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Valhall Oil Field
The Valhall oil field is located near the southeastern most point of the Norwegian sector of the North Sea. It is considered part of the Greater Ekofisk area and resides within the Central Graben along the Lindesnes Ridge. It was discovered by exploratory drilling in 1975 and began production in 1982. The reservoir consists of fractured chalk which retains porosity due to its location within an overpressured zone and is made financially viable due to a relatively high permeability because of fractures within the system. The Valhall field has been particularly difficult to study due to the gas charges within the structure which often hinders the acquisition of accurate seismic profiles. Modeling has also been a challenge, as the reservoir properties, such as thickness, porosity, permeability, and pressure, have continuously changed since its discovery due to subsidence, pore collapse, decrease in hydraulic pressure, and increasing amounts of fractures and faulting. Data collection has become more accurate since a permanent ocean bottom cable array in 2003.

Tectonic History
The North Sea was formed during a multiphase extensional event which began in the Jurassic. The rifting caused a series of horst and graben structures which are used throughout the North Sea Basin as traps for hydrocarbon systems. Horst and grabens form due to basin extension and cause both high areas and low areas to form through normal faulting occurring in opposite directions over a basin. During the late Cretaceous period, orogenic events caused various anticlinal structures to form, one of which makes up the Valhall Structure. The compression of the North Sea basin is one of the reasons for the systems high permeability, as the folding caused a large fracture network to form. With the exception of the seal (Rogaland Formation), all other main components of this hydrocarbon system lie beneath an unconformity showing a gap in time between the Late Cretaceous to the Paleocene. The erosion of the horst and grabens beneath the unconformity suggests that at one time, the chalks had been uplifted to above sea level causing significant weathering to occur.

Structural Geology
This hydrocarbon system is trapped by the Lindesnes Ridge which is a westerly plunging anticlinal structure which trends northwest-southeast which was formed during the Cenomanian to Oligocene. This orogenic event is responsible for the systems high permeability because the systems large fracture network occurred during the formation of the anticline. Because the Valhall oil field produces from an overpressured chalk reservoir, low porosity is expected; however, the pore spaces within the system are being held open by overpressured fluids. While the overpressured fluids are beneficial for oil storage within the system, it has caused many production problems. With the continued production of oil, fluid pressure is decreased as oil is pumped out of the system through the micro fracture network caused by both the formation of the Lindesnes Ridge Anticline and anthropogenic damage associated with drilling techniques and well perforation. Additional fracturing occured later when the system was hydraulically fractured by BP. The resulting lack of fluid pressure led to the system's effective stress reaching the yield stress causing chalk failure. These production induced chalk failures create new fractures which can be beneficial to oil production but can also cause production problems such as damage to existing wells and possible gas escape through larger fractures into the ocean floor.

Reservoir Geology
The reservoir is made up of overpressured and undersaturated chalk formed in the Upper Cretaceous which is overlain by a late Miocene to Pliocene claystone which seals the underlying oil and prevents it from escaping. The overlying claystone averages from 500 to 1000 meters thick and acts as a caprock and is highly saturated with gas. This has led to problems when attempting to use seismic analyses on the system. The production area lies approximately 2400-2600 meters below the seafloor with a water depth of 67 meters. Currently no oil water contact has been defined due to variability in water saturation and changes in formation properties. The reservoir has been measured to be 109 degrees Fahrenheit which is well within the oil window. The fluid itself has an oil gravity of 34.3 degrees API and an oil viscosity of 0.32 cp. This cretaceous chalk is classified as a loose to well-cemented biomicrite and is divided into two oil bearing chalk formations, the Tor and the Hod. Both the Tor and Hod formations exhibit high porosity and low permeability to varying degrees. Lack of silica and proper cementation within the chalk formations make them highly susceptible to failures causing production problems when not properly maintained. Because chalk fields typically exhibit low permeability, formation-wide micro fractures are required to generate permeability high enough to recover oil from within the pore spaces. Rock and Fluid Properties given by York, Peng, and Joslin 1992.

Nordland Group
The Nordland Group is the uppermost section of the Valhall field stratigraphy. It is a mixture of clays and sands which began being deposited during the Miocene and is approximately 1200 to 1600 meters thick. The uppermost section of the Nordland Group is fossiliferous clay which contains abundant layers of both forams and shell debris. The middle section is primarily sand which fades back into clay near the bottom of the section.

Hordaland Group
The Hordaland Group has presented geologists with significant challenges when studying the Valhall field. It is a highly gas-saturated shale composed of both silt and clay which was deposited between the Eocene and the Early Miocene. It remains approximately 1000 meters thick across the system. This gas-saturation is a problem due to its reflective properties which can reflect seismic rays and invalidate seismic profiles.

Rogaland Group (Seal)
The Rogaland Group is a primarily shale formation with interbedded clays which acts as the seal for the Valhall hydrocarbon system. These shales range from light green to grey in the upper section and light brown to light grey in the lower section. The Rogaland Group sits atop an unconformity which can be easily distinguished as the underlying sediment has been structurally altered during the Jurassic Rift. It is approximately 47 meters thick and the uppermost portion has a high tuff content. Some fracturing has occurred in the Rogaland Group due to production-induced faulting associated with drilling, which has allowed a small amount of gas escape through the ocean floor.

Tor Formation (Reservoir Rock)
The Tor formation contains two-thirds of the oil within the Valhall oil field and contributes 85% of the oil production. It is 80 to 100 feet thick and is a highly pure allochthonous chalk composed of 95-98% calcite. As the Tor chalk s allochthonous, the Tor was most likely formed by residue deposited by planktonic feeders which which created a high porosity ooze. It is the youngest of the two chalk formations and was formed during the Danian, Maastrichtian, and Campanian Ages. The Tor formation's proximity to the anticlinal structure overhead has caused the significant structural alteration to the formation. The Tor's properties change across the structure with the most fracturing near the crest of the structure. As structural alteration of the formation has changes, so does the degree of permeability due to fractioning of the chalk. It also has a porosity averaging from 25-30% across the formation, but can be as high as 50% near the crest of the structure. Tor chalk contains more than 90% or greater oil saturation. While most chalk fields within the north sea are water-wet systems, it is suggested that the crestal region of the Tor formation of the Valhall field is intermediate to oil-wet. Overall, the Tor formation has been altered more heavily than the Hod formation which has led to it having higher permeability, and therefore greater productivity.

Hod Formation (Reservoir Rock)
The Hod formation is an autochthonous chalk which is the older of the two formations and was formed in the Santonian, Coniacian, and Turonian Ages. The Lower Hod formation does not act as a reservoir, but rather a transitional stage from shale to chalk. The Middle to Upper Hod Formation has a higher percentage of chalk and begins to act as an oil reservoir due to increased porosity due to overpressured fluids within the system. While the Hod formation is not the primary producing formation in the Valhall field, the Middle to Upper Hod does exhibits higher porosities than the Tor formation averaging 30-35% with oil saturation of up to 80%. This is due in part to its low silica concentration and higher magnesium concentration; however, it is less financially viable to produce because of its lower permeability.

Kimmeridge Clay Formation (Source Rock)
The Kimmeridge Clay Formation is a marine oil shale which was deposited in the in the Upper Jurassic and is the source rock of the Valhall hydrocarbon system. Oil began migrating into the reservoir rock during the Miocene and is currently at peak oil generation. Oil migrates upward into the system through various fractures which formed above the Kimmeridge Clay Formation and likely formed as a result of overpressure in the system.