Dogger Formation Research Articles

Characterization of a carbonate geothermal reservoir using rock-physics-guided deep neural networks

Deep neural networks (DNNs) have the potential to streamline the integration of seismic data for reservoir characterization by providing estimates of rock properties that are directly interpretable by geologists and reservoir engineers instead of elastic attributes like most standard seismic inversion methods. However, they have yet to be applied widely in the energy industry because training DNNs requires a large amount of labeled data that is rarely available. Training set augmentation, routinely used in other scientific fields such as image recognition, can address this issue and open the door to DNNs for geophysical applications. Although this approach has been explored in the past, creating realistic synthetic well and seismic data representative of the variable geology of a reservoir remains challenging. Recently introduced theory-guided techniques can help achieve this goal. A key step in these hybrid techniques is the use of theoretical rock-physics models to derive elastic pseudologs from variations of existing petrophysical logs. Rock-physics theories are already commonly relied on to generalize and extrapolate the relationship between rock and elastic properties. Therefore, they are a useful tool to generate a large catalog of alternative pseudologs representing realistic geologic variations away from the existing well locations. While not directly driven by rock physics, neural networks trained on such synthetic catalogs extract the intrinsic rock-physics relationships and are therefore capable of directly estimating rock properties from seismic amplitudes. Neural networks trained on purely synthetic data are applied to a set of 2D poststack seismic lines to characterize a geothermal reservoir located in the Dogger Formation northeast of Paris, France. The goal of the study is to determine the extent of porous and permeable layers encountered at existing geothermal wells and ultimately guide the location and design of future geothermal wells in the area.

Coupled hydro-chemo-mechanical model for fault activation under reactive fluid injection

Pre-existing faults cross-cutting caprock formations constitute potential leakage pathways during geological CO2 sequestration. When the fault filling material is calcite-rich, dissolution of the fault minerals is expected to occur causing the mechanical weakening of the fault or, even worse, its reactivation. In turn, these chemical-mechanical processes influence the distribution of pore pressure in the vicinity of the fault. Hence, a fully coupled approach including the chemically-induced alteration is necessary while investigating fault’s mechanical stability. In this paper, we present a simplified set of equations that allows the resolution of the coupled hydro-chemo-mechanical problem of reactive fluid flow inside a fault. Empirical relationships are used to describe the change of the fault’s material mechanical properties with porosity. The derived model was used to simulate the evolution of the hydro-chemo-mechanical behavior of a fault following an expected leakage scenario during CO2 storage in the Dogger formation in the Paris Basin. The simulation results showed that under normal conditions, fault reactivation is unlikely to happen even in the long term (∼100 years). Nevertheless, the proposed model is capable of capturing the evolution of fault reactivation and has the potential to become a useful element of the toolbox for assessing the risks related to fault reactivation and leakage for CO2 storage projects.

Fractured reservoir characterization using VSP and surface data: a case study from the Paris basin

Anisotropic characteristics are mainly attributed to fractures in fractured reservoirs. The Dogger formation in the Paris basin is a kind of fractured carbonate reservoir, and the nine-component surface seismic and vertical seismic profiling (VSP) data were acquisited in this area. To characterize the fracture attributes, we analyzed the differential variation of each attribute of the vector wavefield, such as velocity ratio, polarizations, amplitudes and differential travel times. The results described using these parameters had a good match with the overall shear wave anisotropy in the Dogger formation. Two VSP data verified our results. There was about four percent anisotropy in the horizontal direction, which could be interpreted as the porosity and permeability changing along the Dogger layer. The differential shear-wave amplitude from the top of the Dogger shows an overall dimming. There were about six intervals in the shear wave section, which showed that shear wave processes and interpretation provided more information than the P-wave section. The interpretation with shear wave anisotropy showed advantages in revealing detailed lateral variations in fractured reservoirs.

U/Pb dating of geodic calcite: new insights on Western Europe major tectonic events and associated diagenetic fluids

This study presents the first application of the U/Pb dating method to highly Pb-depleted diagenetic geodic calcites of the Jurassic formations of the Paris Basin that leads to a reappraisal of the palaeohydrological history of this region. Composite U/Pb ages from multiple geodes, combined with δ 18 O analyses, reveal two main phases of diagenetic fluid circulations linked with major regional tectonic events. Dogger formations recorded a first diagenetic fluid episode at 147.8 ± 3.8 Ma, i.e. at the very beginning of the emersion of the basin during the Tithonian period and 30 Ma earlier than previously assumed. δ 18 O results confirmed that most of the calcitic cement phases that closed the porosity of these formations precipitated at the beginning of the Cretaceous period. Oxfordian formations recorded another major meteoric fluid circulation at 33.5 ± 2.8 Ma related to the initial stage of the European Cenozoic Rift System (ECRIS). Consequently, the porosity of the Mesozoic formations of the studied area may have been closed sooner than previously thought, before the main ECRIS rifting phase of Oligocene age. This study shows that U/Pb dating of secondary geodic calcite offers a new powerful way for reconstructing the coupled palaeohydrological and diagenetic history of sedimentary basins. Supplementary material: Pb/Pb diagrams, 238 U and 235 U series isochron diagrams and effect of blank correction on U and Pb isotopic compositions and on ages calculation are available at https://doi.org/10.6084/m9.figshare.c.3886885

Palaeopressure reconstruction to explain observed natural hydraulic fractures in the Cleveland Basin

Natural fractures observed within the Lower Jurassic shales of the Cleveland Basin show evidence that pore pressure must have exceeded the lithostatic pressure in order to initiate horizontal fractures observed in cliff sections. Other field localities do not show horizontal fracturing, indicating lower pore pressures there. Deriving the burial history of the basin from outcrop, VR and heat-flow data gives values of sedimentation rates and periods of depositional hiatus which can be used to assess the porosity and pore pressure evolution within the shales. This gives us our estimate of overpressure caused by disequilibrium compaction alone, of 11 MPa, not sufficient to initiate horizontal fractures. However, as the thermal information shows us that temperatures were in excess of 95 °C, secondary overpressure mechanisms such as clay diagenesis and hydrocarbon generation occurred, contributing an extra 11 MPa of overpressure. The remaining 8.5 MPa of overpressure required to initiate horizontal fractures was caused by fluid expansion due to hydrocarbon generation and tectonic compression related to Alpine orogenic and Atlantic opening events. Where horizontal fractures are not present within the Lower Jurassic shales, overpressure was unable to build up as high due to proximity to the lateral draining of pressure within the Dogger Formation. The palaeopressure reconstruction techniques used within this study give a quick assessment of the pressure history of a basin and help to identify shales which may currently have enhanced permeability due to naturally-occurring hydraulic fractures.

Earth tidal and barometric responses observed in the Callovo-Oxfordian clay Formation at Andra Meuse/Haute-Marne Underground Research Laboratory

Abstract Fluid pressures were recorded over 6.5 years in borehole EST207 located at the Andra Meuse/Haute-Marne Underground Research Laboratory in Bure (France). The borehole is equipped with a multipacker system monitoring 11 intervals, including 8 in the Callovo-Oxfordian clay Formation, 2 in the Dogger Formation and 1 at the Oxfordian base. Pressure data were analysed for responses to Earth tides and barometric pressure in the 11intervals. Fourier analyses on the pressure data revealed some of the Earth tidal components. Use of ETERNA software determined a better estimate of Earth tidal wave parameters (amplitudes, phases and their standard deviations) and barometric efficiency. Estimates of the tidal parameters K1, O1, P1, S1 and M2, N2, S2, K2 were made before and after barometric correction. It is suggested that in the Callovo-Oxfordian clay, the greater the clay content of the formation, the greater the barometric efficiency values. A barometric and tidal responses classification using four groups as a function of the 11 borehole intervals is in good agreement with the geological configuration. This preliminary geological interpretation will allow both the estimation of poroelastic and hydrogeological parameters of the clay formation from Earth tidal and barometric responses without classical pump test experiments, and the monitoring of these properties over a long period.

Morphological analysis of stylolites for paleostress estimation in limestones

We develop and test a methodology to infer paleostress from the morphology of stylolites within borehole cores. This non-destructive method is based on the analysis of the stylolite trace along the outer cylindrical surface of the cores. It relies on an automatic digitisation of high-resolution photographs and on the spatial Fourier spectrum analysis of the stylolite traces. We test and show, on both synthetic and natural examples, that the information from this outer cylindrical surface is equivalent to the one obtained from the destructive planar sections traditionally used. The assessment of paleostress from the stylolite morphology analysis is made using a recent theoretical model, which links the morphological properties to the physical processes acting during stylolite evolution. This model shows that two scaling regimes are to be expected for the stylolite height power spectrum, separated by a cross-over length that depends on the magnitude of the paleostress during formation. We develop a non linear fit method to automatically extract the cross-over lengths from the digitised stylolite profiles. Results on cores from boreholes drilled in the surroundings of the Andra Underground Research Laboratory located at Bure, France, show that different groups of sedimentary stylolites can be distinguished, and correspond to different estimated vertical paleostress values. For the Oxfordian formation, one group of stylolites indicates a paleostress of around 10MPa, while another group yields 15MPa. For the Dogger formation, two stylolites indicate a paleostress of around 10MPa, while others appear to have stopped growing at paleostresses between 30 and 22MPa, starting at an erosion phase that initiated in the late Cretaceous and continues today. This method has a high potential for further applications on reservoirs or other geological contexts where stylolites are present.

Determination of the Effect of Geological Reservoir Variability on Carbon Dioxide Storage Using Numerical Experiments

The simulations of carbon dioxide storage in sedimentary reservoirs model the fluid and gas flow and the chemical reactions which occur between the minerals (calcite and dolomite) and the injected CO<sub>2<sub/> [André <i>et al.<i/> (2007) <i>Energy Convers. Manage<i/>. 48, 17821797; Gunter <i>et al.<i/> (1999) <i>Appl. Geochem.<i/> 4, 1-111. However because of the lack of data, these reservoirs are always partially known and the fitted variograms of petrophysical and mineralogical quantities are approximate.The aim is to quantify the impact of uncertainties on reservoir characteristics on the storage predictions. We focus on two operational parameters: the quantity of the stored carbon dioxide and the mean variation of the porosity. Two sources of uncertainties are examined: the draw dispersion and the approximation on the variogram parameters. To study the influence of the draw dispersion, variogram parameters are kept fixed and different simulations are run; the associated variance on the operational parameters then has the meaning of a repeatability error. In the second case, a sensibility analysis is carried out to study the influence of variogram parameters variations (sill, range, nugget effect) on the CO<sub>2<sub/> storage. The chosen methodology is the designs of experiments.The simulations are carried out using reactive transport software. The studied carbonated reservoir is built in reference to the Dogger formation of Paris Basin (France) [Diedro (2009) <i>Thèse<i/>, École Nationale Supérieure des Mines de St- Étienne). This reservoir is composed of several minerals, some of them being disposed in spots. The results show that the impact of the draw dispersion remains lower than the impact of the variogram parameters. The effect of the size of the dolomite spots within the rock on the stored carbon dioxide is to be noticed. Larger spots of the dolomite field with low concentration lead to a greater precipitation of carbonate and reduction of porosity than little ,spots with higher concentrations. The interaction between the sill and the range of the variogram of the initial dolomite concentration is also important. The initial calcite concentration has a slight influence only on the average porosity in the reservoir. Except for the variations of the size of the dolomite spots within the rock, small variations on the variogram parameters of the mineralogical concentration do not involve important modification of stored carbon nor of the final porosity.

Modeling of CO2 Leakage up Through an Abandoned Well from Deep Saline Aquifer to Shallow Fresh Groundwaters

This article presents a numerical modeling application using the code TOUGHREACT of a leakage scenario occurring during a CO2 geological storage performed in the Jurassic Dogger formation in the Paris Basin. This geological formation has been intensively used for geothermal purposes and is now under consideration as a site for the French national program of reducing greenhouse gas emissions and CO2 geological storage. Albian sandstone, situated above the Dogger limestone is a major strategic potable water aquifer; the impacts of leaking CO2 due to potential integrity failure have, therefore, to be investigated. The present case–study illustrates both the capacity and the limitations of numerical tools to address such a critical issue. The physical and chemical processes simulated in this study have been restricted to: (i) supercritical CO2 injection and storage within the Dogger reservoir aquifer, (ii) CO2 upwards migration through the leakage zone represented as a 1D vertical porous medium to simulate the cement–rock formation interface in the abandoned well, and (iii) impacts on the Albian aquifer water quality in terms of chemical composition and the mineral phases representative of the porous rock by estimating fluid–rock interactions in both aquifers. Because of CPU time and memory constraints, approximation and simplification regarding the geometry of the geological structure, the mineralogical assemblages and the injection period (up to 5 years) have been applied to the system, resulting in limited analysis of the estimated impacts. The CO2 migration rate and the quantity of CO2 arriving as free gas and dissolving, firstly in the storage water and secondly in the water of the overlying aquifer, are calculated. CO2 dissolution into the Dogger aquifer induces a pH drop from about 7.3 to 4.9 limited by calcite dissolution buffering. Glauconite present in the Albian aquifer also dissolves, causing an increase of the silicon and aluminum in solution and triggering the precipitation of kaolinite and quartz around the intrusion point. A sensitivity analysis of the leakage rate according to the location of the leaky well and the variability of the petro-physical properties of the reservoir, the leaky well zone and the Albian aquifers is also provided.

Tracing the origin of water and solute transfers in deep groundwater from Oxfordian, Dogger and Trias formations in the east of the Paris Basin – France

In order to assess the feasibility for long lived radioactive wastes storage facilities in deep geological formation, solute transport processes must be investigated in the vicinity of the host formation. In France, the Oxfordian and Dogger limestone layers surrounding the Callovo-Oxfordian (CO x ) argillite in the east of the Paris Basin are investigated for this purpose. More than 60 samples of Oxfordian and Dogger formation groundwater and one sample of Triassic formation groundwater, located at the bottom section of the investigated sedimentary cover, were collected over a 250 km 2 area, and were analysed for major ions, δ 18O and δ 2H of water, 87Sr/ 86Sr, and δ 34S and δ 18O of dissolved sulphate. Oxfordian and Dogger formation water is from meteoric origin, and no direct water flow between Oxfordian, Dogger, nor Triassic formations was evidenced. Mineralization processes of the Dogger limestone groundwater were fully investigated. These processes correspond to a series of geochemical reactions including: calcite dissolution, incongruent dissolution of dolomite, ion addition from upward vertical diffusion through the sedimentary pile inducing cation exchange and further dissolution due to the increased ionic strength of the solutions. A PHREEQC model was developed to simulate inferred chemical processes. The model output is consistent with observed data.

Transport Properties of Lavoux Limestone at Various Stages of CO2-Like Acid-Rock Alteration

Since 2005, the French National Research Agency supports several scientific projects intended to select, validate and implement a geological site for CO2 storage in France. The work presented in this paper was carried out within the Geocarbone-Injectivity project, aimed to study the suitability of a carbonate deep saline aquifer located in the Paris basin for a geological CO2 storage. A geological analog to the aquifer rock was chosen in order to perform several petrophysical and transport characterizations. Measured parameters were intrinsic permeability, inertia coefficient, Klinkenberg coefficient, relative permeabilities and transport properties such as the dispersion coefficient and the stratification factor. Samples of Lavoux limestone were characterized in their original state and also after a phase of CO2 -like acid-rock alteration. Results show sensible changes in porosity, permeability and inertia coefficient and almost no change in relative permeabilities, dispersion coefficient and stratification coefficient. These results mean that the CO2 injectivity would be maintained and even enhanced during its injection in the considered Dogger formation. Nevertheless, to confirm this observation, additional experimental and modeling work should be performed, taking into account the potential fine migration. A permeability alteration could be expected due to these fine displacements.

Jurassic sedimentation in the Cleveland Basin: a review

Jurassic sedimentation in the Cleveland Basin: a review

Coupled Large Scale Hydromechanical Modelling for Caprock Failure Risk Assessment of CO2Storage in Deep Saline Aquifers

This work presents a numerical strategy of large scale hydromechanical simulations to assess the risk of damage in caprock formations during a CO<sub>2<sub/> injection process. The proposed methodology is based on the development of a sequential coupling between a multiphase fluid flow (TOUGH2) and a hydromechanical calculation code (Code_Aster) that enables us to perform coupled hydromechanical simulation at a regional scale. The likelihood of different caprock damage mechanisms can then be evaluated based on the results of the coupled simulations. A scenario based approach is proposed to take into account the effect of the uncertainty of model parameters on damage likelihood. The developed methodology is applied for the caprock failure analysis of deep aquifer of the Dogger formation in the context of the Paris basin multilayered geological system as a demonstration example. The simulation is carried out at a regional scale (100 km) considering an industrial mass injection rate of CO<sub>2<sub/> of 10 Mt/y. The assessment of the stress state after 10 years of injection is conducted through the developed sequential coupling. Two failure mechanisms have been taken into account, namely the tensile fracturing and the shear slip reactivation of pre-existing fractures. To deal with the large uncertainties due to sparse data on the layer formations, a scenariobased strategy is undertaken. It consists in defining a first reference modelling scenario considering the mean values of the hydromechanical properties for each layer. A sensitivity analysis is then carried out and shows the importance of both the initial stress state and the reservoir hydraulic properties on the caprock failure tendency. On this basis, a second scenario denoted “critical” is defined so that the most influential model parameters are taken in their worst configuration. None of these failure criteria is activated for the considered conditions. At a phenomenological level, this study points out three key aspects for risk management. The maximum overpressure is reached rapidly after a couple of years, the lateral extension of the “overpressurized” zone induced by the injection is very large (> 50 km) and the most critical zone is the injection near zone (distance < 100 m) at the interface between the caprock and the reservoir layer.

Results of investigations to design a monitoring program for a CO2 storage project in the Paris Basin (France)

The future development of safe CO2 storage on land needs a particular effort on monitoring methods. In the specific geological context of the sedimentary Paris Basin (France), the Geocarbone-Monitoring project carried out from 2006 to 2008 the evaluation and testing of different monitoring methods. The targeted reservoirs are either depleted reservoirs in the carbonate Dogger formation or saline aquifers in the silico-clastic formations of the Triassic. To map the migration of the CO2 plume, the project evaluated geophysical methods as active seismic, electrical resistivity and gravity. Results show that the geological context implies further research to be conducted to overcome problems related either to data acquisition in the case of gravity or to the geological context in the case of seismic and electrical resistivity. The InSAR remote sensing method was not able to detect ground deformation over seasonal gas storage, mainly because of the dense vegetation cover. Methods for soil gas measurement at the surface were successfully tested on natural analogs, revealing the complexity of mechanisms involved in the origin of the CO2 flux, its temporal and spatial variations, as well as the need for continuous surface measurements in order to control seasonal variations

Basin scale evolution of formation waters: a diagenetic and formation water study of the Triassic Chaunoy Formation, Paris Basin

Formation waters and their movements within basins are commonly attributed with responsibility for patterns of cementation and porosity-loss within reservoirs and aquifers. It is thus important to understand when and how waters move in the subsurface. We have studied the evolution and movement of formation water in the Triassic Chaunoy Formation of the Paris Basin, NW Europe to define the way in which the water has evolved and to interpret water movement patterns using mineral isotope and fluid inclusion data in conjunction with detailed formation water analyses. The Chaunoy Formation is cemented with different types of dolomite, calcite and quartz cement. We have studied the evolution of waters in terms of oxygen, carbon and strontium isotopes and salinity. Connate waters were meteoric in origin but strongly influenced by the proximity of a playa lake. Input of palaeo-meteoric water, which entered the Chaunoy via eastern halite bodies, resulted in highly saline formation water for the majority of the Chaunoy Formation from about 100 Ma until (and for some time after) maximum burial. Saline waters spread into the aquifer initially taking a west-north west trajectory and then switching to a west-south west pattern in the Eocene. Following the Alpine orogeny and localised uplift of the basin margin, the southern portion of Chaunoy received fresh, low salinity, meteoric water. It is likely that formation water was lost from the Chaunoy in a more central part of the basin, via sub-vertical cross formational flow to the Mid Jurassic Dogger Formation, to accommodate the influx of fresh water.

The Seasonal Climate of the Early-Middle Jurassic, Cleveland Basin, England

Diverse aspects of the fossil floras of the Early to Middle Jurassic (Cleveland Ironstone and Dogger Formations and Ravenscar Group; Pliensbachian-Bathonian stages) of North Yorkshire, England, have been investigated in order to better define the prevailing Early-Middle Jurassic paleoclimate. Growth rings in fossil woods have been characterized in terms of their 'mean sensitivity'; overall woods are 'complacent' and grew well within the bounds of their climatic tolerance, but woods from the oldest and, particularly, youngest parts of the succession analyzed (the Dogger and Scalby Formations, respectively) show greater interseasonal variation in growth. Intraseasonal variability in growth is indicated by the ubiquitous presence of false rings. Further evidence of seasonality from the fossil floras is the occurrence of leaf beds (ginkgophytes and Nilssoniopteris) interpreted as resulting from the deposition of leaves shed from deciduous plants during flood seasons. Charcoal is ubiquitous throughout the succession and wood samples often show 'checking' of cell walls as a microscopic feature. These observations, combined with a consideration of the depositional and diagenetic history of the host sedimentary rocks, indicate the predominance of a winter-wet seasonal paleoclimate, which was characterized by greater aridity in the late Bajocian-Bathonian relative to the Pliensbachianmid Bajocian. The increasing aridity towards the close of this interval may have been partly a function of regional withdrawal of the extensive seaway that characterized the area for most of the preceding Early Jurassic, but also may have been an early manifestation of the continent-wide arid phase that affected Eurasia during the Late Jurassic.

Palynological variations and sedimentary cycles in the Jurassic from the Paris basin

Two studies, one on Hettangian dolomitic cap rocks over “Gaz de France” gas storage reservoir rocks at Sologne, and the other on two separate sections in Dogger formations in Burgundy, conducted at the Palynology Laboratory of Strasbourg, illustrate that a palynological approach can lead to a considerable refinement of knowledge in sequence stratigraphy. The palynological study of the dolomitic cap rocks at Sologne dates them as Hettangian, distinguishes microfloristical assemblage types within the sequence, and establishes palynofacies variations with basin evolution. The evidence suggests that palynological variations in the Hettangian rocks are attenuated, reflecting restricted marine conditions and weak eustatic changes. The two sections in Dogger formations in Burgundy, one in Buffon with Callovian sediments deposited in an inner neritic environment, and the other in Varzy with Bathonian sediments deposited in an outer neritic environment, show an alternation between a sapropelic palynofacies, with abundant amorphous organic matter and numerous acritarchs, and a humic palynofacies, rich in dinoflagellate cysts, and spores and pollen. Such alternations indicate a cyclic palynological evolution even within the sedimentary parasequences (4th–6th order), as recorded in the section at Buffon.

Cross formational flow in the Paris Basin

AbstractCross formational flow of water has occurred across several hundred metres of Liassic mudstone from the Triassic Chaunoy Formation sandstone to the Middle Jurassic Dogger Formation carbonate in the Paris Basin, France. This has been demonstrated by chemical and isotopic data from rocks and formation waters, sampled on a basin scale, from the Dogger and Chaunoy formations. Present‐day and palaeoformation waters in the Dogger record input of exotic water in terms of salinity and carbon and strontium isotopes. The exotic water was highly saline and contained isotopically light carbon and 87Sr‐enriched strontium in comparison to the indigenous Dogger Formation water. The source of the exotic water can only have been the Chaunoy Formation. Salinity and isotope data show that present‐day Dogger Formation water contains 10–15% invasion Chaunoy water. Modelling strontium isotope ratio and concentration data from mineral cements shows that over the duration of Dogger cementation, Dogger palaeoformation waters were replaced by 1–5% Chaunoy water. Cross formational flow of water must have occurred by convective mass flow, rather than diffusion, in order to preserve the characteristics of water input from the Chaunoy. Flow probably occurred via the major Hercynian fault and fracture systems that run through the centre of the Paris Basin. Upward flow in this setting is most likely in a compressional tectonic regime, the situation during much of the Tertiary in the Paris Basin. Thermodynamic modelling indicates that cross formational flow from the Chaunoy has probably caused carbonate cementation in the Dogger.

The Blea Wyke Sandstone Formation (Jurassic, Toarcian) of Rosedale, North Yorkshire, UK

SUMMARY Revision of the stratigraphy and field relationships of the late Toarcian succession of Rosedale in general, and of Hollins Mines in particular, has clarified the stratigraphy of the Blea Wyke Formation and given new evidence for the nature of the pre-Dogger Formation deformation. Four new members of the Blea Wyke Sandstone Formation are introduced for the succession in Rosedale: the Yew Grain Member, the High House Ironstone Member, the Low Baring Member and the Rosedale Ironstone Member. The Yew Grain Member comprises a coarsening-up succession, dominated by siltstones, but with bioturbated sandstones at its top. The siltstones yield ammonites indicative of a fallaciosum Subzone age. The ironstones of the High House Ironstone Member are bioturbated silty berthierine ooidal mud- and wacke-ironstones, with abundant sideritic and phosphatic concretions, and are of dispansum Subzone age. The Low Baring Member comprises bioturbated fine-grained sandstones and siltstones of dispansum Subzone age, and is largely equivalent to the ‘Serpula Sandstone’ of previous authors. The Rosedale Ironstone Member is characterized by ooidal grain-ironstones, with berthieroidal ooids and a siderite cement. The Rosedale Ironstone Member is of uncertain age, constrained by the dispansum Subzone age of the Low Baring Member below and the probable opalinum Biozone age of the unconformable base of the Dogger Formation above. The magnetitic ironstones of the Hollins Mines are referred to the Rosedale Ironstone Member. Their different mineralogy reflects a different diagenetic evolution. The magnetitic ironstones are locally more than 25 m thick and are not channel-fills, as previously believed, but are contained within truncated synclines, probably roll-overs associated with extensional growth faulting. East of a north–south line passing just east of Hollins Mines the lowest part of the Dogger Formation is the Glaisdale Ironstone Member (of Aalenian, opalinum Biozone age). West of this line the Low House Sandstone Member is the lowest element of the Dogger Formation. The Low House Sandstone Member and the western part of the outcrop of the Glaisdale Ironstone Member are overlain by the ‘Blakey Series’, forming the upper part of the Dogger Formation (of Aalenian age, murchisonae Biozone), which also contains ooidal grain ironstones in its upper part in the south-east of Rosedale. These were also worked, although on a smaller scale, in the area of Hollins Mines and a little to the north near Abbey Bank.

Effects of the “pays de bray” fault on fluid paleocirculations in the Paris basin dogger reservoir, France

Clay fractions and fluid inclusions were studied in the Aulnay sous Bois and Cerneux (CER-1 and CER-P6) wells located 10 km, 1.3 and 0.6 km, respectively, from the “Pays de Bray” fault (Paris Basin, France). It was shown that a connection was probably established between the Dogger and Triassic reservoirs during the active period of the fault. Chemical and heat transfers have locally modified the diagenesis conditions in the Dogger formation, inducing an overmaturation of clay minerals. These effects, although attenuated, are still identifiable in the Aulnay and CER-1 wells. A maximum temperature of about 90°C was maintained in CER-P6 site even during the uplift stage of the basin.