Mesozoic resource potential in the Southern Permian Basin area: the geological key to exploiting remaining hydrocarbons whilst unlocking geothermal potential

It is generally accepted that hydrocarbon exploration in northern Europe has reached a mature stage. A basin’s maturity is defined by the underlying number of new discoveries and the declining production rate of mature fields (SPE 2015). For geoscientists, a mature basin has well-defined characteristics in terms of, for example, reservoir presence or trap formation (e.g. Byrne 2012). It is interesting, therefore, to note how much is still unknown about certain stratigraphic intervals in northern Europe. The Mesozoic overburden of the Southern Permian Basin (sensu Maystrenko et al. 2008; Doornenbal & Stevenson 2010) continues to provide fresh insights into the geological history of an area where, as the name suggests, historical hydrocarbon exploration has focused on the Paleozoic. The aim of this Special Publication is to increase knowledge of the Mesozoic overburden as a driver for further hydrocarbon exploration/production and the development of new geothermal energy sources. The succeeding chapters are introduced by tectonic framework overviews that give a context for the papers that follow. The remaining articles are organized in approximate stratigraphic order, from old to young, and include a variety of examples from semi-regional to localized field or sub-basin studies. An overview of the study area for all the following chapters is given in Figure 1. It is recognized that the overprinting Mesozoic systems have different naming conventions across the study area (e.g. the Central European Basin System sensu Littke et al. 2008; Maystrenko et al. 2008). Here we use the title ‘Southern Permian Basin area’ to emphasize the resource opportunities associated with a region that has, at least in large part, been associated with hydrocarbon exploration, and very large gas fields, in the Permian Rotliegend. For our purposes, this includes part of the Polish Trough, the highs surrounding the basin (e.g. the Ringkobing-Fyn High), and analogous elements of the Danish and Norwegian offshore. In this introduction, an overview of the geological history and resource base of the area is presented as a framework for the chapters that follow. A description of further prospective resources is then offered (oil and gas plus geothermal potential).

It is generally accepted that hydrocarbon exploration in northern Europe has reached a mature stage. A basin's maturity is defined by the underlying number of new discoveries and the declining production rate of mature fields (SPE 2015). For geoscientists, a mature basin has well-defined characteristics in terms of, for example, reservoir presence or trap formation (e.g. Byrne 2012). It is interesting, therefore, to note how much is still unknown about certain stratigraphic intervals in northern Europe. The Mesozoic overburden of the Southern Permian Basin (sensu Maystrenko et al. 2008;Doornenbal & Stevenson 2010) continues to provide fresh insights into the geological history of an area where, as the name suggests, historical hydrocarbon exploration has focused on the Paleozoic. The aim of this Special Publication is to increase knowledge of the Mesozoic overburden as a driver for further hydrocarbon exploration/production and the development of new geothermal energy sources.
The succeeding chapters are introduced by tectonic framework overviews that give a context for the papers that follow. The remaining articles are organized in approximate stratigraphic order, from old to young, and include a variety of examples from semi-regional to localized field or sub-basin studies. An overview of the study area for all the following chapters is given in Figure 1.
It is recognized that the overprinting Mesozoic systems have different naming conventions across the study area (e.g. the Central European Basin System sensu Littke et al. 2008;Maystrenko et al. 2008). Here we use the title 'Southern Permian Basin area' to emphasize the resource opportunities associated with a region that has, at least in large part, been associated with hydrocarbon exploration, and very large gas fields, in the Permian Rotliegend. For our purposes, this includes part of the Polish Trough, the highs surrounding the basin (e.g. the Ringkøbing-Fyn High), and analogous elements of the Danish and Norwegian offshore.
In this introduction, an overview of the geological history and resource base of the area is presented as a framework for the chapters that follow. A description of further prospective resources is then offered (oil and gas plus geothermal potential). generalized and simplified overview is presented as a framework for the following papers.
The time period from 500 Ma (Cambro-Ordovician boundary) to 300 Ma (latest Carboniferous) saw a period of large-scale plate reorganization which was fundamental to the development of the NW European basins (e.g. Ziegler & Dèzes 2006;Krawczyk et al. 2008;McCann 2008c;Pharaoh et al. 2010). In Ordovician-Devonian times (c. 460-380 Ma), the Caledonian collision led, through a series of phased events, to the amalgamation of the Avalonian, Laurentian and Baltica plates which formed Laurussia (e.g. Cocks et al. 1997;Cocks 2000;Holdsworth et al. 2002;Krawczyk et al. 2008;Smit et al. 2016). To make a simplistic statement, for what was in reality a complex structural situation, the Caledonian Front between Avalonia and Baltica is expressed as the Trans-European Fault Zone in northern Germany/southern Denmark and as the Tornquist-Teisseyre Zone through NE Germany and Poland, with the relict crustal domains defined by gravity/magnetic data and long-offset seismic studies (e.g. EUGENO-S 1988;Zielhuis & Nolet 1994;Thybo 1997;Abramovitz & Thybo 2000;Thybo 2001;Banka et al. 2002;Grad et al. 2002Grad et al. , 2009Yegerova et al. 2007;Krawczyk et al. 2008;Kaban et al. 2010;Maystrenko & Scheck-Wenderoth 2013). This sutured margin and its related structures continued to have a strong influence on the subsequent Mesozoic sequence, as discussed in the Pomerian area by Deutschmann et al. (2018) and Seidel et al. (2018). In Late Devonian-Permian times (c. 370-270 Ma), the final stages of collision between Gondwana to the south and Laurussia to the north occurred, leading to the amalgamation of Pangaea and the generation of the Variscan mountain belt, and associated structures, in Central Europe (e.g. Gast 1988;Ziegler 1990b;Kroner et al. 2008;Maystrenko et al. 2008). The foreland of the Variscides became the main Carboniferous basin, allowing the deposition of large-scale deltaic and marine systems associated with the coal-rich Westphalian intervals and their related gas-prone source rocks (e.g. Kombrink et al. 2010). The gradual fill of that basin, combined with changes in palaeogeographical setting, access to marine gateways and global climate shifts, led to the deposition of continental sequences of the Rotliegend in the Mid-Late Permian followed by thick evaporites in the late Permian-Early Triassic (e.g . Ziegler 1990a;Verdier 1996;Geluk 2005;Peryt et al. 2010;Fryberger et al. 2011;McKie 2017). These evaporite sequences later (typically, from the Early-Mid Triassic onwards) mobilized into significant salt pillows, walls and diapirs which impacted Mesozoic depocentres and allowed hydrocarbon trap formation (as discussed by Bouroullec et al. 2018;Hernandez et al. 2018;van Winden et al. 2018).
The Mesozoic structural evolution of the Southern Permian Basin area is defined by the relict foreland basin and subsequent tectonic movement along the terrane boundaries (Thybo 2001;Pharaoh et al. 2010). For example, the Triassic is notable for the generation, or accelerated evolution, of various overprinting rift systems (Thybo 2001). These led to thick Triassic sequences in areas such as the Glückstadt Graben (Maystrenko et al. 2005) and the Horn Graben (e.g. Kilhams et al. 2018) with subsequent thick Jurassic sequences in the Central Graben and adjacent areas (e.g. Verreussel et al. 2018). These fast rifting phases, and variable filling of the associated basins, have an impact on aspects such as overpressure distribution related to depth, reservoir quality and fluid fills (e.g. Peeters et al. 2018). Subsequent inversion and erosion phases led to the exposure of some Triassic intervals. . The Jurassic sequence is, however, only preserved in areas of the basin that were protected from a late Jurassic-Early Cretaceous inversion episode (related to the Alpine collisional phase: e.g. Rosenbaum et al. 2002;Pharaoh et al. 2010) or areas that only suffered from subsequent Late Cretaceous-Cenozoic inversion (related to further southern European plate convergence: Dèzes et al. 2004;Kley & Voigt 2008;Pharaoh et al. 2010;Kley 2018). In The Netherlands, this includes discrete rifts such as the Roer Valley Graben/West Netherlands Basin and the Central Graben (e.g. Winstanley 1993;de Jager et al. 1996;de Jager 2003;Verreussel et al. 2018). In Germany, Jurassic preservation is often associated with halokinetic rim synclines and other localized areas such as the Lower Saxony Basin (Schwarzkopf 1990;Baldschuhn et al. 1996;Doehler 2005;Lott et al. 2010;Bruns et al. 2014). Although the Cretaceous interval suffered various discrete inversion phases (e.g. Vejbaek et al. 2010;Kley 2018;Krzywiec et al. 2018;Wolf et al. 2018), which allowed hydrocarbon traps to form, there is a general basin sag during this time.
This, coupled with rising eustatic sea level (e.g. Miller et al. 2005;Ramkumar 2016) and generally warm global temperatures (e.g. Steuber et al. 2005), led to an evolution from a clastic-dominated succession (marine shorefaces/pelagic mudstones/deepwater sandstones: e.g. Milton-Worssell et al. 2006;Jeremiah et al. 2010;Vis et al. 2018;Zwaan 2018) to chalk-rich seas (including important reservoir units across the region: e.g. van Lochem 2018). The Cenozoic then saw a switch back to clastic conditions. Shallow-marine sands were deposited around the basin edges (e.g. Knox et al. 2010)  Historical resource focus: conventional hydrocarbons Boigk (1981), van Hulten (2009) and Breunese et al. (2010) provide detailed overviews of the hydrocarbon exploration and production history of this basin. Here a short, simplified summary is presented to illustrate the relative historical importance of the Paleozoic and Mesozoic intervals to hydrocarbon exploration. It is recognized that there are further Mesozoic geological resources that are not covered here or in subsequent chapters. These include, but are not limited to, quarrying (e.g. limestone at Winterswijk in the eastern Netherlands; Faber 1959) and salt extraction (e.g. Wassmann & Brouwer 1987).
The discovery and use of hydrocarbons in NW Europe can be traced back to at least 1628 when oil products were used as cart lubrication and medical remedies in the region of Wietze, Lower Saxony (Boigk 1981; Arndt 2017). Although oil remained a useful product throughout the subsequent centuries, it was not until the late 1930s that hydrocarbon exploration started to boom in the region with the discovery of the Bentheim gas field (Zechstein), and subsequently (in 1943) the Schoonebeek oil and gas accumulations (van Hulten 2009). At that time exploration was focused on Mesozoic and Permian Zechstein targets with, for example, discoveries at the Goldenstedt (in 1959: Zechstein), Adorf (in 1959: Triassic) and Hengstlage (in 1963: Triassic) fields (Breunese et al. 2010). However, the discovery of a series of giant Rotliegend gas fields was to change the focus of explorers across the basin. In 1959, the Slochteren-1 well proved up 2900 Bcm (billion cubic metres) (c. 102 Tcf (trillion cubic ft)) at Groningen, at that time the largest gas field in the world (Grötsch et al. 2011). This was followed up by the 1965 Groothusen and 1969 Salzwedel discoveries, the latter containing 200 Bcm (c. 7.1 Tcf) of gas, the largest field in Germany (Breunese et al. 2010). The very large hydrocarbon volumes associated with the Paleozoic have subsequently made it the key economic driver for operators. However, the Mesozoic has also had an important role to play in the Southern Permian Basin area. Figure 2a illustrates that around 10% of gas reserves are hosted within Mesozoic reservoirs. These are typically, but not always, associated with the Triassic interval, being charged by Westphalian Type III coals. Field examples include Apeldorn (Kus et al. 2005), Caister (Ritchie & Pratsides 1993), De Wijk (Bruijn 1996Goswami et al. 2018) and F15-A (Fontaine et al. 1993). Perhaps of more consequence, Figure 2b illustrates that around 90% of the basin's oil reserves are found in the Mesozoic interval.  Figure 2c, d illustrates that these Mesozoic conventional gas and oil reserves, within the Southern Permian Basin, are predominantly found in The Netherlands, Germany and Denmark (97% for gas and 99% for oil), with only minor amounts associated with the UK and other countries such as Poland. This geographical distribution is a result of the geological history described above, including the spatial extent of rift systems (e.g. the Central Graben and associated Jurassic source rocks in a discrete area running through the Dutch, German and Danish offshore), areas of inversion, source-rock presence and charge timing (cf. Pletsch et al. 2010). However, this does not rule out further conventional hydrocarbon discoveries in the Mesozoic of, for example, Poland or other historically less attractive areas (e.g. Kilhams et al. 2018;Kortekaas et al. 2018).
An example, from The Netherlands, of the number of historical exploration wells and their stratigraphic targets (post-1940) is shown in Figure 3a. Early exploration, as described above, focused on the Triassic and Zechstein intervals, with the former considered the most attractive. However, after the 1959 Groningen discovery, there was a distinct upswing in both the annual total number of exploration wells and the proportion of Paleozoic targets which continued throughout the 1960s and 1970s.
In the period from 1980 to 1990, a combination of high to moderate oil prices, new 3D seismic technology and a desire to explore new plays saw an increase in the annual total number of exploration wells to record numbers. In that period, the proportion of Mesozoic targets also increased, and the success rate of all exploration wells jumped from around 35 to 65% (Breunese & Rispens 1996). Figure 3b illustrates which Mesozoic interval these Dutch wells targeted (post-1940). Historically,  the early (pre-1986) discoveries in The Netherlands were associated with the relatively shallow Cretaceous structures (and, to a lesser extent, the Jurassic) which could be defined and drilled using 2D seismic data (Breunese & Rispens 1996). With the advent of 3D seismic technology, there was a switch to deeper Triassic targets and those prospects that could be understood and de-risked by new techniques such as amplitude analysis (e.g. Breunese & Rispens 1996;Bruijn 1996). Note that in the period 2011-16 there was a resurgence in Cretaceous exploration drilling. This is associated with the chalk interval in the Dutch Central Graben where the application of advanced seismic inversion techniques, developed in the adjacent Danish offshore, gave confidence to chalk reservoir quality and fluid fill predictions For Germany, the historical discovered recoverable volumes per era is shown in Figure 4a and per Mesozoic period in Figure 4b. This illustrates a similar early (pre-1964) focus on Mesozoic exploration before a switch to Paleozoic targets and subsequent discoveries. With the exception of the 1980 Mittelplate discovery (cf. Doehler 2005), only very small volumes (<10 MMboe UR (million barrels of oil equivalent ultimate recovery)) have been discovered in the Mesozoic since 1968. This is likely to be due to a combination of geological factors (e.g. limited Jurassic source-rock presence: cf. Pletsch et al. 2010) and the more limited acquisition of 3D seismic data in comparison to The Netherlands. The historical perception of relatively small-volume promise in Germany has also led to high oil-price sensitivity, with an increase in discoveries related to peaks in global oil prices (e.g. between 1978 and 1980 the Brent crude price (inflation adjusted) rose from $50/bbl (barrel) to $105/bbl, by 1985 the inflation adjusted price was around $31/bbl: Macrotrends 2017). Prior to the latest 2014-16 oil price fall (from c. $105/bbl to $30/bbl) there had been significant interest from smaller operators to redevelop (mainly Jurassic) historical oil fields and to chase exploration upside (including unconventional plays). The oil-price fall and general social movement against unconventionals caused companies to downscale activities or exit the country. Examples include PRD Energy at Volkensen (Arndt 2015;Market-Wired 2015), Kimmeridge GmbH in Lower Saxony (iDeals 2016) and Central Anglia A/S in the Sterup licence, Schleswig-Holstein (Central Anglia 2016).

Future resource focus: hydrocarbons and geothermal energy
It is clear after at least 70 years of intense exploration across the Southern Permian Basin area that there is increased pressure to apply new geological ideas or new techniques to the exploration and exploitation of conventional hydrocarbons. The development of unconventional oil plays also gives an opportunity to extend hydrocarbon production within the basin. However, environmental concerns and societal pressure has, in recent years, seen an upsurge in government support for geothermal projects. In all these resource scenarios, a sound geological understanding is key to successful exploitation. Here, each of these existing and future resources is considered within the framework of the chapters that follow.

Maximizing production efficiency from existing hydrocarbon fields
It has become standard practice in conventional oil and gas fields across the Southern Permian Basin area and elsewhere, if considered economic, to drill extra infill wells and apply enhanced recovery techniques to achieve the highest possible hydrocarbon yield (e.g. Lake 2010). Such techniques require a good geological understanding of how, for example, sedimentary layers cause differential fluid flow. Porter et al. (2018) give an example from the Cretaceous shoreface reservoirs of the Rotterdam oil field, including a comparison to analogous outcrop examples in southern England. Additionally, Vis et al. (2018) consider how local tectonic phases influenced reservoir distribution (which, therefore, has a possible impact on hydrocarbon extraction strategies) in the Dutch Schoonebeek Field.
Typically, enhanced recovery techniques have been focused on oil reservoirs via water injection. However, gas injection is increasingly being used. Goswami et al. (2018) give an example of enhanced recovery via nitrogen injection at the De Wijk Field (Triassic reservoir interval), illustrating how new technologies can be utilized to increase gas yield. It is possible that existing or future technologies, such as cheaper drilling or enhanced recovery techniques, could unlock further resources.

Exploring for further conventional hydrocarbon resources
An estimate of the remaining prospective conventional hydrocarbon resources for The Netherlands is shown in Figure 5, which shows considerable potential left in all the Mesozoic intervals, including Triassic gas (Fig. 5a) and both Jurassic (Fig. 5b) and Cretaceous oil (Fig. 5c). Chasing further exploration opportunities in a mature basin demands a high level of geological understanding of all the play elements. This starts with consideration of the potential of large existing datasets. An example is presented by van Kempen et al. (2018), utilizing public well log data to consider Triassic Bunter interval reservoir properties and trends across The Netherlands. Detailed consideration of existing seismic data can also reveal interesting features. For example, Strozyk et al. (2018) identify a series of pockmarks which could refine the gas-charge timing story in the eastern Netherlands. It is also possible to question the fundamental tectonic framework of an area and the impact this might have on exploration models. An example is presented by Krzywiec et al. (2018) for the Polish Trough. Dogmas around reservoir (e.g. Kortekaas et al. 2018 for the Bunter of the  Dutch Northern Offshore; Zwaan 2018 for the Cretaceous of the adjacent Norwegian and Danish offshore) or source-rock development (e.g. Kilhams et al. 2018 for the Triassic play of the German Horn Graben) can be challenged in areas perceived to be fallow through the integrated evaluation of well results and conceptual geological models. Conventional hydrocarbon opportunities remain. For example, for the aforementioned German Jurassic oil play opportunities, the geological elements remain the same even if the economic boundaries have shifted.

Exploring for unconventional hydrocarbon resources
The presence of various Mesozoic source rock intervals and associated high TOC shale units (in addition to similar Paleozoic intervals) suggested that the recent unconventional oil and gas boom in the USA could be transferred to NW Europe (e.g. Schulz et al. 2010). A combination of societal pressure, economics and geological factors mean that this has not yet happened (e.g. Selley 2012;Johnson & Boersma 2013;Weijermars 2013). However, geoscience is central to the identification of resources and can contribute to the continuing debate over its extraction. Stock & Littke (2018) give an example of how the Posidonia shale may be associated with unconventional resources in the Lower Saxony Basin.

Efficient exploration for, and exploitation of, geothermal energy resources
Northern Europe is often considered to be at the forefront of the energy transition from hydrocarbons to renewable sources. Here, we consider the role that geoscience can play in unlocking further geothermal energy reserves. In the area of the Southern Permian Basin, there are a number of ongoing initiatives to promote geothermal energy. Figure 6 shows an estimate of the geothermal district heating production and targets for various countries in the study area and, although not specific to the Mesozoic, gives an example of the gap to potential for this energy  source. As suggested by Franz et al. (2018), there is considerable potential for the use of geothermal energy in Germany, a statement which can also be extended to The Netherlands and Poland. The resources of, for example, Denmark and the UK are not yet fully defined. In some areas of the basin, there is social and economic demand for clean energy (mainly for urban greenhouse heating). Pilot projects, often with a combination of Triassic and Cretaceous targets, have been undertaken which have highlighted the importance of sound geological understanding, through facies and porosity prediction in achieving economic water production rates (e.g. Pluymaekers et al. 2012;De Vaal 2017;Franz et al. 2018;Vondrak et al. 2018). For example, Figure 7a illustrates the growth of geothermal systems in The Netherlands (both in operational projects (2016, n = 12) and associated energy production). This effort began with a 2005-07 pilot project by A+G van den Bosch horticultural company at Bleiswijk (West Netherlands Basin), with the aim of utilizing heat for growing via water production from the Upper Jurassic-Lower Cretaceous interval (Platform Geothermie 2017; VleesTomat 2017) The success of this project has led to an acceleration of geothermal investment (focused on the Mesozoic, as demonstrated by all the drilled projects (2016, n = 16) in Fig 7b) aided by a wealth of subsurface data in the public domain (see also Vondrak et al. 2018). Various projects have also been undertaken in NE Germany. For example, in Neubrandenburg, production has been achieved from Triassic Rhaetian sandstones (Wolfgramm et al. 2009;Franz et al. 2018), with further potential in the Middle Jurassic fluvial sandstone sequence Franz et al. 2018). Various regional and local governmental organizations are supporting both further research into geothermal energy (with a number of technical and mapping tools now available: e.g. TNO 2013; Peta 2015) and subsidizing projects (e.g. EBN 2017). It is clear that a large range of techniques originally developed for the hydrocarbon industry, such as basin modelling (e.g. Nelskamp & Verweij 2012) and seismic attribute analysis (e.g. Dierkhising 2015), can also be applied to geothermal projects to improve pre-drill prediction of, for example, reservoir temperature and fluid fill.

Conclusions
The Mesozoic of the Southern Permian Basin area continues to provide fresh insights which can be applied to energy resource exploitation and identification. The general tectonic history of this area is considered well known, but the papers that follow illustrate that new observations can still be made. It is demonstrated throughout this publication that the key to unlocking remaining resources is built on a foundation of solid geological understanding. This applies to the efficient production of discovered hydrocarbons, especially when attempting to apply new engineering techniques, as well as defining and exploring for new conventional reserves. If the exploitation of unconventional hydrocarbons becomes socially acceptable in northern Europe, an underlying geological understanding of various Mesozoic age units will become increasingly important. However, this mantra also applies to the development of geothermal energy reserves, where accurate predictions of porosity and geothermal gradients are one of the many keys to a successful project. It is hoped that this Special Publication will spur further exploration for, and efficient extraction of, the remaining resources in the basin.
The editorial team extends its appreciation to all the authors and collaborators that contributed to this book. The time and effort taken represents considerable determination and perseverance. Everyone that contributed to the original Geological Society conference is also appreciated, the open and collaborative atmosphere formed the basis of this book, with special thanks going to: Laura Griffiths, Gary Hampson, Howard Johnson, James Maynard, Robert Schöner, Martin Wells and Sarah Woodcock. We thank various managers who allowed the editors and authors to spend time making a significant contribution. Ben Kilhams particularly thanks Max Brouwers, Ramon Loosveld, Carlo Nicolai and Edwin Verdonk, who have supported the venture through their patience and sponsorship. Sincere thanks go to all the reviewers who took time and care to give excellent, constructive feedback: Oscar Abbink, Kresten