Southern Permian Basin Research Articles

Giant polygonal anhydrite ridges in the Southern Permian Basin

Giant polygonal anhydrite ridges with diameters of 2−4 km have been identified in the NE margin of the Southern Permian Basin in Poland. The primary ridges reach heights of 80−120 m, and secondary ridges, <40 m in height, subdivide them into smaller cells ∼1 km in diameter. The ridges were up to ∼40% higher before the alteration of gypsum to anhydrite. Both seismic and gravity surveys indicate the presence of similar structures over an area of thousands of square kilometers. Two alternative hypotheses, gypsum diapirism and free water convection, are proposed to explain their formation.

Hydrocarbon Potential and Basin Analysis of the Southern Permian Basin, North Sea: Evaluating Key Plays in the Turonian and Carboniferous

This research provides an in-depth basin analysis and evaluates the hydrocarbon potential in the Southern Permian Basin, North Sea, Netherlands. It employs a thorough approach, including interpreting well logs, analysing petrophysical data, mapping faults and horizons, converting time to depth, and calculating reserves. The study highlights two key plays: the Turonian play (Barremian Formation) and the Carboniferous play (Rotliegend Formation). Petrophysical analysis of the Turonian Formation shows porosity values between 8.9% and 16% and net-to-gross ratios ranging from 0.6 to 1.0, indicating variable reservoir quality with a 45% geological chance of success. In contrast, the Carboniferous play, which includes Rotliegend marine sandstones, has a 45% chance of success but benefits from better porosity and permeability than the Turonian Formation. The research produced time and depth structure maps, clarifying trap mechanisms and identifying potential leads. With its promising results, the Southern Permian Basin presents a significant opportunity for future exploration and development. However, the study notes several limitations, such as issues with power stability, the need for more 3D seismic data and missing logs like neutron and density for some wells. Addressing these issues by improving data collection and operational conditions is crucial, as it could enhance exploration accuracy and increase the chances of success in the Southern Permian Basin. It also highlights the potential for further research in this area.

Lithium Prospectivity in the Northeast German and Thuringian Ba-sins

Over the years many boreholes have been drilled into the Northeast German Basin (NEGB) in pursuit of the exploration of hydrocarbons. As well as gaining important information regarding petrophysical and geophysical data, legacy boreholes also provide information regarding total dissolved solids in formation water, which sometimes includes lithium concentrations. With the rapid increase in demand for electric vehicles in combination with EU net-zero targets, lithium is a vital component of the future of the energy industry as it transitions away from fossil fuel dependence.
 So, where can we find lithium, how much of it is there, and where does it come from? After extracting lithium content data from formation water trace element data, we can answer two of these questions. The data gathered [1] is from boreholes drilled in the NEGB (in the states of Brandenburg, Mecklenburg-Vorpommern, and Sachsen-Anhalt) and the Thuringian Basin. The Northeast German Basin makes up the eastern part of the North German Basin, which, in turn, is part of the Southern Permian Basin. It is located between the Baltic Shield to the north and the Variscan belt to the south [2]. The basin contains up to 12km of Phanerozoic strata [2], the evolution of which began with Late Carboniferous/Early Permian (Stephanian/Autunian) volcanism [3, 4, 5, 6]. In the NEGB increased concentrations of lithium can be found within the Zechstein Staßfurt Carbonate (Ca2) and the Rotliegend. In Brandenburg the highest values of 230 mg/L can be found in the geothermal well doublet Groß Schönebeck which was drilled into sedimentary Rotliegend and Permo-Carboniferous volcanics. Comparable values (190 mg/L) can be found in formation water samples of the Ca2 in the well Lübben 103. In Mecklenburg-Vorpommern, lithium contents of 600 mg/L were encountered in the Rotliegend of the well Gristow 7, with lithium contents in the Ca2 of the nearby well Gristow 6h2 reaching 300 mg/L. The Reinkenhagen field (which targeted the hydrocarbons of the Ca2) has several wells which exhibit lithium contents in the Ca2 of up to 140 mg/L. In Sachsen-Anhalt elevated lithium values are limited to the Ca2. The Fallstein field, originally drilled for the exploration of hydrocarbons, encountered lithium contents of up to 189 mg/L. Spatial distribution of the lithium data can be seen in Figure 1 along with current geothermal projects. Also displayed are areas of proven and suspected geothermal potential.
 The Thuringian Basin is located between the Harz Mountains and the Thuringian Forest Mountains, the formation of which was part of the intramontane sudsidence zone of the Variscan mountains [7]. The trough shaped sedimentation area of the basin was formed in the early Mesozoic [8]. In Thuringia, as in Sachsen-Anhalt, elevated lithium values are limited to the Ca2. The fields of Kirchheilingen, Langensalza, and Mühlhausen all have multiple wells which show lithium contents of up to 200 mg/L.
 The source of lithium in formation water is not well understood and therefore cannot be clearly defined. A case study in Thurinigia focusing on Zechstein salts in Gorleben and Morlseben made the assumption that lithium content in the salt brines were leached from phyllosilicate bearing units (T4, A3, Ca3, T3) in the Zechstein [9]. Other sources discussed include relict metamorphic brines and organic compounds. A different study focusing on Rotliegend Ca-Cl brines in the
 North German Basin found that brines rich in lithium originated from the leaching of lithium rich mica in the Lower Rotliegend (volcanics) [10]. If lithium accumulation in formation waters is related to Permo/Carboniferous rocks, it is possible that deep faults may have acted as conduits for lithium rich brines to migrate into overlying formations [11]. This may be a plausible theory as this dataset shows that elevated lithium contents in the Ca2 can be found within the carbonate margin facies in areas through which major faults run. This is especially evident on the northern basin margin.
 Ultimately the data compiled from both basins will be used to perform a lithium in place analysis.

THE ZECHSTEIN Z2 HAUPTDOLOMIT PLATFORM IN THE SOUTHERN UK MID NORTH SEA HIGH AND ITS ASSOCIATED PETROLEUM PLAYS, POTENTIAL AND PROSPECTIVITY

The Mid North Sea High (MNSH) region represents one of the least explored areas for the Late Permian Zechstein Hauptdolomit play in the Southern Permian Basin although some of the first offshore wells drilled in the UK were located here. In other parts of the basin such as onshore Poland, the Hauptdolomit Formation (“Hauptdolomit”) is an active and attractive exploration target, with oil and gas production from commercial‐sized fields. In the UK, the play has been overshadowed by drilling campaigns in areas to the south of the MNSH which tested plays in the underlying Rotliegend and Carboniferous successions. However, with these areas now in decline, there is increased exploration interest in the Hauptdolomit in the MNSH region, particularly since 2019 when 3D seismic data were acquired and the first hydrocarbon discovery was made at Ossian (well 42/04‐01/1Z). Geochemical data from the latter discovery have pointed to the presence of a prolific petroleum system with the potential for Hauptdolomit reservoirs to be charged both by Zechstein‐generated oils and Carboniferous condensate/gas. With regard to hydrocarbon migration and preservation in the southern MNSH, a detailed evaluation of the effects of the Mid Miocene Unconformity has allowed for a greater understanding of the main factors controlling hydrocarbon preservation and remigration. Reservoir characterization of the Hauptdolomit play has been achieved by integrating petrographic microfacies analyses, core data and petrophysical interpretations. The most important factors controlling reservoir quality are the presence and extent of anhydrite cementation and the presence of high energy shoal facies. Thicker and coarser grained shoal facies are expected to occur along the yet‐to‐be explored Orchard platform margin where numerous prospects have been mapped and refined using recently acquired 3D seismic data.

Diagenetic History and Timing of Cu and Zn-Pb Sulfide Mineralization in the Permian Kupferschiefer System, Saale Subbasin, Eastern Germany

Abstract The Southern Permian basin in central Europe contains a number of important high-grade sediment-hosted Cu deposits. Laterally extensive stratabound Cu and Zn-Pb sulfide mineralized rocks are located at a major stratigraphic redox boundary, where coarse-grained continental sandstones of the uppermost Rotliegend Group are overlain by carbonaceous mudstones (T1) and limestones (Ca1) of the Zechstein Formation. This study investigates the diagenetic evolution and style of sulfide mineralization in three drill cores that intersect Cu and Zn-Pb sulfide mineralized rocks at three locations (Sangerhausen, Allstedt, and Wallendorf) in the Saale subbasin (Eastern Germany), which is located at the southern margin of the Southern Permian basin. We combine macro- to microscale petrographic data (binocular, transmitted and reflected light, and scanning electron microscopy) with quantitative X-ray diffractometry and bulk-rock geochemical analyses. Petrographic results show extensive, primary-porosity-occluding, early diagenetic calcite cementation that predates both the diagenetic alteration of detrital clasts and sulfide mineralization. The highest-grade Cu and Zn-Pb sulfides (bornite, sphalerite, and galena) replace the calcite cement, with subordinate replacement of dolomite and detrital clasts. Quantitative mineralogical and geochemical data demonstrate that the highest base metal (Cu, Zn, and Pb) concentrations are associated with carbonate-rich samples, mostly as disseminated mineralization in the middle T1. Bulk-rock geochemical results show enrichment and covariation of redox-sensitive trace elements (RSTEs, e.g., Mo) with total organic carbon content toward the lower T1, consistent with highly reducing depositional conditions. Overall, the distribution and dissolution of calcite cement across this stratigraphic redox boundary provided the main control on the lateral migration of base metal-bearing fluids and high-grade Cu and Zn-Pb sulfide mineralization in the Saale subbasin.

Palustrine limestones and calcretes as the sedimentary record of paleoenvironmental changes in a Late Permian semi-arid climate at the SE periphery of the Southern Permian Basin (the Holy Cross Mountains, Poland)

An important part of the Upper Permian (Zechstein) sequence in the Holy Cross Mountains (HCM) in Poland comprises continental carbonate facies where dryland and wetland depositional environments dominated the southeastern part of the Southern Permian Basin. Sedimentological and thin-section microfacies analysis of over 860 m of core from seven wells, supplemented with field observations from several outcrops from the HCM area, has allowed recognition of numerous siliciclastic and mixed silici‑carbonate facies which are related to alluvial fan, proximal and distal floodplains, and ephemeral lacustrine environments. Due to climatic seasonality and subsequent groundwater oscillations, the latter were frequently exposed, causing widespread desiccation cracking and pedogenesis. Continuous aggradation controlled by steady tectonic subsidence allowed the deposition and preservation of thick (> 30 m) units with numerous palustrine intervals. In such a low aggradation rate system, vertical and lateral changes in the continental facies associations reflect the climatic evolution from arid to semi-arid and then to sub-humid environments with enhanced climatic seasonality. These facies changes indicate a gradual increase in the water availability towards the Permian-Triassic boundary, which may support the hypothesis of generally increased wetting at the low-latitudes of Pangea, linked with the development of greenhouse conditions and enhancement of the intertropical convergence zone at that time. Continental carbonates are therefore recognised as high-resolution recorders of palaeoclimate and tectonics.

Source rock evaluation and fluid inclusion reconnaissance study of Carboniferous and Zechstein rocks in the northern margin of the Southern Permian Basin, onshore Denmark

Hydrocarbon exploration onshore Denmark has been unsuccessful with no commercial discoveries. Several potential plays have been identified of which the Zechstein carbonate play relying on organic-rich Zechstein (Z2) carbonates as source rock is considered the most promising. Additional source rocks might be lower Carboniferous calcareous and carbonaceous units. This study aims at documenting Palaeozoic source rock(s) in the Upper Mississippian/Lower Pennsylvanian and Zechstein Z2 cored sections in wells drilled in the northern margin of the Southern Permian Basin in the Danish Baltic Sea area. The source rock potential, organic matter composition, and thermal maturity was examined using Rock-Eval pyrolysis, vitrinite reflectance measurements, and organic petrography of cuttings and core samples from two wells. In addition, a fluid inclusion reconnaissance study was conducted on samples from four wells. The examined Upper Mississippian/Lower Pennsylvanian marine calcareous mudstones are thermally immature to marginally mature with a generally low TOC content consisting of minor amounts of dispersed vitrinite and inertinite and scattered yellowish fluorescing mainly terrigenous liptinite macerals. The source rock potential is nil to poor. The analysed Zechstein Z2 section is also thermally immature and consists of resedimented carbonate mudstones deposited in a marine environment on a slope. The mudstones are organic-lean with identifiable tiny vitrinite particles and some telalginite. The source rock quality is poor with a potential for some gas generation. Thus, identification of rich source rocks failed, but the occurrence of solid bitumen in the Zechstein Z2 suggests the presence of restricted source rock levels in the mudstones or migration from deeper richer source rock sections. Further, hydrocarbon fluid inclusions are direct evidence for petroleum generation and migration. The dark appearance, high vapour/liquid ratio at room temperature and their homogenisation behaviour clearly indicate a mixed hydrocarbon composition with methane and higher hydrocarbons, like gas condensate. This demonstrates the presence of a working petroleum system in the northern margin of the Southern Permian Basin in southern Denmark.

Redox conditions in Late Permian seawater based on trace metal ratios in fluid inclusions in halite from the Polish Zechstein Basin

Previous studies have confirmed that primary fluid inclusions in halite record and preserve information concerning the major ion chemistry of seawater. Here, we determine ratios of redox sensitive trace metals (Fe, Mn, V, Mo, U) in paleo-seawater trapped as primary fluid inclusions in halite from two boreholes (Przyborów IG3, Gorzów Wielkopolski IG1) from the Polish sector of the European Southern Permian Basin and estimate the original seawater concentrations of the redox sensitive trace metals.Our results show generally higher ratios of Fe/U, Fe/Mo, and Fe/V in fluid inclusions in halites associated with sulfate-depleted (calcium-rich) brines, compared to inclusions in halites formed in association with sulfate-rich brines in the Przyborów IG3 borehole. This behavior is consistent with the interpretation of García-Veigas et al. (2011) to reflect changes in redox conditions from oxidizing (represented by sulfate-rich brines) to reducing (represented by sulfate-depleted and calcium-rich brines) in the Zechstein epicontinental sea during the Late Permian. The correlation is more complex in halites from the Gorzów Wielkopolski IG1 borehole, likely reflecting local controls on redox state.Results suggest that ratios of Mn/U, Mn/V, and Mn/Mo in fluid inclusions may be used to refine redox conditions in the water column when these data are evaluated in combination with Fe/U, Fe/Mo, Fe/V, and Mn/Fe ratios because reduction of Mn(III, IV) to Mn(II) occurs at a higher redox potential compared to that required to reduce Fe(III) to Fe(II).The estimated original Mn and Mo concentrations in the seawater in the Late Permian Zechstein Basin are generally within the concentration range observed in modern anoxic basins while concentrations of V and Fe are generally higher and U is generally lower. Our results suggest that trace metal abundances in fluid inclusions in halite vary in response to redox changes in seawater and provide a potential redox proxy. To better understand and apply results obtained from fluid inclusions in halite as a paleoredox proxy, however, further work is required. These include the study of fluid inclusions in halite from more locations worldwide and the more detailed characterization of redox sensitive trace metal behavior during seawater evaporation.

Palustrine Limestones and Calcretes as the Sedimentary Record of Paleoenvironmental Changes in the Late Permian Semi-Arid Climate at the Se Periphery of the Southern Permian Basin (the Holy Cross Mountains, Poland)

Palustrine Limestones and Calcretes as the Sedimentary Record of Paleoenvironmental Changes in the Late Permian Semi-Arid Climate at the Se Periphery of the Southern Permian Basin (the Holy Cross Mountains, Poland)

Illite and chlorite cementation of siliciclastic sandstones influenced by clay grain cutans

The distribution and amount of clay rim cements are different for Permian Rotliegend and Lower Triassic Bunter sandstones in the Southern Permian and German Triassic Basins, respectively. Both are similar fluvial-aeolian deposits in hot-arid endorheic basins. In both Permian and Triassic sandstones clay grain cutans can be present, but clay rim cement is often lacking or rare in Bunter sandstones. At first sight it would appear that the presence of cutans is thus not relevant for the development of clay cementation. However, at closer inspection, it appears that authigenic clay is indeed present in both cases. In Rotliegend sandstones, the authigenic clay mainly developed as rims around the clay grain cutans, which are often thin and well microlaminated with few intra-micropores and composed of platy particles. The clay crystals in the rim decrease the pore interconnectivity and lower permeability significantly. In Bunter sandstones, most of the authigenic clay developed within the cutans. These cutans are less well microlaminated, have ample micropores, and are composed of more equant-shaped particles. The latter looser structure facilitated clay authigenesis within the cutans and their micropores. This often led to exfoliation of the cutan laminae and expansion of the entire cutan. These expanded cutans also lower pore connectivity. The presence and thickness of clay rim cement on top of cutans and grains is thus not correlated with the thickness of the cutans but with the texture of the cutans. The latter determined where authigenic clays precipitated and the thickness of cutans is partly the result of clay authigenesis within the cutans. This demonstrates that the composition and the texture of sedimentary components constrained and controlled burial diagenesis.

New approaches to distinguish shale-sourced and coal-sourced gases in petroleum systems

Current approaches relating thermogenic gases to either shale source rocks (predominantly type II kerogen) or coal source rocks (predominantly type III kerogen) are not reliable and not globally applicable. This is because these mostly empirical approaches were developed using small poorly-constrained datasets from limited locations. The evaluation of a large global dataset of molecular and isotopic properties of gases from unconventional shale and coal reservoirs suggests that two genetic diagrams based on stable carbon isotopes of methane and ethane, δ13C-C2H6versus δ13C-CH4 and δ13C-CH4versus Δ(δ13C-C2H6 – δ13C-CH4), provide the best separation of shale-sourced and coal-sourced gases. Newly designated genetic fields and shale/coal separation lines on these diagrams were tested and validated using data from five petroleum systems with, likely, only shale (class B and A organofacies) source rocks (the Maracaibo Basin in Venezuela, the Guajira Basin in Colombia and the Rub Al Khali Basin in Iran) and only coal (class F organofacies) source rocks (the Southern Permian Basin in Germany and the Sichuan Basin in China). The practical usefulness of this new approach to gas-source correlations was demonstrated in two case studies from petroleum systems with debated source rock organofacies (the Mozambique Basin in Mozambique and the Indus Basin in Pakistan). These better constrained and more reliable diagrams with genetic fields and shale/coal separation lines represent a new tool for the evaluation of petroleum systems.

Small-scale diagenetic facies heterogeneity controls porosity and permeability pattern in reservoir sandstones

The fluvial-aeolian Upper Rotliegend sandstones from the Bebertal outcrop (Flechtingen High, Germany) are the famous reservoir analog for the deeply buried Upper Rotliegend gas reservoirs of the Southern Permian Basin. While most diagenetic and reservoir quality investigations are conducted on a meter scale, there is an emerging consensus that significant reservoir heterogeneity is inherited from diagenetic complexity at smaller scales. In this study, we utilize information about diagenetic products and processes at the pore- and plug-scale and analyze their impact on the heterogeneity of porosity, permeability, and cement patterns. Eodiagenetic poikilitic calcite cements, illite/iron oxide grain coatings, and the amount of infiltrated clay are responsible for mm- to cm-scale reservoir heterogeneities in the Parchim formation of the Upper Rotliegend sandstones. Using the Petrel E&P software platform, spatial fluctuations and spatial variations of permeability, porosity, and calcite cements are modeled and compared, offering opportunities for predicting small-scale reservoir rock properties based on diagenetic constraints.

Origin of hydrocarbon and non-hydrocarbon (H2S, CO2 and N2) components of natural gas accumulated in the Zechstein Main Dolomite carbonate reservoir of the western part of the Polish sector of the Southern Permian Basin

The origin and migration of hydrocarbon and non-hydrocarbon (hydrogen sulphide, carbon dioxide and molecular nitrogen) components of natural gas accumulated within the Zechstein PZ2 (Stassfurt) Main Dolomite (Ca2) carbonate reservoir in the western part of the Polish sector of the Southern Permian Basin (SPB) were established based on molecular and isotopic compositions (δ13C[CH4, C2H6, C3H8, n-C4H10, i-C4H10, n-C5H12, i-C5H12 and CO2], δ2H[CH4], δ15N[N2] and δ34S[H2S]) related to geological settings and 1-D modelling of generation processes. Hydrocarbon gases were generated during both microbial and thermogenic processes. Primary microbial processes ended at the turn of the Late Permian and Early Triassic, when oil and thermogenic gas generation processes began. The reconstruction of the thermal evolution and transformation of organic matter in this part of the SPB shows that thermogenic processes ended in the Late Jurassic in the Grotów Peninsula (Zone C), and in the Late Cretaceous in the western part of the Wielkopolska Platform and Gorzów Platform (zones A and B). The presence of the microbial component in analysed accumulations indicates that traps within the Ca2 carbonate strata were formed and sealed by the overlying impermeable anhydrites and salts of PZ2 (Stassfurt), PZ3 (Leine) and PZ4 (Aller) cycles preventing its escape. Accumulation of natural gas in Ca2 traps took place in at least two stages and had a cumulative character. First, microbial gases accumulated, then thermogenic gases associated with oil were generated from type II kerogen in the “oil window”. An insignificant methane component was generated from type III kerogen and migrated to Ca2 reservoir from Carboniferous strata. H2S was most probably generated during both microbial (MSR) and thermochemical sulphate reduction (TSR) of anhydrites in the PZ1 (Werra) cycle, and additionally from thermal decomposition of organic sulphur components of oil and kerogen in the Ca2 carbonate rocks. MSR ended during the Late Permian. TSR was temporarily halted as a result of Early Cretaceous uplift and probably resumed on a minor scale after the Late Cretaceous. CO2 was predominantly generated during thermogenic processes of kerogen decarboxylation, carbonate dissolution processes, and TSR and MSR together with H2S. A lesser contribution of CO2 from microbial fermentation was also observed. To a large extent N2 was mainly generated during thermal transformation of type II kerogen dispersed in the Ca2 carbonate strata and probably partly migrated from Carboniferous high-maturity type III kerogen to Ca2 reservoirs through fault systems.

An andesitic sill complex in the Southern Permian Basin: volcanogenetic model and stratigraphic implications

Subvolcanic intrusions are highly variable in shape and structure, and occur in nearly all parts of the upper crust, as a result of extensive volcanic activity. Processes of subvolcanics interacting with the host rock are insufficiently understood, as they are rarely exposed. In the southernmost part of the Flechtingen-Altmark Subprovince, (sub)volcanic rocks of the Flechtingen Volcanic Complex (FVC) are exposed in several quarries. It is built up of silicic tuffs, ignimbrites and lava flows, but also of intermediate lavas and extended sill sheets. Additionally, major granitic intrusions exposed by drillings are associated with the FVC. In the Mammendorf quarry, a sill intruded in between lithified turbiditic series of early Carboniferous (Visean–Serphukovian) age at the base, and widely consolidated volcanoclastic deposits of late Carboniferous (late Pennsylvanian) age at the top. Various magma-host rock interactions were found indicating brittle and ductile deformation patterns occurring at the basal contact, and secondary fluidal mixing predominantly occurring at the top contact, most probably caused by fluids accompanying the intruding magma. We present an extended volcanogenetic model for the FVC. Volcanic activity initiated at 302 ± 3 Ma with fallout deposits represented by mostly re-deposited silicic ashfall deposits of the Flechtingen Formation, and cumulated in depositing major ignimbrite series, most likely forming a caldera. Later, the sills intruded at the rheological boundary of the lithified Mississippian turbiditic series and the partly consolidated volcanoclastic series. Finally, major granitic intrusions emplaced in the basement rocks at around 298 ± 4 Ma. The study contributes to clarify stratigraphic constraints of late Carboniferous to early Permian continental deposits and sheds light on stratigraphy of significant late Paleozoic volcanic deposits of the Flechtingen-Altmark Subprovince in the Southern Permian Basin.

Long-range, high-resolution stratigraphic correlation of Rotliegend fluvial-fan deposits in the central Dutch offshore

The Rotliegend feather-edge area in the central part of the endorheic Southern Permian Basin in the Dutch offshore is characterized by a predominance of mud-prone, evaporite-bearing playa and lake deposits with a subordinate amount of interbedded, thin, fluvial sheet sandstones. The distribution and lateral facies changes of the sandstone bodies have been analyzed by generating a long-range, high-resolution chronostratigraphic correlation framework. The correlation technique of pattern matching of GR logs was applied, supported by calculating spectral trend curves. Flooding events are the primary near-synchronous correlation surfaces, which can be traced up to and over 100 km. The basin setting of the Southern Permian Basin, the studied sandstone depositional architecture (logs) and sedimentary characteristics (core) are analogous to the depositional setting of laterally-amalgamated terminal lobes of dryland-river systems in an endorheic basin, such as the Holocene Altiplano Basin in Bolivia, present-day Lake Eyre (Australia) and the Miocene Ebro Basin (Spain). The integrated approach has yielded a stratigraphic reservoir-architecture framework in which the reservoir sandstones, with net sand up to 10 m, have been identified as amalgamated terminal-splay sandstone sheets formed at the end of dryland-river pathways, alternating with lacustrine mudstone layers deposited during short-duration, high-magnitude flooding in intermittent wet climate periods.

Mudstone/sandstone ratio control on carbonate cementation and reservoir quality in Upper Permian Rotliegend sandstones, offshore the Netherlands

Abstract The eolian-fluvial sandstones of the Upper Permian Rotliegend formation, which were deposited in the Southern Permian Basin, are today deeply buried (~3–4 km) and constitute important gas reservoirs in the Netherlands and the southern North Sea. The reservoir properties of the sandstones have been documented to be strongly affected by diagenesis, but the primary diagenetic factors impairing reservoir quality and their cause remain variably interpreted in the literature. Here, we present the results of a detailed investigation on the diagenetic processes controlling reservoir quality in the Lower Slochteren formation in the L and K blocks offshore the Netherlands, where fluvial and aeolian sandstones intercalate with playa lake muds in a delta setting. Quantitative analysis of the diagenetic mineral phases occurring in all main depositional facies (eolian, fluvial, playa-lake) was carried out on more than 200 samples from 21 wells, with the authigenic mineral composition of an additional 500 samples being evaluated qualitatively. The integration of petrographical observations with log data and core descriptions reveals that cement distribution/abundance is not dominantly driven by depositional facies, nor are reservoir properties. Early pore-filling dolomite cement can be as high as 40% and represents the main control on reservoir properties. Detailed analysis of its spatial distribution shows it to be distinctly related to mudstone proximity and mudstone/sandstone (M/S) ratio. Sandstones occurring as thin beds in mudstone-rich depositional sequences (high M/S ratio) typically exhibit strong pervasive carbonate cement regardless of sedimentary facies. In contrast, sandstones forming thick beds in mudstone-poor sequences (low M/S ratio) are commonly free/low in dolomite cement. Our results demonstrate, for the first time, that reservoir quality in the Rotliegend sandstones in the delta setting of the Netherlands is primarily controlled by early dolomite cement. The latter is most developed in areas with high (>70%) vertical M/S ratio, where it may be a devastating factor for reservoir quality. Best reservoir sandstones should be expected where the depositional stacking pattern is poor in shaly deposits (playa-lake and distal sheet-flood sediments), such as in the southern/south-eastern part of the studied area.

Chiswick and Kew fields, Blocks 49/4a, 49/4b, 49/4c, 49/5a and 49/5b, UK North Sea

Abstract The Chiswick Field is a Carboniferous gas field located in UK Blocks 49/4a and 49/4b in the Southern North Sea, approximately 18 km NW of the Markham Field, close to the UK–Netherlands median line. The Kew Field is situated approximately 3 km NE of the Chiswick Field. The Kew structure is a NW–SE-trending horst separated from the Chiswick Field, a large anticlinal domal structure, by a major NW–SE fault and a structural low. The productive reservoir units are Carboniferous (Westphalian A and B) fluvial sandstones. Both fields are situated on the eastern edge of the Silverpit Basin (part of the Southern Permian Basin). The initial exploration drilling had Leman Sandstone Formation as the primary objective, but the first wells encountered a tight Permian reservoir with gas-bearing Carboniferous reservoirs, subsequently appraised and developed. The current estimate for the gas initially in place of Chiswick and Kew is respectively 687 bcf and 85 bcf in the Carboniferous reservoir. The fields to date (Q4 2018) have produced respectively 220 bcf and 33 bcf sales gas. Gas recovery is through natural depletion from hydraulically fractured, horizontal development wells.

Fault seal behaviour in Permian Rotliegend reservoir sequences: case studies from the Dutch Southern North Sea

Abstract Permian Rotliegend reservoir rocks are generally characterized by high net/gross (N/G) ratios, and faults in such sand-dominated lithologies are typically not considered likely to seal. Nevertheless, many examples of membrane sealing are present in Rotliegend gas fields in the Southern Permian Basin. This manuscript reviews examples of membrane sealing in the Dutch Rotliegend; it presents an extensive dataset of petrophysical properties of Rotliegend fault rocks and analyses two case studies using commonly used workflows. Fault (membrane) seal studies have been carried out on two Rotliegend fields to test the level of confidence and uncertainty of prediction of ‘across fault pressure differences’ (AFPD) based on existing SGR-based algorithms. From the field studies it is concluded that observable small AFPDs are present and that these are likely pre-production AFPDs due to exploration-time scale trapping and retention of hydrocarbons. Two shale gouge ratio (SGR)-based empirical algorithms have been used here to estimate AFPDs in lower N/G reservoir intervals with the aim of predicting membrane seal behaviour, and these results are compared to field data. It is concluded the selected SGR-based tools predict AFPD for Upper Rotliegend lower N/G reservoir rocks with reasonable results. Nonetheless, the core sample datasets show a much wider range of permeability and capillary entry pressure than predicted by the selected SGR transforms. This highlights the potential to modify existing workflows for application to faults in high N/G lithologies. Data sharing and collaboration between industry and academics is encouraged, so that in the long run workflows can be developed specifically for faults in high N/G lithologies.

Quantifying the late-to post-Variscan pervasive heat flow, central Netherlands, Southern Permian Basin

The Southern Permian Basin is marked by significant Latest Carboniferous-Early Permian magmatism attributed to mantle plume emplacement. This intense magmatic activity is commonly assumed to have impacted the heat flow in this area. In the central Netherlands a large number of wells show evidence of magmatic activity dated as Permo-Carboniferous. In addition, high maturity values have been measured for the Carboniferous and below. Theoretical models for tectonic heat flow and maturity evolution presented in this paper show that mantle upwelling, underplating, and intrusions have a significant effect on maturity-depth trends. Tectonic modelling of five selected wells shows that tectonic subsidence and exhumation can be correlated with a significant heat flow pulse at Latest Carboniferous-Early Permian time. This could well explain the widespread elevated maturity/depth gradient in Carboniferous rocks. Quantitative assessment of heat flow, based on a model of mantle plume emplacement, shows that mantle upwelling and underplating at the base of the crust, proposed by previous studies, provides insufficient heat flow to explain strongly elevated maturity-depth trends. In contrast, widespread Permo-Carboniferous calc-alkaline magmatism at shallow crustal levels provides a mechanism for elevated heat flow with a regional impact, consistent with observed high maturity-depth trends. The model and maturity data demonstrate that elevated maturity at shallow burial-depth levels of 500–1000 m is probably sited in the gas window during a heat pulse, occurring at Late Carboniferous times. • Late-to post-Variscan thermal perturbation related to magmatic event. • High maturity in central Netherlands by deep lithospheric processes. • Distinct, short lived thermal peak in carboniferous sediments. • Result fit with regional geodynamic models and magmatic occurrences across the Southern Permian Basin.

Origin and distribution of dolomite in Permian Rotliegend siliciclastic sandstones (Dutch Southern Permian Basin)

ABSTRACTDolomite is a common and volumetrically important mineral in many siliciclastic sandstones, including Permian Rotliegend sandstones (the Slochteren Formation). These sandstones form extensive gas reservoirs in the Southern Permian Basin in the Netherlands, Germany, Poland, and the UK. The reservoir quality of these sandstones is negatively influenced by the content and distribution of dolomite. The origin and the stratigraphic distribution of the dolomite is not yet fully understood. The aim of this study is to identify the origin of carbonate. The main methods used to achieve those aims are a combination of thin-section petrography, scanning electron microscopy (SEM and EDX), and XRD analyses.The present study shows that the typical dispersed occurrence of the dolomite is a consequence of dispersed detrital carbonate grains that served both as nuclei and source for authigenic dolomite cement. The dolomite cement formed syntaxial outgrowths and overgrowths around detrital carbonate grains. The study also shows that dolomite cement, often in combination with ankerite and siderite, precipitated during burial after mechanical compaction. Most of the carbonate grains consisted of dolomite before deposition. The carbonate grains were affected by compaction and pressure dissolution, and commonly have no well-defined outlines anymore.The distribution of dolomite cement in the Rotliegend sandstones was controlled by the presence of stable carbonate grains. Due to the restricted and variable content of carbonate grains and their dispersed occurrence, the cement is also dispersed and the degree of cementation heterogeneous.Our findings have important implications on diagenesis modeling. The presence of detrital carbonate excludes the need for external supply by any large-scale advective flow of diagenetic fluids. By knowing that the carbonate source is local and related to detrital grains instead of being externally derived from an unknown source, the presence of carbonate cement can be linked to a paleogeographic and sedimentological model.