North german basin

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Stratigraphic map portraying the depositional history of the North German Basin along with its respective time period

Introduction

The North German Basin (also known as Blind Valley) is a passive- active rift basin, located along the northwest corner of Germany, and dipping into Denmark, the Netherlands, the southwest portion of the Baltic Sea, and the southeast portion of the North Sea. As a sub-basin of the Southern Permian Basin, the North German Basin accounts for a sizeable composite of intra-continental basins composed of sediments from the Cenozoic to the Permian era. These sediments have accumulated to a thickness of roughly seven miles. The geometry of the basin has been influenced by many different tectonic events, but mainly several stages of subsidence and rifting. The North German Basin includes the Groningen gas field, one of the largest reservoirs of natural gas in the world. The North German Basin also accounts for a sizeable portion of Western Europe’s natural gas.[1]

Rifting

Initial Rifting

The North German Basin’s initiation began around two hundred and ninety years ago, around the end of the Carboniferous era. The formation began by the crustal rifting and wrenching, along with large amounts of magmatism and volcanism. This event can only be dated by the long poly-phased subsidence of the area. The SHRIMP (sensitive high-resolution ion microbe) method has been shown to be the most accurate dating evidence method for graphing the reservoir[1]. This allows for the dating of sediments created during the magmatic flare up of the Permian era. An increase in crustal erosion occurred due to wrench tectonics, mantle lithosphere erosion, and magmatic inflation.

Secondary rifting

Around 200 million years ago, during the early Jurassic era, due to the breaking of Pangea, there were new rifting events coming from both the North and the South that caused an East to West extension covering the basin. The Triassic grabens became the result of the extensions, while simultaneously creating the salt tectonics that can be observed throughout the region. This response spurred another phase of subsidence due to lithospheric thermal relaxation and sedimentary loading.[1]

Tertiary Rifting

The third rifting event took place during the late Jurassic Era in response to the North Sea’s doming process. Mudstones from the Kimmeridge Clay Formation make up for most of the hydrocarbons at play in the North German Basin.[1] These hydrocarbons are restricted from migrating upward due to the Zechstein salt formation. The Zechstein salt formed approximately 155 million years ago from the faulting and rifting of the North Sea doing.

Doming Process

The North Sea underwent a doming period during the middle to late Jurassic period. During the middle Jurassic, the dome rose above sea level and soon began to deflate due to rifting in the later Jurassic years. The development of the dome seems to be consistent with that of an active rift model, having a broad- based plume head.[1]

Subsidence

Roughly 20 million years after its rifting, the North German Basin endured a great deal of accumulation of sediments. Roughly 8900 feet of strata came from the Upper Rotliegend Unit to the Bunter Unit. This event caused massive thermal subsidence from the Permian period to the middle of the Triassic period. This rapid burial of sediments led to subsidence rates of 220 m/ million years due to the extreme increase of crustal load. The thermal relaxation of the lithospheric magmatic inflation influenced the subsidence by allowing the basin to deepen itself along with the accumulation of sediments.[1] The final phase of subsidence occurred during the Cenozoic period, when many basement faults were reactivated via strike-slip faults during the late Cretaceous. This reactivation led to more Halokinesis, which is the presence of salt tectonics. This reversal, due to salt tectonics, allowed for small amounts of Pliocene and Miocene deposits. These deposits were later buried by widespread deposits, resulting in rapid subsidence.

Risks and Uncertainties

Stratigraphy

The North German Basin's stratigraphy can be seen in the figure shown below. The figure is mainly divided into three different eras, Paleozoic, Mesozoic, and Cenozoic. Amond the periods they are divided into more specific time periods, such as the Jurassic and Permian periods. The type of sediments formed from each period can be seen as well. The majority of the sediments found can be classified as interbedded sand and mudstone.[1]

The Paleozoic Era

The Paleozoic is the oldest period shown in the figure, hence why it is at the bottom. This part only covers the Permian period and is subdivided into two different groups, the Rotliegend, and the Zechstein. The Rotliegend group consists of rhyolites, conglomerate sandstones, and andesites. These sediments are most thin in the South and most thick in the East, resulting in a 5200-5200 ft range of thickness.[1] It wasn't until 10 million years before the end of the Paleozoic period that the Zechstein formation began to form atop the Rotliegend. Although there is an increase of thickness in the northwest region, Zechstein has quite a diverse amount of thickness. This is mostly due to post-depositional salt tectonics.

The Mesozoic Era

The biggest era on the chart, the Mesozoic Era, is divided into three different sub-periods: Triassic, Jurassic, and Cretaceous. The Bunter unit is the first unit of the Mesozoic era, thus depositing over the Zechstein unit. The Bunter unit consists mostly of red sandstone beds, along with some clay and conglomerates. Salt tectonics have deformed the initial thickness of the section. Muschelkalk makes up for most of the middle Triassic period, making up around 330 feet of mussel shells found in limestone and dolomite. The name "Muschelkalk" is German for mussel chalk.[1] The Keuper unit comes on top of Muschelkalk. This unit is composed of 3900 ft of shale and dolomite. Keuper is split up into three subdivisions: the Kohlenkeuper, which is mainly sandstone and clay, the Hauptkeuper, which is dolomite, gypsum, and marls, and the upper Keuper, which is mainly impure coals mixed in with grey dolomite. The Lias unit covers the late Triassic to early Jurassic times. Comprised of clay, limestone, shale, and sandstone, this unit is specifically difficult to determine thickness accurately due to the hiatus that comes before it. This hiatus is known as the Late Cimmerian Unconformity and lasted almost 100 million years. After the Late Cimmerian Unconformity comes the Valhall formation, which occurs during the early years of the Cretaceous period. This unit varies from 30-130 feet in thickness and primarily consists of sandstone, lime, and shale.[1] The final formation of the Mesozoic era is the Cenomanian transgression. Most of this unit's makeup is a chalky limestone with some marks accumulated from 1300 to 1800 feet of thickness.

The Cenozoic Era

The third and final era of the chart depicts the Cenozoic era. The two main units of this era are the Chattian unit and the Quaternary unit, as the other two units are hiatus'. The Chattian unit is composed of layers of mudstone and sandstone, while the Quaternary unit is made up of glacial sediments.

Hydrocarbon Resources

The North German Basin yields a large number of hydrocarbons, specifically natural gas. These hydrocarbons are here because they have been morphed together via a singular total petroleum system (TPS) known as Carboniferous-Rotliegend TPS.[1] The basin's biggest natural gas reserve is the Groningen Gas Field, holding a maximum of 100 trillion cubic feet of natural gas, making it one of the biggest gas fields in the world.

References

External Links

  1. 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 “North German Basin.” Wikipedia, Wikimedia Foundation, 25 July 2021, https://en.wikipedia.org/wiki/North_German_basin.