The North Sea Petroleum Province is made up of Northern and Central North Sea. This is the greatest oil producing region in the world. It has been, for the last 30 years, playing a major role as Britain’s resource base and a contributor to the country’s wealth. This region also touches Norway and the Shetland Islands which are part of the Northern arms of the Late Jurassic system. This report gives an account of much of what is already known about Petroleum geology, Palaeogeography and tectonostratigraphic development of the Upper Jurassic, Middle Jurassic and the Lower Cretaceous rocks in the province. The recent “Buzzard” discovered in the Upper Jurassic sandstones, in the Central North Sea area, have about 800 to 1100 million oil barrel according to experts (Verweij, 2003). This is, therefore, the largest discovery in 25 years for the Continental shelf of the United Kingdom. Significant finds continue to be obtained in the Moray Firth.
Introduction
The north part of the North Sea is between Norway and the Shetland Islands, forming the northern arm of the Late Jurassic system. The northern arm spans northern Norway and offshore Eastern England. (Michaelson et al, 1998).Explorers discovered Petroleum in the North Sea in the mid 1960s with the Danish well A-1 in 1966 and the Norwegian well 25/11-1 in 1967. The petroleum exploration initially focused on the prospects of the Tertiary and Mesozoic ages (Pedersen et al. 2006). Examples of reservoir rocks that were prominent there include cretaceous chalk, deltaic sandstones of the middle Jurassic times and Paleocene turbiditic sandstones. Rich marine type II shales of upper Jurassic times, which are rich in organic matter, helped charge the reservoirs almost exclusively. Such shales include the Draupne, Tau and Mandal formations which are Kimmeridge clay equivalents. These are located in the Central Graben and Mesozoic Viking systems f the North Sea (Verweij, 2003). Extensive exploration in this area renders it now a “mature” area. Some parts, however, remain vastly unexplored. These are the east-northern North Sea and parts of the Skagerrak in the south of Norway. Since upper Jurassic organic Shales are likely to be immature or absent in this area, the importance of Paleozoic source rocks is great. Onshore Norway and parts of Sweden have numerous locations with petroleum generated from the marine type II source rocks of the Lower Paleozoic stage.
Stratigraphy
The petroleum basin in the North Sea has a stratigraphy divided into six main tectono-stratigraphic sequences. The Variscan sequence contains the Carboniferous coal. These are the main source rocks for gas in the area. Pre- and early rift developments brought about a sedimentary sequence that was highly conformable (Germanic Trias, Zechstein, the gas-prone Aalburg, Upper Rotliegend groups and Posidonia Shale Formation). Sedimentation of the Schieland Group, during the main syn-rift phase, continued on a large scale in the fault basin. The late Cretaceous inversion interrupted the post-rift deposition of Rijnland, associated with thermal subsidence. The inversion caused the uplifting of the basin’s syn-rift sedimentary fill, substantially eroding it.
Tectonic history
The basement in northwestern Europe came to existence due to the crustal accretion which happened after the Caledonian collision. This collision happened between the ancient continents of Avalonia, Baltica and Laurentia. During this period, the North Sea’s location coincides with the closing of the Iapetus and the Tornquist Sea. Strike-slip tectonics dominated the North Sea region as the Laurentia and Baltica continents continued to collide in the Appalachians. These collisions led to attrition, and lateral extrusion of Europe. Consequently, a broad pull-apart basin in the same location came to exist in the late Jurassic. This came to be known as the Viking Graben. At roughly the same time, the extension half-Graben formed. This development extended to the Early carboniferous. Fold structures in the North Sea and Scotland show that there was structural inversion in the Late Carboniferous.
These were more than likely related to the orogenic Variscan tectonics in the northwest of Europe. Basin subsidence as a result of thermal cooling and extension characterized the Permian in the North Sea. (Fossen et al, 2003). This, however, was subdued during the Permian. Greater rifting would then follow in the Triassic where extension occurred in an east-westerly direction, in the Moray basin and the central and northern parts of the North Sea. These regions at the time had not been connected to form a unit rift system. The most notable phase of deformation started in the Late Jurassic. The extension occurring across the Moray basin and the central and northern North Sea produced a triple-rift junction giving three linked but distinct rifts. These are the Central Graben, the Viking Graben and the Moray Firth. The linkage of the rift systems, namely the Central Atlantic and Arctic led to the rifting in the North Sea during this period. The separation of North America and Europe as well as the oceanic spreading would then occur along these rift systems.
Petroleum Occurrence
This rift event indicated above is the most significant in the North Sea. This is because it exposes the hydrocarbon-rich reservoirs as well as its dense sampling via boreholes and significant seismic data imaging. According to Knaap & Coenen (1987), Mobil Oil Company drilled the first successful well in the Broad Fourteens Basin. Mobil encountered gas in Triassic sandstones and Zechstein carbonates in 1968. These are in the P6 block. Union oil later discovered the first oil fields in block Q1 in 1979.
Gas system
There are significant gas accumulations in the Zechstein Carbonate Members, the Slochteren Formation and the Buntsandtstein subgroup found in the Broad Fourteens Basin. The Slochteren reservoir’s principal top seal has Zechstein evaporates (Oele et al, 1981). In this basin, the main gas source rock is the largely gas-prone Westphalian Coal. This is a type III kerogen source rock with a coal content of 1.5% of the Baarlo Formation. The Ruurlo Formation has a coal content of 4% while that of the Maurits Formation is 6%. The total organic carbon content (TOC) of these coals is 70%.
Oil system
Commercially viable accumulations of oil are principally in the Delfland Subgroup and the Vlieland Sandstone formations. In blocks Q1 and K18, the top seal for oil accumulations is Vlieland Claystone Formation (Roelofsen & De Boer, 1991). The faulted anticlinal traps present in Vlieland Sandstone Formation sprung in the Early tertiary (or Late Cretaceous) inversion. In the Hoorn, Helm and Helder fields (block Q1) are water washed and biodegraded oils linked to the Lower Cretaceous-Posidonia Shale oil system. The most prominent oil source rock in the Broad Fourteens is the Kerogen type II (Cornford, 1998). For the Posidonia shale, the TOC content is approximately 5%. Roelofsen & De Boer (1991) considers the secondary oil source rock in this region to be the bituminous shale that falls under the Delfland subgroup.
Eldfisk Petroleum Field
Eldfisk is one of the two largest fields found in the greater Ekofisk region and started functions in 1979. Norwegians pride themselves in the history of the greater Ekofisk field which transformed their country into an oil nation. The reservoir is at a depth of 2700 meters to 2900 meters beneath the seabed. Both gas and oil flow from Eldfisk via pipes passing through the Ekofisk Centre. Aker solutions have acquired a new contract to modify the present platforms.
Reservoir rocks
The Eldfisk petroleum system came up from sedimentary rocks that deposited in a rift system that failed. This happened between Norway, Denmark and Great Britain in the North Sea. The pre-rift rocks serve as reservoir rocks for a small amount of petroleum but are not part of this petroleum system. They are mostly underburden rocks.
Oil-source rock
The biological marker analyses by Mackenzie et al. (1983) shows that the oils present in the Eldfisk field have their origins in the Mandal Formation source-rock. The source rock exists in the synrift sedimentary section. The figure below is a burial history chart which helps to better determine when the source rock (Mandal) was active in petroleum generation. It can be seen that peak generation is at approximately 30 Ma which serves as the critical moment.
Lithology and Facies
Folding, deposition, faulting and post-depositional changes occurring in reservoir lithology significantly affect wellbore pressure response. We have small-scale heterogeinities and large-scale ones. Small-scale heterogeinities include vugs and fractures while large scale ones include faults, thickness changes, fluid-fluid contacts, physical barriers and lithology changes. Permeable beds, which correspond to different facies, are formed from tectonic movements.
Works Cited
Fossena, H et al., 2003. A Major Tilted Fault Block at the Eastern edge of the Horda Platform: Structural geology of the Huldra Field, northern North Sea. Norway. Department of
Earth Sciences, University of Bergen.
Knaap, A. & Coenen, J., 1987. Exploration for Oil and Natural gas. Seventy-five Years of Geology and Mining in the Netherlands, p.207-230.
Michelsen et al., 1998. Cenozoic sequence stratigraphy in the eastern North Sea. Society for Sedimentary Geology (SEPM)
Oele, J & Tiemens, J., 1981. Some Rotliegend gas fields of the K and L Blocks, Netherlands Offshore (1968-1978).
Roelofsen, J. & De Boer D., 1991. Geology of the Lower Cretaceous Q/1 oil-fields, Broad Fourteen Basin, The Netherlands. In: Spencer, M. (eds.). Generation, accumulation and
production of Europe’s hydrocarbons: Special Publication of the European Association of Petroleum Geoscientists 1. Oxford: Oxford University, p. 203-216.
Verweij, J., 2003. Fluid flow Systems Analysis on Geological Time Scales in Onshore and
Offshore Netherlands; with special reference to the Broad Fourteens Basin (in press). Oxford: Oxford University Press.
