Below is a overview of the geological history of the North Sea Basin, specifically the southern part in the Dutch Sector. I wrote it up a while back so it may not be up to date anymore and please understand that no images can be provided. Comments and additions are welcome!Geological History of The North Sea BasinIntroduction
The North Sea Basin covers and area of 625,000 km2 and is lies north of the Netherlands and in between Norway and Great Britain. In the north it is bounded by the continental shelf edge. It is a topographical low and can be divided into several sub-basins. The area has a active tectonic history and deposition has varied between the different basins. In this study the focus will be on structural development and the stratigraphy from the Silesian (125 Ma) until recent. A link to the depositional history and the paleo environment will be made. This is done to be able to indicate possible source rocks, reservoirs and seals for oil production, the main industry of the region.
A series of seismic data, well data, subcrop maps and stratigraphic correlation charts from previous research were used to create a geological overview. The focus in this research will the North Sea Graben and the Terschelling Basin and it’s hydrocarbon production potention. Both basins are situated just North of the Netherlands and are of great importance for the oil industry in this region.Geological Setting
The present North Sea Basin lies between three bounding massifs; the Norwegian Caledonides, the London Brabant Massif and the topographic high of Great Britain. The Norwegian Caledonides have been a topographic high since the Carboniferous, whereas the other two massifs were uplifted above sea level during the Early Tertiary.
The crustal configuration of the north sea is inferred from detailed gravity measurements. The resulting contour maps clearly show a North sea anomaly 500 km NNW of the Netherlands: Thin crust in center of basin. Moho depth at 20 km depth, this is 10 km less thick crust as rest of basin. The structure and depth map of the base Cenozoic clastic sediments evidently show a deep depocenter above the gravitational anomaly. (profiles through this depocenter confirm this). This indicates a thinning of the crust after which isostatic subsidence created a basin. This is probably the result of rifting activity before the Cenozoic. The Cenozoic basin fill shows little deformation or fault influence on the seismic sections. Also the horizontal reflectors of Cenozoic layers can be clearly distinguished from the curved reflectors of pre-Cenozoic deposits. This suggests that the basin experienced little tectonic activity in the Cenozoic. Some sources suggest several episodes of minor activity in the late Ypresian to the Bartonian. But, most of these sediments have been removed in the early Oligocene. North Sea Central Graben/Terschelling Basin system
The North Sea Central Graben is a structural low and displays North to South trending listric faults. The Graben borders the Dutch coast and extends for 400 km o the North and is about 25 km wide. In the South the Terschelling Basin borders the Central Graben in the West. The Terschelling Basin has a rectangular shape with North to South orientated listric faults on the East and West margins and East to West orientated faults in the North and South. The Terschelling basin is about 50 km wide and long. Zechstein salt diapirs primarily have a North South orientation and are mainly found near the mayor faults of both the Central Graben and the Terschelling Basin.
Not only the structure differs between the two basins also the stratigraphy varies indicating variation in tectonic activity. A lot of research has been done on both basins, because of their hydrocarbon potential and stratigraphic columns are readily available. Using these stratigraphic columns and well logs seismic interpretation could be done.
A seismic profile cutting across the Central Graben in a SW
to NE direction has been used to find structures, lithology thickness and unconformities. The profile clearly shows that the Lower Jurassic is only present in the Central Graben. The Upper Jurassic has a larger deposition range and extends beyond the Grabens margins for about 5 kms. Indicating subsidence of the Central Graben during the Late-Jurrasic. This rifting event coincides with the rifting of the Broad Fourteens Basin, just south of the Central Graben. The burial diagram of a well in the Central Graben shows an increased burial rate from 170 to 140 Ma, also coinciding with the rifting phase.
Also the Triassic deposits can not be found near the South Western edge of the profile. Reflectors of Triassic and Upper Jurassic layers are cut off by overlying Cretaceous layers in the SW and erosion by the Kimmerian tectonic phase must have caused these unconformities. In the Central Graben the reflectors of the Cretaceous layers are cut-off by tertiary sediments, outside the Central Graben the Cretaceous and Tertiary sediments show no unconformities. This indicates local uplift and thus inversion of the Central Graben during the Subhercynian. This is in one line with other research, stating that the Broad Fourteens Basin show traces of tectonic inversion during the Subhercynian.
The seismic profile of the Terschelling Basin runs from West to East. The Lower Jurassic is not present in the Terschelling Basin and the Triassic deposits seem to wedge out towards the East. This indicates slower subsidence during the Jurrasic in the Terschelling Basin than in the Central Graben. The burial diagram of a well in the Terschelling Basin shows a slower burial rate than the burial diagram from the Central graben during this time. Instead, the burial rate in the Terschelling Basin seems to increase later; from 150 to 130 Ma
The Cretaceous reflectors are cut-off by Tertiary deposits and vary thickness, but are present everywhere. This indicates smaller deformation and erosion during the Subhercynian than the Central Graben experienced.
Both profiles show that most faults within the basin or graben extend to the Base of the Tertiary while most faults outside the basins can not be followed beyond the base Zechstein.
Furthermore it can be clearly seen that Zechstein salt diapirs, deposited in the Late Permian, formed near faults. The timing of this halokinesis is not universal throughout the system. Some diapirs show breakthrough after the Upper Jurassic, whilst others, near the edge of the Central Graben seem to have broken through during or after the Upper Triassic. None of the diapirs cut through the Cenozoic deposits, butt offset due to diapirism can be seen in these layers.
The lithostratigaphy of the Central Graben and Terschelling Basin is constructed out of several wells that have been drilled and logged with Gamma ray and sonic. Also several lithological features like hiatus, coal, dolomite, organic matter and glauconite were distinguished.
From this potential hydocarbonate sources can be distinguished in the Clay Deep Member in the North of the Central Graben. This Member contains organic matter, but has been eroded towards the South, which may have caused the leakage of hydrocarbonates. Another good source rock could be the Kimmeridge Clay Formation, it has been deposited over a large area (but is thickest in North of Central Graben) and may have been enriched in organic carbon.
Overlying both Formations is the Scruff Greensand Formation, which may be a good porous reservoir rock. In the central Central graben this formation has been eroded, but has good coverage over the remaining Kimmeridge Clay and the entire Terschelling Basin.
The Chalk deposition of the Cretaceous would provide a excellent seal and traps caused by salt diapirism continuing till Cenozoic times and tectonic inversion of the Cental Graben during the Subhercynian make this an area of great oil production potential. Where the North of the Central Graben, due to thick Kimmeridge Clay deposition, and the South of the Central Graben, due to thick Scruff Greensand deposition, are the most promising.
By cross correlating the different well logs and using previously made Stratigraphic diagrams detailed lithostratigraphic diagrams can be made. This could only be done because previous research had already dated most North Sea lithologies.
A lithostratigraphic diagrams running from North to South, covering both the Central Graben and the Terschelling Basin shows four hiatus: In the Oxfordian of the North Central Graben, the Kimmeridgian till Ryazanian of the Central Central Graben, the Kimmeridgian of the South Central Graben and the Ryazanian of the North Central Graben. Most of these hiatus can be coupled to erosion in the Mid- and Late Kimmerian and show that the area was tectonically very active during this time. Furthermore it can be noted that a shale lithology is deposited during the Callovian (164-161 Ma) in the North while in the South it’s deposited during Early Kimmeridgian (155-151 Ma). This is followed by a Long period of clay deposition in the North (157-144 Ma) and a shorter period in South (150-144 Ma). This indicates a period of transgression to the South with a sediment input from the South.
The sudden deposition of sand of the Late Portlandian ( 144-142 Ma) indicates a sudden regression, followed by transgression to the South. This sudden relative sea-level lowering could have been the result of tectonics or global sea level rise, but as mentioned earlier, the Late Jurrasic coincided with rifting of the Central Graben. It is therefore believed that the rifting of the Central Graben was the result of a dome structure forming due to high rates of extension, temporary local failure of the lithosphere and the diapiric rise of asthenospheric material to upper mantle levels. This would cause relative lowering of the Central Graben while the basin in a whole must have been uplifted pushing the Central Graben in a more energetic paleo environment, so the deposition of sand was possible. The presence of Late Jurassic volcanoes may be proof for this.
The distribution maps of these lithologies show a similar story of transgression to the south and sudden regression causing deposition of the Scruff Greensand formation in both the North and the South.
By analysing Pre Cretaceous and Pre Tertiary subcrop maps the tectonic inversion of the Cental Graben can also be proven. The Pre Cretaceous subcrop map clearly show the most recent deposits to be located within the structural low of the Central Graben and the Terschelling Basin. In a Pre-Tertiary subcrop map the most recent deposits are located outside the Central Graben structure indicating an inversion between the Cretaceous and Tertiary. The Base Cretaceous subcrop map also shows a transgression from the North through the Central graben into the Terschelling Basin and the Broad Fourteens Basin further in the South. In the structural lows of these basins Basal sandstone is found, indicating a long reworked paleo-shoreline, whereas the areas outside these basins seem to have flooded quite rapidly. This post-rift rapid flooding could have been the result of the previously mentioned dome structure deflating causing a sudden subsidence of the entire North Sea Basin.
From the Cretaceous onwards the sediment seems to have been inputted from the South, the Rheinish-Bohemian Massif. Off course, since the Norwegian Caledonides had been a topographic high near the basin from Carboniferous time, a large amount of sediment must have been provided from this source.Conclusions
By analysing seismic data, well data, subcrop maps and stratigraphic correlation charts a geological history of the North Sea Basin and in particular the North Sea Cental Graben and the Terschelling Basin can be constructed.
In the Late Permian Zechstein salts are deposited in the entire basin. The are locally covered by Triassic deposits and at the start of the Jurassic rifting of the basin starts resulting in the formation of the Central Graben and a little later the Terschelling Basin. The Central Graben subside faster and more resulting in the deposition of thick deep marine Kimmeridge Clays during the Late Jurrassic. The Kimmeridge Clay is deposited thicker in the North of the graben as subsidence rate is faster here. Also, in the South the Clay is eroded. During this time differential loading of the Zechstein salt forms diapirs that will continue to develop into the Cenozoic. As rifting continues diapiric rise of asthenospheric material into the upper mantle probably causes doming and uplift of the entire system causing thick layers of Scruff Greensand to be deposited in both the Central graben and the Terschelling Basin. Afterwords transgression from the North to the South occurs until the Barremian and deflation of the asthenosperic dome probably causes sudden subsidence causing post-rift flooding of the entire North Sea Basin.
During the Cretaceous, chalk is deposited but is partly eroded during the Austrian tectonic event. This event may have initiated tectonic inversion of the Central Graben, but is followed by the Subhercynian Tectonic event that has been widely accepted as cause of the inversion.
Seismic sections clearly show that most faults stop on the base of the Cenozoic deposits and deformation and faulting must have ceased after these layers were deposited. Because salt diapirism still continues today, some small structures due to this can be observed in Cenozoic sediments.
The reconstruction of this geological history helps predict the location of possible oil reservoirs through the analysis of possible source, reservoir and seal rocks. The Kimmeridge Clay is a good source rock and it is thickest in the North of the Central Graben and has been deposited less in the Terschelling Basin. A good reservoir rock can be found in the porous Scruff Greensand formation, which has also been, which has been widely deposited throughout the Central Graben, but is thickest in the South. A good seal are the Cretaceous chalk formations, that together with deformation due to (inversion) tectonics and salt diapirism can create perfect oil and gas traps. Most of these layers have been eroded in the centre of the Central Graben though. So the oil potential (at this depth) in this area is not very good.
But as the Zechstein salt covers a very good reservoir rock (Rotliegendes) throughout the north sea, and it has not been eroded, the oil potential below the Zechstein is very good. The tectonic activity and faults near the Graben has caused salt movement creating traps at the base of the Zechstein resulting in great reservoirs at deeper levels. Off course migration of the oil and gas may have resulted in oil filled reservoirs at shallower depths.