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= Gyda Oil Field = Gyda (Norwegian: Gydafeltet) is an offshore oil field off the coast of Norway in the North Sea. Gyda Field is in the southern Norwegian sector between Ula and Ekofisk oil fields in block 2/1. The water depth is around 65m and the reservoir depth is around 4,000m. Gyda field is geologically located near the Central Graben formation in the North Sea which is apart of a trilete system which includes the Viking Graben, Central Graben, and Moray Firth Basin. Periods of extension, cooling, and subsidence during the Permian-Triassic and Jurassic-Cretaceous caused the present day structure of the basin. The source and seal of Gyda field is the Kimmeridgian Clay Formation and the reservoir is the Ula Formation.

Petroleum Exploration
Gyda Field was first discovered in 1980, but production started in 1990 by Repsol Norge AS. Gas production ceased in 2016 and oil production has been shut down since February 2020. Company partners with production licenses include Repsol Norge AS, INEOS E&P Norge AS, and KUFPEC Norway AS. Repsol Norge AS owns most of the company shares at 61%, while INEOS E&P Norge AS owns 34% and KUFPEC Norway AS owns 5%. The oil field platform included a steel jacket, offshore drilling and processing facilities, as well as housing for crew. Recovery of oil and gas was produced by water injection as well as pressure support from aquifer points. Once recovered, the hydrocarbons were transported through a dedicated pipeline to Ekofisk field which could then be transported via Norpipe to either Teesside in the UK (for oil) and Emden in Germany (for gas). Plugging and abandonment of past wells are in progress and plan to be completed by 2023.

Regional Geology
Carboniferous-Permian: Major rifting occurred with volcanism. Two evaporite basins developed that were eventually overlain with sediment. Buoyancy forces from the overlain sediment caused halokinesis to occur uplifting the salt deposits. Triassic: Rifting occurred in the N-S, NE-SW direction. Coarse fluvial sediments were deposited along the rift margins, while finer grained river and lake deposits formed near the center of the basins. Jurassic: The Triassic-Jurassic boundary was marked by a north to south marine transgressive system. Growth of a volcanic dome (Mid-North Sea dome) occurred over the trilete point (Viking Graben, Central Graben, Moray Firth Basin) which resulted in erosion and rifting of the uppermost Triassic sediments. Then large deltaic systems containing sand, shale, and coal developed. The most important rifting occurred in the Late Jurassic and lasted into the early Cretaceous. During this rifting sequence, block faulting caused major uplift and tilting, which resulted in erosion and increased rates of sediment supply, along with shale accumulation in anoxic basins. Also as a result of this uplift, some locations such as the Central Graben do not show stratigraphic formations from the Jurassic due to erosion. Net transgressive, shallow-marine systems formed during this time and are the main sources of reservoirs near the Central Graben.

Cretaceous: Overall, uplift and rifting began to subside and deposition of mudstones and chalks occurred. The reservoirs that formed during the Jurassic period were capped by Upper Jurassic to Lower Cretaceous mudstones. Thermal maturation also started to occur during this time.

Stratigraphy
The Ula formation is a net transgressive shallow marine sandstone system that formed during the Middle-Late Jurassic. The sedimentology of the system contains fine to medium grained sandstones, is moderately sorted, homogenous, and has a high porosity level. During burial with increasing temperatures, sponge spicules from the system dissolved and precipitated as microquartz overgrowth among the quartz grains. Due to this overgrowth of microquartz, the presence of macroquartz growth was inhibited which then caused higher levels of porosity among the sandstones. The porosity count of these sandstones are around 20%-27%.

The Kimmeridgian Clay Formation formed during the Late Jurassic and sealed the Ula Formation with marine shales. The Kimmeridgian Clay Formation contained highly organic-rich mudstones which is a large source of oil in the North Sea. The Paleogene was a critical time for thermal maturation and migration of the hydrocarbons from the Kimmeridgian Clay Formation to the shallow marine Upper Jurassic reservoirs such as the Ula Formation.

Source and Seal
The reservoir of Gyda Field formed in the Ula Formation during the Late Jurassic and is relatively deep (3650m-4165m), hot at (155°C at 4155m) and over-pressured around (605 bar). The Lower-Middle Jurassic in this image contains the Ula Formation and the Upper Jurassic contains the Kimmeridgian Clay Formation. The source and seal of Gyda field is the Kimmeridgian Clay Formation and the reservoir of the field is the Ula Formation. As you can see on the diagram, the Viking/Central Graben is located at the lowest Two-way Time interval, which is the lowest depth interval on the diagram. The Lower-Middle Jurassic, which contains the seal is below the Upper Jurassic, which contains the source. The faulting that occurred from rifting and uplift during the Late Jurassic formed migrational pathways for the hydrocarbons in the Upper Jurassic to travel upwards along the faults and into the reservoirs at slightly higher depths. This allowed the Kimmeridgian Clay Formation to be the source and seal of the reservoir.

Mini-Basin Development Model


Mini-Basin Development Model: This model is a proposed model of the rise and fall of salt diapirs, which could potentially explain the architecture of the Upper Jurassic Strata along the Central Graben in the North Sea. The cross sectional view of the profile cuts along the Central Graben from NW-SE between Ula Field and Gyda Field. (A) During the Early Triassic, subparallel deposition occurred then (B) moved into the Early-Middle Triassic when extension and reactive diapir rise occurred. Listric faults formed along the eastern side of the diapirs, while salt dissolution started to occur on top of the diapirs. In between the growth of the two diapirs, a Triassic depocenter formed holding the thickest section of Triassic deposits in the model. (C) During the Late Triassic, differential loading began which caused different sections of the stratigraphy to gain varying amounts of deposition. Also, diapirs were buried due to new positions, and salt welds formed. The salt weld can be seen below the Triassic depocenter section and due to the increased amount of deposition on top of the stratigraphy, the salt weld will continue to thin and potentially weld completely together due to pressure. (D) During the Early-to-Late Jurassic, uplift of the Central Graben, or called the Mid North Sea dome, causes erosion of the uppermost Triassic sediments. (E) During the Late Jurassic (Oxfordian-Portlandian), extension began, along with salt withdrawal, forming turtle structures. The Triassic turtle structure was once the Triassic depocenter. A turtle structure occurs when accommodation is created above recently collapsed diapir crests. Younger strata then is deposited, but most of the younger strata fills the new depocenters, while less young strata gets deposited in the structures with initial diapir rise (i.e. between diapirs). (F) Postrift infill of topography occurred during the Late Jurassic (Portlandian) to Early Cretaceous. As you can see in the image, more sediment was deposited. (G) During the Late Cretaceous, rejuvenation of diapir growth occurred due to regional shortening in the area. Overall, the tectono-stratigraphic model described above suggests that the main mechanism to provide accommodation space of shallow-marine reservoirs around the Central Graben was extensional diapir-collapse during the Late Jurassic.