Lower saxony petroleum basin

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Figure 1: Location of the Lower Saxony Basin (Lueders_etal_2012)

The Lower Saxony Basin is located in the northwestern region of Germany. It is an important petroleum basin for Germany as it provides as a principle oil province. The petroleum production yield for the Lower Saxony Basin is calculated to eighty-five percent of Germany’s total oil production [1].

Background Information

The Lower Saxony Basin ranges from three-hundred kilometers long and sixty-five kilometers wide [1]. It is characterized by researchers as a sedimentary basin that underwent large tectonic changes including strong inversion and uplift. The first extraction and production of the basin began in 1864 [2]. The beginning of the basin formation and evolution of the LSB has been dated to the Late Permian. A majority of source rocks used in German oil and gas production are localized in the Upper Carboniferous coals, Jurassic shales, and Cretaceous shales of the basin [2].

Formation

The Lower Saxony Basin is located north of the Rhenish Massif with a projected East-West strike line [3]. The basin is characterized by rifting and thermal subsidence of the lithosphere during the Permian [4].

Stratigraphy

Figure 3: Formation History of the Lower Saxony Basin (Senglaub_etal_2006)

The region of northern Germany is dominated by the Stephanaian, Permian, and Mesozoic depositional packages including intermittent unconformities throughout the series. These geologic units lay on top of Westphalian deposits which underwent deformation throughout the Varisan orogeny [1]. A large portion of LSB study areas includes the units deep within the previous formation of the uplifted Variscan mountain belt. Within this area lies a thickly-bedded sedimentary sequence containing coal that was eroded partially during the orogeny [4]. Throughout the Late Carboniferous, the depositional environment was deltaic system, indicated by the fluvial and marine sedimentary rocks. Geologically, the Late Carboniferous unit acts as the basement of the basin, but remains a major source of gas rocks [4]. The basal sedimentary units covering the Late Carboniferous contain salt, anhydrite, dolomite, and limestone. These sedimentary rocks were deposited by the presence of the Zechstein sea. The entirety of the Triassic geologic units contains clastic sediment deposits with the absence of an inland sea [4]. With the introduction of the Jurassic time period, marine conditions were reintroduced to the LSB area, depositing marine shales with an abundant Ammonite fauna. Throughout the Aalenian time period, shales were deposited and interlaced with sand units made of clastic deposits stemming from the emersion and erosion of the Rhenish Massif to the south [1].  The increase of sandy units within the Jurassic shale deposits also reflects the abnormal shallowing of the sea level during the Middle Jurassic, producing a series of transgressive-regressive sequences [3]. The last major gas source rock units for the Lower Saxony Basin can be found in the Cretaceous geologic unit. Within the Cretaceous, the units evolved from marine deposits with clastic inclusions to completely shallow marine in the Mid to Late Cretaceous. Due to a period of inversion in the Late Cretaceous, a large sequence of erosion of the Cretaceous units occurred [3]. Partially-eroded Tertiary clastic deposits lie on top of the remaining late Cretaceous.   

Structure

Figure 3: Kimmeridgian depositional setting for the Lower Saxony Basin (Zuo_etal_2018)

Highly-integrated research of the Lower Saxony Basin predicted the connection between high maturity and both gravity highs and magnetic anomalies. This data proved presence of igneous intrusive bodies including the Bramsche Massif within the southern region of the Basin [3].  The structural characteristics of the LSB consist primarily of rift and wrench tectonics in the late Jurassic causing rapid subsidence [4]. The tectonic geologic history of the LSB begins with the Permian and Triassic at the formation of three main sub-basins due to deformation of the Central European Basin System. Following this, the Late Carboniferous to Early Permian was characterized by major faulting and igneous intrusive activity [2]. In the Triassic, global extension tectonics rifted apart the supercontinent, leading to the ultimate rift phase sequence of the CEBS [2]. The Kimmeridgian time period, rapid subsidence within the center of the basin coincided with the uplift of rising features surrounding the outer areas of the basin including the Rhenish Massif, thus creating what geologists termed as an “internally faulted graben” [2]. The Coniacian and Santonian time periods experienced inversion of existing fault blocks, reactivating them into steep angle thrust faults and reverse faults [4].  Finally towards more recent tectonic events of the LSB, the Late Cretaceous to Early Tertiary time period resembled a sequence of uplift in the basin [1].

Research Methods

Gravity and Magnetic Data

The Lower Saxony Basin is one of the most well-researched petroleum basins in the world due to its high productivity and supply of oil and gas source rocks. Early studies in the 1980's utilized gravity data for distinguishing major gravity highs within the LSB to indicate intrusive bodies such as the Bramasche Massif and outline the basin as a whole [1]. Another source of information for interpretation of the basin used early on was magnetic data. Magnetic data did not have the capability to outline the LSB, however, it indicated negative anomalies which represented the partial edges of the basin [1].

Numerical Modeling and 3-D Basin Modeling

The second, prominent form of conducting research on the Lower Saxony Basin was numerical modeling using burial and temperature history. Anomalies found within this data were correlated to earlier data such as gravity or magnetic. Supported research stemming from the use of this data includes substantiating the understanding of the origins of such prominent anomalies. For example, there is recent dispute of the anomalies that were thought to have indicated the presence of intrusive igneous bodies surrounding the basin, however a recent study in numerical modeling challenged this with the idea that these anomalies were indicative of an eroded cretaceous rock unit within the basin [4]. A more recent basin model research type is 3-D basin modeling which has used temperature and maturity data calibrated by downhole temperature data and vitrinite reflectance in order to successfully model the Lower Saxony Basin [2].

Production

The Lower Saxony Basin has been under extraction and production for over one-hundred and fifty years and it has been studied as one of the number one suppliers of oil and gas source rocks within the German oil and gas industry [1]. The largest source for extraction within the basin is the Middle Jurassic siliciclastic reservoirs with the two main productive units being the Wealden and Posidonia Shale [5] .

References

  1. 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 Betz, D., Fuehrer, F., Greiner, G., Plein, E. (1987) Evolution of the Lower Saxony Basin
  2. 2.0 2.1 2.2 2.3 2.4 2.5 Bruns, B., Di Primio, R., Berner, U., Littke, R., (2013) Petroleum system evolution in the inverted Lower Saxony Basin, northwest Germany: a 3D basin modeling study
  3. 3.0 3.1 3.2 3.3 Petmecky, S., Meier, L., Reiser, H., Littke, R., (1999) High thermal maturity in the Lower Saxony Basin: intrusion or deep burial
  4. 4.0 4.1 4.2 4.3 4.4 4.5 4.6 Senglaub, Y., Littke, R., Brix, M. R., (2006) Numerical modeling of burial and temperature history as an approach for an alternative interpretation of the Bramsche anomaly, Lower Saxony Basin
  5. Ziegs, V., Mahlstedt, N., Bruns, B., Horsfield, B. (2015) Predicted bulk composition of petroleum generated by Lower Cretaceous Wealden black shales, Lower Saxony Basin, Germany