Chalk Formation Research Articles

Novel Temperature-Responsive Organic Polymer Gel for Efficient Abandonment of Aged Porous Chalk Formations Characterized by X-ray Computed Tomography

Novel Temperature-Responsive Organic Polymer Gel for Efficient Abandonment of Aged Porous Chalk Formations Characterized by X-ray Computed Tomography

Technology Focus: Unconventional and Tight Reservoirs (July 2024)

Unconventional reservoirs bear a unique perplexity in that, at every scale, they are different from their conventional counterparts and even one another. Small nuances in any one parameter can result in a vastly different well result, which may affect how an area, or even an entire play, is interpreted. This month’s selection of papers is all about those differences, in recognition that it is technology that drives innovation through understanding what these differences mean and how best to extract value from these vast resources. Starting at the well scale and looking at the benefits of knowing the efficacy of each stimulation stage, paper URTeC 3864145 is from the Ordos Basin in China. Investigating hydraulic fracture performance on a stage-by-stage basis proves time and time again that understanding the details can improve overall development outcomes. The case study reviews two hydraulic fracture diagnostic techniques used in combination to determine water breakthrough and shutoff plans. Zooming out to the play scale, paper SPE 216292 is focused on the most variable of unconventional reservoirs: the carbonate reservoir. This case study of the Austin Chalk formation in Texas analyses the production uplift from application of horizontal, multistage fracture stimulation technology in tight or fractured carbonate reservoirs. The application of unconventional technology in carbonate reservoirs can extended field life, and the deployment of this technology should be considered for any redevelopment or reassessment of potential resources trapped in tight carbonate reservoirs. Finally, paper OTC 34832 draws on the concept of innovation that is required to unlock unconventional resources. Novel methods of stimulation are the cornerstone of unconventional technology in that well stimulation is a basic requirement of all low-permeability reservoirs. The paper describes a trial pilot from idea through execution from Kuwait Oil Company for enhanced oil recovery in tight formations using downhole hydrogen generation from in-situ exothermic multistage chemical reactions between two unique hydroreacting agents. The paper also provides lessons learned and optimization insights from the trial, concluding that a successful, low-cost alternative stimulation technique was demonstrated and repeatable. Recommended additional reading at OnePetro: www.onepetro.org. SPE 216139 Tight Gas Reservoir Characterization and Comparison of PLT Methods: Microseismic Monitoring, Fiber-Optical Production Logging, and Tracer-Coated Sand Monitoring Applied in the Same Well by Xiao Yao, Petrochina, et al. SPE 216149 New Insight in Developing Tight Fractured Carbonate Reservoirs, Mishrif Formation, West Kuwait by M. El-Jeaan, Kuwait Oil Company, et al. SPE 215712 Successful Utilization of In-Situ Dune Sands in Saudi Arabian Unconventional Frac Operations by Nahar Qahtani, Saudi Aramco, et al.

Temperature, pressure, and duration impacts on the optimal stiffening of carbonates aged in diammonium phosphate solution

Diammonium phosphate (DAP) has been proven effective in improving the stiffness of weak or acid-damaged carbonates, thereby preserving hydraulic fracture conductivity. The reaction between DAP and calcite in chalk formations primarily produces hydroxyapatite (HAP), which is stiffer than calcite. However, the optimal reaction outcomes vary greatly with factors such as DAP concentration and reaction conditions. This study investigated the DAP-calcite reaction duration, pressure, and temperature effects on the stiffness magnitude of soft Austin chalk. Also, the catalyst effect and depth of HAP formation were examined. The study involved the assessment of stiffness non-destructively (impulse hammering), mineralogy (XRD, SEM), and elemental composition (XRF). The study tested 15 different DAP-chalk reaction variations, where the pressure, temperature, aging time and catalyst addition were modified in each case. The samples' elastic stiffness distributions were then collected and compared to the pre-reaction ones. The results showed that the elastic stiffness increased in all treated samples, with an 181% maximum increase achieved after 72 h at 6.9 MPa and 75 °C. However, the pressure effect was minor compared to the temperature. The SEM images revealed different HAP morphology corresponding to different treatment conditions. Although the treated samples showed an increased intensity of phosphorus throughout the entire sample, the near-surface zone (4–6 mm) was the most affected, as inferred from the XRF elemental analysis. The study's findings can help optimize hydraulic fracturing operations in weak carbonate reservoirs, improving production rates and overall well performance.

The Effect of Salt Concentration on Dielectric Permittivity and Interfacial Polarization in Carbonate Rocks with Complex Pore Structure

Summary Broadband dielectric dispersion measurements are attractive options for the assessment of water-filled porosity. Dielectric permittivity is influenced by salinity as well as other rock/fluid properties. However, the effect of salinity on Maxwell-Wagner polarization (i.e., interfacial polarization) and dielectric permittivity in rock samples with complex pore structures requires further investigation. The objectives of this work are (a) to perform frequency-domain dielectric permittivity numerical simulations on 3D pore-scale rock samples at different salt concentration levels, (b) to quantify the effect of salinity on dielectric permittivity and interfacial polarization in the frequency range between 20 MHz and 5 GHz, and (c) to quantify the critical frequency (i.e., the frequency at which the relative permittivity becomes frequency-independent). We first perform pore-scale frequency domain dielectric permittivity simulations in fully water-saturated carbonate samples with complex pore structures to obtain the complex dielectric permittivity in the frequency range of 0.01–5 GHz and at different salinity levels. Next, we numerically create partially water/hydrocarbon-saturated water-wet samples and perform simulations at different salinity and water saturation levels to investigate the combined effect of salinity and water saturation on dielectric permittivity. Finally, we investigate how reliable conventional mixing models, such as the complex refractive index model (CRIM) and Hanai-Bruggeman (HB), are in the assessment of water saturation at different salinity levels. We used 3D pore-scale rock samples with complex pore structures from Austin Chalk, Estaillades Limestone, and Happy Spraberry formations. The increase in salinity from 2 to 50 parts per thousand (PPT) resulted in the relative permittivity to increase by 18% at 20 MHz. Similarly, an increase in salinity from 2 PPT to 50 PPT resulted in electrical conductivity to increase by 15 times at 20 MHz. However, at 5 GHz, the difference between the relative permittivity of the samples at different salinities was negligible. We demonstrated that the critical frequency was above 1 GHz. Thus, if complex dielectric permittivity at 1 GHz is being used, an accurate salinity assumption is required in the interpretation of conventional dielectric mixture models in carbonate formations. Finally, we observed 52% and 42% average relative errors in water saturation quantification when applying CRIM and HB models at all the frequencies of interest, respectively. The results also indicated that conventional models should not be used in the presence of uncertainty in salinity at lower frequencies. The results of this work quantified the frequency at which the water-filled pore volume rather than the Maxwell-Wagner polarization controls the relative permittivity of rock samples saturated with a wide range of brine salinity. Moreover, the results demonstrated that the relative permittivity of the rock samples with complex pore structures may still be significantly affected by the interfacial polarization even at 1 GHz. Moreover, the results suggested that the conventional mixture methods cannot reliably take into account the salt concentration of formation water, and this can lead to significant errors in reserves assessment.

The Late Maastrichtian Rørdal Member (Denmark): a remarkable window of climatic, environmental and biotic changes at orbital timescales

Abstract The Late Maastrichtian Rørdal Member of the Møns Klint Formation is a remarkable lithological unit of the Danish Basin, characterized by chalk–marl cyclicity, standing out from pure Maastrichtian white chalk within the Chalk Group. A cyclostratigraphic analysis across this unit suggests a control by orbital precession and a strong amplitude modulation of the precession by the 405 kyr eccentricity cycle. Oxygen isotope data from bulk carbonate, benthic foraminifera and brachiopods indicate a prominent cooling commencing at the base of the unit, with maximum cooling aligned with a 405 kyr amplitude modulation maximum. This unit thus represents a rare example of a climatic cooling associated with a 405 kyr insolation maximum. However, geochemical and calcareous nannofossil data support lower productivity levels across the unit, in contradiction to an expected ocean fertilization accompanying enhanced continental weathering as the source of the clay material. An alternative model of deposition for this lithological unit is thus proposed via dense water cascading favoured by a sea-level low, subsequent restriction of the Chalk Sea and associated gravity currents responsible for the resuspension of fine clastic deposits from the margins of the Baltic Shield.

Natural attenuation of dissolved petroleum fuel constituents in a fractured Chalk aquifer: contaminant mass balance with probabilistic analysis

A plume-scale mass balance is developed to assess the natural attenuation (NA) of dissolved organic contaminants in fractured, dual-porosity aquifers. This methodology can be used to evaluate contaminant distribution within the aquifer, plume source term, contaminant biodegradation and plume status. The approach is illustrated for a site on the UK Upper Chalk aquifer impacted by petroleum fuel containing methyl tert -butyl ether (MTBE) and tert -amyl methyl ether (TAME). Variability in site investigation data and uncertainty in the mass balance were assessed using probabilistic analysis. The analysis shows that benzene, toluene, m/p-xylene and o-xylene (BTEX) compounds are biodegraded primarily by denitrification and sulfate reduction in the aquifer, with an equivalent plume-scale first-order biodegradation rate of 0.49 a −1 . Other biodegradation processes are less important. Sorption contributes to hydrocarbon attenuation in the aquifer but is less important for MTBE and TAME. Uncertainty in the plume source term and site hydrogeological parameters had the greatest effect on the mass balance. The probabilistic analysis enabled the most likely long-term composition of the plume source term to be deduced and provided a site-specific estimate of contaminant mass flux for the prediction of plume development. The mass balance methodology provides a novel approach to improve NA assessments for petroleum hydrocarbons and other organic contaminants in these aquifer settings. Supplementary material: Plume organic chemistry, aquifer hydraulic conductivity and probability distribution functions used for mass balance inputs are available at https://doi.org/10.6084/m9.figshare.c.7016429 Thematic collection: This article is part of the Monitoring the aquifers collection available at: https://www.lyellcollection.org/topic/collections/monitoring-the-aquifers

Diammonium Phosphate Treatment for Sustained Hydraulic/Acid Fracture Conductivity in Chalk and Limestone Formations

Summary The sustained conductivity of hydraulic/acid fractures is crucial for the continuous and effective production of hydrocarbons. However, hydraulic fractures in soft carbonate formations often experience a reduction in conductivity due to rock deformation and creeping under in-situ stresses. One approach to resolve this issue is to stiffen the fracture surfaces using a consolidating agent. This study examines the application of diammonium hydrogen phosphate (DAP) as an additive to improve the hydraulic/acid fracture conductivity in Indiana limestone and Austin chalk slabs. Initially, flat slabs of Indiana limestone and Austin chalk were subjected to acidization using 15% and 10% hydrochloric acid (HCl) for 10 minutes and 5 minutes, respectively. The resulting surface texture changes were measured using a profilometer. Subsequently, half of the samples underwent treatment in 1 M DAP under 1,000 psi and 75°C for 72 hours. The surface stiffness of the samples was evaluated using nondestructive impulse hammering before and after acid injection and DAP treatment, while hydraulic/acid fracture conductivity was determined using an API conductivity setup. Following acid injection, the experimental results demonstrate that the stiffness of all the samples was diminished to different degrees. However, intact Indiana limestone samples exhibited increased stiffness after treatment with DAP. Acidized Indiana limestone samples showed partial restoration of their original stiffness with DAP treatment. In contrast, DAP treatment fully restored the stiffness of Austin chalk samples and further enhanced it, resulting in a twofold to fivefold increase. The increased stiffness observed in the treated samples had a direct impact on short- and long-term conductivity. Treated Indiana limestone fractures exhibited double the conductivity of untreated fractures. The improvement in Austin chalk conductivity was even more significant after DAP treatment, resulting in seven to eight times higher conductivity in the treated hydraulic/acid fractures. It is worth noting that there was little to no correlation between surface roughness and conductivity, highlighting the significant influence of fracture stiffness on conductivity.

Old chalk, new tricks: revisiting the Lower Cretaceous carbonates of the Danish Central Graben

Abstract The Lower Cretaceous (upper Hauterivian–Albian) pelagic and hemipelagic carbonates of the Tuxen and Sola Formations in the Danish Central Graben, North Sea, constitute one of the oldest chalk successions recorded globally, but have received less attention than the Upper Cretaceous–Danian Chalk Group. This paper presents an updated depositional model for the succession drawn from synthesis of the latest published sedimentological and stratigraphic results, and correlation of 11 wells in the Valdemar, Boje, Adda and Tyra Fields. Four depositional sequences, deposited on a relatively deep subphotic shelf, record c. 20 Myr of transgressive–regressive cycles, including: (1) late Hauterivian–earliest Barremian highstand and differential subsidence, resulting in aggradation across a westward-dipping ramp; (2) early Barremian eastern (Adda Field) inversion causing plateau condensation, sediment bypass and sourcing of gravity flows, followed by lowstand-controlled basin isolation and associated anoxia (Munk Marl Bed), and finally late Barremian tectonic quiescence and highstand with deposition of clean reservoir chalk; (3) latest Barremian lowstand causing filling of local depocentres, interrupted by early Aptian transgression-controlled anoxia during the global Oceanic Anoxic Event 1a (Fischschiefer Member), and finally late Aptian highstand; and (4) latest Aptian–earliest Albian lowstand causing local erosion and heightened influx of clay.

Sedimentary environment and benthic oxygenation history of the Upper Cretaceous Austin Chalk Group, south Texas: An integrated ichnological, sedimentological and geochemical approach

ABSTRACTOxygen concentration in the ocean is vital for sustaining marine ecosystems. While the potential impacts of deoxygenation on modern oceans are hard to predict, lessons can be learned from better characterizing past geological intervals formed under a greenhouse climate. The greenhouse Cretaceous containing several oceanic anoxic events characterized by widespread oxygen‐deficient water is ideal in this regard. The Austin Chalk Group in south Texas (USA) shows organic‐rich intervals that can be linked to oxygen depletion in the ocean, but the exact bottom water oxygenation conditions have not been estimated. This study aims to reconstruct both sediment interstitial and bottom water oxygenation history during Austin Chalk Group deposition by integrating detailed ichnological, sedimentological and geochemical (X‐ray fluorescence and X‐ray diffraction) analyses, thereby providing a consistent model that may be applicable across a range of marine shelf settings. The 141.12 m Gise #1 core contains a continuous record of the Austin Chalk Group, providing an opportunity for unravelling oxygenation and deoxygenation events. Whereas the anaerobic–exaerobic deposits are essentially nonbioturbated, four oxygen‐related ichnocoenoses are defined, further refining the transition of aerobic to dysaerobic conditions in the sediment interstitial water. Omission surfaces and glauconitic grains, products of current‐induced scouring and condensation, suggest sporadic high‐energy events in the Austin Chalk Group ramp that drove elevated terrestrial inputs. Geochemical data further help to identify anoxic bottom water conditions within the anaerobic facies. Additionally, the lowermost part of the Austin Chalk Group illustrates redox cycles, whereas dilution events characterized by elevated terrestrial input are identified throughout the rest of the Austin Chalk Group. The evolution of oxygenation levels in sediment interstitial water and bottom water disputes the existence of a long‐lasting oxygen‐deficient sea in south Texas. The refined depositional model may be applicable to coeval shelfal settings. Moreover, the results provide insights into variable, evolving palaeoclimatic and palaeoceanographic conditions of the greenhouse Late Cretaceous.

How can an integrated multi-method geophysical approach optimise the groundwater management in the Chalk aquifer? Case study of the Seine fault system (France, Normandy)

The Cretaceous chalk aquifer is a strategic water resource in the north of France, especially in Normandy where it faces increasing pressure in terms of its quality and availability. The effective and sustainable manage of this resource relies on a deep understanding of its complex geological and hydrogeological nature. However, modelling this hydrosystem has its limitations due to the spatial and qualitative heterogeneity of the data available. The integration of geophysical data using various methods and scales has improved the geological knowledge by identifying the different chalk formation and highlighting the geometry of the Seine fault system near Rouen. Geophysical analysis has revealed highly complex fault geometries in this area and clarified their impact on the chalk formations. Additionally, it has provided detailed information about the vertical heterogeneity of the chalk, both geologically and hydrogeologically. This new knowledge feeds into the existing 3-D regional geological model, especially in areas with limited data. Supporting hydrogeological modelling, this updated model has already enabled to drill new boreholes. In the future, it will contribute to a better understanding and management of groundwater resources in this region.

Facies characterisation and stratigraphy of the upper Maastrichtian to lower Danian Maastricht Formation, South Limburg, the Netherlands

Abstract The Maastricht Formation is a mostly calcarenitic unit that belongs to the Chalk Group but is unlike the typical North Sea chalk in that it is much coarser and at times contains a significant terrigenous component. The formation was deposited between the late Maastrichtian and the early Danian in a proximal zone of the Chalk Sea immediately north of the Anglo-Brabant and Rhenish massifs. The formation crops out in South Limburg, the Netherlands, in the German state of North Rhine–Westphalia, near Aix-la-Chapelle (Aachen), and in the Belgian provinces of Liège and Limburg, and it is present in the subsurface in the Campine Basin and in the Roer Valley Graben, in Belgium and the Netherlands. The often spectacular fossil content of the Maastricht Formation has been extensively studied but its sedimentological aspects remain understudied. Indeed, the lithostratigraphy of the formation is largely informal and based on the abundance and morphology of flint and on the quantification of fossil content. The paucity of facies studies and the lack of a lithostratigraphic framework based on modern facies studies hampers stratigraphic correlation between outcrops and, more importantly, boreholes. A facies characterisation and facies-based lithostratigraphic framework of the Maastricht Formation in South Limburg is particularly urgent because groundwater is abstracted from aquifers in the formation and geological models currently in place fail to predict facies heterogeneity and, consequently, aquifer properties. We studied eight outcrops of the Maastricht Formation across South Limburg and carried out a (micro)facies analysis of the outcrops. We show that the Maastricht Formation can be subdivided into three lithofacies and five microfacies. The lithofacies reflect the traditional subdivision of the formation into Maastricht and Kunrade limestones. Our results suggest that the current subdivision of the Maastricht Formation into six members is untenable. The formation is best subdivided into lower and upper members. The Kunrade limestone should be afforded the status of formation. We interpret the Maastricht Formation as having been deposited in an epeiric ramp, in which facies distribution was controlled by water temperature, nutrient levels and storminess. The (micro)facies of the Maastricht Formation can be organised into two depositional stages: stage 1, representing the lower part of the formation, is characterised by heterozoan carbonates deposited under cooler, mesotrophic conditions in a nutrient-rich, more proximal region of the epeiric sea; stage 2 is characterised by heterozoan-photozoan carbonates deposited in a warmer and stormier environment with slightly lower nutrient levels.

Deposition in the northwest Porcupine Basin during the Cretaceous

Abstract: The paper details the sedimentary infill of the northwest part of the Porcupine Basin, offshore west Ireland, during the Cretaceous. This is a critical area for understanding the sediment input and incremental infilling of the basin, particularly during the early Cretaceous. Late Jurassic rifting and early Berriasian erosion produced a northward narrowing valley on the Base Cretaceous unconformity surface, which provided an entry point for sediment into the basin and also constrained the lateral extent of initial sedimentation. Following Berriasian marine transgression, the first deposits in the Porcupine Basin were turbidites located in localised depressions on the unconformity surface (named here informally as Basal Deposits 1–4). As the unconformity surface became smoother, the succeeding layers were more widespread (Sequences 1–3, the top being of probable Barremian age). Several features, interpreted as mounded fans sourced from the basin flanks, are developed in the Valanginian-Barremian section. Where drilled, the Berriasian to Barremian section is predominantly fine-grained. The rifted marginal horsts were covered in late Barremian time, and Aptian and later sediments onlapped the flanks of the Porcupine High to the west. The 'Near Base Aptian' is a well-defined prograding surface and the 'Near Top Aptian' horizon is a marked unconformity. The Aptian section is comprised predominantly of deep-water mudrocks and siltstones, although a distinctive, possibly shallower, erosive Intra-Aptian event was also mapped. A renewed sediment influx, comprising of sand prone deltaics, occurred at the northern margin of the basin during the Albian. Sand content decreases southwards into the basin. Sedimentation in the late Cretaceous was dominated by the basinwide deposition of the Chalk Group (Cenomanian-Selandian) deposited in a stable thermal subsidence setting. At the northern basin margin Upper Cretaceous deposits are thin and lithologically variable. The well-defined 'Top Chalk' reflector, which shows mounded features, is overlain unconformably by Paleocene clastics.

Triassic and Jurassic possible planktonic foraminifera and the assemblages recovered from the Ogrodzieniec Glauconitic Marls Formation (uppermost Callovian and lowermost Oxfordian, Jurassic) of the Polish Basin

Abstract. In the 1960s and 1970s Werner Fuchs of the Austrian Geological Survey (Vienna) described a significant number of new foraminiferal taxa that he considered ancestral to the planktonic foraminifera. All these taxa are well-curated in the collections of the Austrian Geological Survey and have been studied by one of us (Malcolm B. Hart). Some of these taxa, from the Triassic and lowermost Jurassic strata of Austria and northern Italy, are poorly preserved, possibly the result of having an original aragonitic wall structure. None of these taxa possess characters which give the appearance of a planktonic mode of life, although some of them (e.g. Oberhauserella, Praegubkinella) may well have been ancestral to the holoplanktonic foraminifera that appeared in the Toarcian and younger strata. Other taxa in the collections of the Austrian Geological Survey (part of GeoSphere Austria), from the Jurassic of Poland, are preserved as glauconitic steinkerns and are either unidentifiable as foraminifera or suspect in terms of their stratigraphical and evolutionary significance.

Deposition in the Northwest Porcupine Basin During the Cretaceous

The paper details the sedimentary infill of the northwest part of the Porcupine Basin, offshore west Ireland, during the Cretaceous. This is a critical area for understanding the sediment input and incremental infilling of the basin, particularly during the early Cretaceous. Late Jurassic rifting and early Berriasian erosion produced a northward narrowing valley on the Base Cretaceous unconformity surface, which provided an entry point for sediment into the basin and also constrained the lateral extent of initial sedimentation. Following Berriasian marine transgression, the first deposits in the Porcupine Basin were turbidites located in localised depressions on the unconformity surface (named here informally as Basal Deposits 1–4). As the unconformity surface became smoother, the succeeding layers were more widespread (Sequences 1–3, the top being of probable Barremian age). Several features, interpreted as mounded fans sourced from the basin flanks, are developed in the Valanginian-Barremian section. Where drilled, the Berriasian to Barremian section is predominantly fine-grained. The rifted marginal horsts were covered in late Barremian time, and Aptian and later sediments onlapped the flanks of the Porcupine High to the west. The ‘Near Base Aptian’ is a well-defined prograding surface and the ‘Near Top Aptian’ horizon is a marked unconformity. The Aptian section is comprised predominantly of deep-water mudrocks and siltstones, although a distinctive, possibly shallower, erosive Intra-Aptian event was also mapped. A renewed sediment influx, comprising of sand prone deltaics, occurred at the northern margin of the basin during the Albian. Sand content decreases southwards into the basin. Sedimentation in the late Cretaceous was dominated by the basinwide deposition of the Chalk Group (Cenomanian-Selandian) deposited in a stable thermal subsidence setting. At the northern basin margin Upper Cretaceous deposits are thin and lithologically variable. The well-defined ‘Top Chalk’ reflector, which shows mounded features, is overlain unconformably by Paleocene clastics.

The interplay between siliciclastic and carbonate depositional systems: Maastrichtian to Danian basin‐floor sediments of the mid‐Norwegian Møre Basin

AbstractSource‐to‐sink sedimentary systems associated with volcanic rifted margins serve as important archives for basin development by recording lithospheric changes affecting the depositional systems. Distinguishing between sediment transport processes and their sediment source(s) can guide the interpretation of a basin's history, and thereby inform regional paleogeographic reconstructions. In this contribution, we integrate and utilize wireline geophysical logs, detailed petrographic observations from side‐wall cores, and seismic analysis to describe and decipher a Maastrichtian to Danian‐aged basin‐floor depositional system in the deep outer Møre Basin, mid‐Norwegian margin. Well 6302/6‐1 (Tulipan) is a spatially isolated borehole drilled in 2001 that penetrates Maastrichtian and younger strata. A succession of hitherto undescribed carbonates and sandstones in the outer Møre Basin was discovered. It is investigated for sediment transport, provenance, and depositional processes on the basin floor surrounded by structural highs and ridges. The strata from the lower parts form a basin‐floor apron consisting of redeposited carbonate sourced from a westerly sub‐aerial high. The apron transitions vertically from mixed siliciclastic and carbonate into a purely siliciclastic fan with intercalated sandstone and mudstone, providing a rare high‐resolution record of how depositional environments experience a complete shift in dominant processes. The development coincides with similar latest Cretaceous‐earliest Palaeocene sequences recorded south of this region (e.g., well 219/20‐1) and may have been influenced by regional uplift associated with the onset of magmatism in the Northeast Atlantic. This study improves our understanding of a late, pre‐breakup source‐to‐sink sedimentary system developed near the breakup axis of an infant ocean, and documents what is possibly the northernmost chalk deposit in the Chalk Group.

Contrasting influence of salt structures and faults on the geothermal potential of regional structural highs: The Cleaver Bank High, Southern North Sea

High-quality 3D seismic reflection data, complemented by 448 bottom-hole temperatures (BHTs) from 48 boreholes, are used to investigate the influence of salt structures and faults on the geothermal potential of the Cleaver Bank High, Southern North Sea. Developed salt structures include multiple salt diapirs, salt pillows and a salt wall, with their presence influencing local geothermal potential. Strata deposited above the Zechstein Group record geothermal gradients that are enhanced proportionally to the thickness of this salt unit. Conversely, strata buried below the Zechstein Group reveal a moderate decreasing trend in geothermal gradients as salt thickens. Large supra-salt faults can act as fluid paths to deep and hot fluid into shallow strata, resulting in the presence of high geothermal gradients in shallow strata. Importantly, geothermal gradients on the footwall of these faults are much higher than that on the corresponding hanging-wall, decreasing as one moves away from them. For example, average geothermal gradients on the footwall of the largest supra-salt fault (Fault A) are, relative to its immediate hanging-wall, 105 % higher in the North Sea Group, 26 % higher in the Chalk Group, and 41 % higher in the Rijnland, Upper and Lower Germanic Trias Groups. Additionally, sub-salt faults influence the geothermal gradient of supra-salt strata in parts of the study area where there is very thin, or even absent, salt (<100 ms; or ∼230 m), forming distinct low-amplitude trails of fluid above these same faults. They also indirectly influence geothermal gradient by controlling the position, geometry and distribution pattern of salt structures. As a corollary, three potential geothermal exploration targets are suggested on the Cleaver Bank High, one located on the footwall of a large supra-salt fault, one above thick salt, and a third target above very thin Zechstein strata where low-amplitude fluid chimneys are found. The results in this work can be applied to similar salt-bearing structural highs in Northern Europe and worldwide.

Calcareous dinoflagellate blooms during the Late Cretaceous 'greenhouse' world-a case study from western Ukraine.

The Late Cretaceous was a unique period in the history of the Earth characterized by elevated sea levels, reduced land area, and significantly high concentrations of atmospheric CO2 resulting in increased temperatures across the globe-a 'Greenhouse World'. During this period, calcareous dinoflagellate cysts (c-dinocysts) flourished and became a ubiquitous constituent of calcifying plankton around the world. An acme in calcareous dinocysts during the Albian to the Turonian coincided with the highest recorded seawater surface temperatures and was possibly linked to conditions that favored calcification and a highly oligotrophic system in European shelf seas. This study examines the potential applicability of c-dinocysts as a proxy for paleoenvironmental conditions based on their assemblage changes plotted against foraminiferal occurrences and microfacies analysis. The material was extracted from the upper Turonian chalk of the Dubivtsi region in western Ukraine. An inverse correlation was observed between species diversity and the number of c-dinocyst specimens. Nutrient availability gradients apparently determined important changes in the calcareous dinocysts distribution. These trophic changes were likely caused by the interplay of eustatic sea-level fluctuations and Subhercynian tectonic activity leading to changeable nutrient inputs from the nearby land.

Microcrinoids (Roveacrinidae) from the middle–upper Cenomanian Grey Chalk Subgroup, Dover (Kent, United Kingdom): biostratigraphy and re-evaluation of cup structure in roveacrinids

New material of Roveacrinidae from the middle–upper Cenomanian Grey Chalk Group of the Kent coast (Folkestone-Dover) is described. The fauna includes 10 taxa, including a new genus and species ( Dubrisicrinus minutus) and three new species ( Styracocrinus shakespearensis, Roveacrinus aboudaensis and Dentatocrinus serratus). The biostratigraphical significance of roveacrinid faunas is placed in a global context, and it is demonstrated that the roveacrinid zone CeR5, previously recorded only from Morocco, is approximately equivalent to the upper middle Cenomanian Acanthoceras jukesbrownei ammonite Zone, and zone CeR6 – to the Calycoceras guerangeri ammonite Zone. The new material also provides novel information on the cup structure of roveacrinids, which is reviewed and placed in a phylogenetic context.

Joint inversion of temperature, vitrinite reflectance and fission tracks in apatite with examples from the eastern North Sea area—Discussion

AbstractNielsen and Balling (2022) studied the thermo‐tectonic history of the eastern North Sea Basin from inversion of temperature, vitrinite reflectance and apatite fission tracks in boreholes, and found no evidence of Neogene exhumation. This contrasts with previous studies that reported the removal of 500–950 m of Cenozoic sediments during Neogene exhumation, partly based on velocity‐depth data in the Upper Cretaceous—Danian Chalk Group. Their result led Nielsen and Balling (2022) to ‘question the accuracy and precision of the sonic velocity method’. We submit that the use of velocity‐depth data to measure exhumation is a reliable method, and that the interruption of Cenozoic burial in the eastern North Sea Basin by Neogene exhumation is a well‐established geological fact. We suggest that the failure of Balling and Nielsen (2022) to detect Neogene exhumation is diagnostic of problems with their own methods, rather than reflecting on the reliability of the sonic velocity method.

Enhancing Fracture Conductivity in Soft Chalk Formations With Diammonium Phosphate Treatment: A Study at High Temperature, Pressure, and Stresses

Summary This study aims to address the problem of fracture hydraulic conductivity decline in soft formations using a diammonium hydrogen phosphate (DAP) solution. A naturally weak carbonate, Austin chalk was chosen as an ideal specimen. Flat chalk samples with reduced elastic modulus and roughness were evaluated before and after aging with 1 M DAP for 72 hours at 75°C and 1,000 psi. The fracture gas conductivity of DAP-aged and untreated samples was measured at various flow rates and stresses while recording sample compaction using linear variable differential transformers (LVDTs). The study found that DAP aging increased the reduced elastic modulus of chalk specimens up to 330% of the original value, improving their resistance to deformation and failure under stress by 200 psi. The hydraulic conductivity of DAP-aged samples was at least twice that of untreated samples, with an extended hydraulic fracture conductivity seven times higher than that of the untreated ones. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) analysis revealed that DAP reacted with the chalk to form hydroxyapatite (HAP), which binds the calcite grains, yielding a stiffer, more deformation-resisting rock surface. Overall, the study demonstrates the potential of chemically enhancing and extending the fracture hydraulic conductivity of weak carbonates using DAP.