Trap
The structure is monoclinal and the strata dip about 5º SW. Closure on the updip side is caused by pinching-out of the reef. The reservoir forms a combination structural-stratigraphic trap and is positioned within a NE-vergent thrust sheet that developed during the Laramide Orogeny (Late Cretaceous to Early Tertiary). Oil is trapped stratigraphically and structurally in an Upper Middle Devonian aged Kee Scarp.