Broad Fourteens Basin Research Articles

Effect of tectonic inversion on supra-salt fault geometry and reactivation histories in the Southern North Sea

High-resolution 3D seismic and borehole data in the Broad Fourteens Basin, Southern North Sea, are used to investigate the relationship between lithology and fault throw distribution, as well as to understand the reactivation and growth histories of faults developed due to tectonic inversion. Two (2) distinct tiers of faults are identified, and their geometry analysed in detail. Tier 1 faults comprise closely spaced sets of normal faults that resulted from the progressive buckling and stretching of Upper Mesozoic strata during Late Cretaceous to Paleogene tectonic inversion. They have been reactivated, but still show net normal throw separations, even though they were formed during a period of regional compression. Tier 2 faults comprise densely spaced sets of normal faults in Paleogene strata with a broad range of strikes, forming a polygonal pattern. These faults relate to early diagenesis but still record the effect of the Paleogene inversion episode. An important characteristic is that Tier 1 faults are highly segmented, and show differences in throw distribution between shale-rich and sandy intervals. These faults are more segmented with relatively small throw maxima of 14 ms (17.7 m) in shale-rich intervals, while sandy intervals are less segmented with larger throw maxima of 32 ms (40.3 m). Discrepancies in fault throw distribution and segmentation increase the chances of compartmentalisation or localised fluid flow through fault linkage zones, presenting at the same time significant risks when injecting CO2 in subsurface traps. Recognising the effect of tectonic inversion on supra-salt fault geometry, and reactivation histories, can be crucial to the characterisation of faulted hydrocarbon and carbon capture and storage (CCS) reservoirs in tectonically inverted basins such as the Southern North Sea.

Discovery of Lower Cretaceous hydrothermal vent complexes in a late rifting setting, southern North Sea: insights from 3D imaging

We present data for 31 kilometre-scale hydrothermal vent complexes and associated sand remobilization that have formed as a consequence of magmatic intrusions within salt strata (southern North Sea, Dutch Sector). This study uses 3D seismic data tied to three wells and artificial neural networks to describe conical and cylindrical vent structures located in the NE part of the Broad Fourteens Basin. The structures are composites of 58 subvertical pipes that form complexes, and are 951 ± 364 m in height and 487 ± 426 m in diameter. Interactions of magma and fluids at depth triggered successive fluid pressure build-ups in Triassic sandstones, resulting in upward-propagated ruptures, followed by the collapse of overlying deposits and the injection and extrusion of fluidized sand. The relationship between vent tops and their overlying deposits indicates a Mid-Hauterivian age, unusual for volcanic activity in the North Sea area. This event could have a significant impact on hydrocarbon exploration, as the hydrothermal vent complexes bypass a regional source-rock and seal, offering new migration routes and connectivity between potential reservoirs. The intrusions also raise questions about the influence of transform tectonics on the focusing of volcanic activity during the late rifting phase of the North Sea. Supplementary material: Tables of seismic attributes and the geometry of the vent complexes as well as a 3D visualisation of the vents are available at https://doi.org/10.6084/m9.figshare.c.3571593

Microstructures of Early Jurassic (Toarcian) shales of Northern Europe

The Toarcian (Early Jurassic) Posidonia Shale Formation is a possible unconventional gas source in Northern Europe and occurs within the Cleveland Basin (United Kingdom), the Anglo-Paris Basin (France), the Lower Saxony Basin and the Southwest Germany Basin (Germany), and the Roer Valley Graben, the West Netherlands Basin, Broad Fourteens Basin, the Central Netherlands Basin and the Dutch Central Graben in The Netherlands. Outcrops can be found in the United Kingdom and Germany. Since the Posidonia Shale Formation does not outcrop in the Netherlands, sample material suitable for experimental studies is not easily available. Here we have investigated lateral equivalent shale samples from six different locations across Northern Europe (Germany, The Netherlands, The North sea and United Kingdom) to compare the microstructure and composition of Toarcian shales. The objective is to determine how homogeneous or heterogeneous the shale deposits are across the basins, using a combination of Ion Beam polishing, Scanning Electron Microscopy and X-ray diffraction. The work presented here shows that the Toarcian shales of Northern Europe display considerable homogeneity in mineralogy and microstructure in the different investigated samples and formations, where the largest variability is the carbonate content ranging from almost zero up to 80%. We conclude that the outcrop locations in Germany and the United Kingdom are suitable analogues with respect to their mineralogy and microstructure for experimental studies on the Posidonia Shale in the Dutch subsurface.

Reservoir leakage along concentric faults in the Southern North Sea: Implications for the deployment of CCS and EOR techniques

High-quality 3D seismic and borehole data in the Broad Fourteens Basin, Southern North Sea, is used to investigate newly recognised concentric faults formed in salt-withdrawal basins flanking reactivated salt structures. Throw-depth and throw-distance plots were used to understand the growth histories of individual faults. As a result, three families of concentric faults are identified: a) intra-seal faults within a salt-withdrawal basin, b) faults connecting the seal and the reservoir on the crest of an inverted anticline, c) raft-bounding faults propagating into reservoir units. They have moved obliquely and show normal throws, even though they formed during a period of regional compression. Faults in the salt-withdrawal basin and on the inverted anticline are highly segmented, increasing the chances of compartmentalisation or localised fluid flow through fault linkages. Slip tendency analysis was carried out on the distinct fault families to compare the likelihood of slip along a fault at different pore fluid pressures and within different lithologies. Our results show that sections of the faults are optimally oriented with regards to maximum horizontal stresses (σHmax), increasing the slip tendency. The identified faults cut through a variety of lithologies, allowing different values of pore fluid pressures to build up before faults reactivate. Within the Vlieland Sandstones, pore fluid pressures of 30MPa are not sufficient to reactivate pre-existing faults, whereas in the deeper Posidonia Shales faults might reactivate at pore fluid pressures of 25MPa. Fluid flow features preferentially occur near fault segments close to failure. Heterogeneity in slip tendency along concentric faults, and high degrees of fault segmentation, present serious hazards when injecting CO2 into the subsurface. This study stresses the importance of high-quality 3D seismic data and the need to evaluate individual fault systems when investigating potential reservoirs for carbon capture and storage and enhanced oil recovery.

Influence of fault geometries and mechanical anisotropies on the growth and inversion of hanging-wall synclinal basins: insights from sandbox models and natural examples

Abstract Salt is mechanically weaker than other sedimentary rocks in rift basins. It commonly acts as a strain localizer, and decouples supra- and sub-salt deformation. In the rift basins discussed in this paper, sub-salt faults commonly form wide and deep ramp synclines controlled by the thickness and strength of the overlying salt section, as well as by the shapes of the extensional faults, and the magnitudes and slip rates along the faults. Upon inversion of these rift basins, the inherited extensional architectures, and particularly the continuity of the salt section, significantly controls the later contractional deformation. This paper utilizes scaled sandbox models to analyse the interplay between sub-salt structures and supra-salt units during both extension and inversion. Series 1 experiments involved baseline models run using isotropic sand packs for simple and ramp-flat listric faults, as well as for simple planar and kinked planar faults. Series 2 experiments involved the same fault geometries but also included a pre-extension polymer layer to simulate salt in the stratigraphy. In these experiments, the polymer layer decoupled the extensional and contractional strains, and inhibited the upwards propagation of sub-polymer faults. In all Series 2 experiments, the extension produced a synclinal hanging-wall basin above the polymer layer as a result of polymer migration during the deformation. During inversion, the supra-polymer synclinal basin was uplifted, folded and detached above the polymer layer. Changes in thickness of the polymer layer during the inversion produced primary welds and these permitted the sub-polymer deformation to propagate upwards into the supra-salt layers. The experimental results are compared with examples from the Parentis Basin (Bay of Biscay), the Broad Fourteens Basin (southern North Sea), the Feda Graben (central North Sea) and the Cameros Basin (Iberian Range, Spain).

Fluid flow during early compartmentalisation of rafts: A North Sea analogue for divergent continental margins

High-quality 3D seismic data tied to eighteen (18) boreholes are used to investigate the styles of faulting and associated fluid flow features in Triassic–early Jurassic rafts of the Broad Fourteens Basin, Southern North Sea. The study area is presented as an analogue for continental margins experiencing early stage gravitational gliding, i.e. prior to complete separation and downslope translation of individual rafts. In such a setting, and for present-day stress conditions, fault slip data indicate that chasms and faults separating rafts in the Broad Fourteens Basin comprise structures subject to dip slip and strike-slip reactivation. Chasms and faults sub-parallel to these latter chasms comprise the most significant bypass areas for fluid sourced from pre-salt strata. Faults sub-parallel to the main chasms show limited propagation into Early Cretaceous and Cenozoic strata draping the rafts, a character further stressed by the depth of occurrence of fluid pipes and dim spots. This is an important observation, and leads us to postulate that faults formed during early stage rafting control fluid flow in regions where gravitational gliding is limited such as West and Equatorial Africa, Southeast Brazil and parts of the Gulf of Mexico.

Kaolinite transformation into dickite during burial diagenesis

The mechanism of kaolinite transformation into dickite has been investigating using 13 samples from the Frøy and Rind oil fields (Broad Fourteens basin, North Sea), 3 kaolinite specimens with different crystal order (KGa-2, Kaolinite API 17, Keokuk kaolinite), and 2 dickite-rich samples (Natural History Museum collection). Detailed analysis of XRD, thermal analysis, and SEM data show that: (1) as dickite content increases, there is also an increase of the crystal order of kaolinite; (2) in dickite-rich specimens kaolinite and dickite have crystals (or XRD-coherent domains) of the same size; (3) there is no specific dehydroxylation temperature for each polytype, rather particle size and crystal order control dehydroxylation temperature independently of polytype; (4) with progressive dickite content, the development of both particle size and the size of the coherent crystal domains within particles is greater in the c direction than in the a - b plane; (5) the growth of defect-free segments in the c direction is not connected with the growth in the a and b directions, as would be expected in crystallization from solution; (6) textural features indicate coalescence of kaolin plates with burial; (7) there is a very weak positive correlation between particle dimensions and relative kaolinite-dickite content. These results are interpreted as resulting from a double reaction taking place in the solid state with burial. Some kaolinite domains grow in size and crystal order while other domains are transformed into dickite. Presumably, also the dickite domains formed early in the transformation grow in crystal order. The transformation into dickite stops at 90-95% dickite because the remaining kaolinite domains are so large and stable that the stability increase produced by the polytype transformation would be negligible.

The tectonic evolution of the southern Dutch North Sea during the Palaeogene: basin inversion in distinct pulses

In this paper, we discuss the response of a sedimentary basin to compressional reactivation of extensional faults in distinct pulses. The inversion of the Broad Fourteens Basin in the southern Dutch North Sea is used as a case study. The first pulse of inversion occurred during the Late Mesozoic as a result of the sub-Hercynean phase. A second pulse occurred during the Mid-Palaeocene Laramide phase. A final inversion pulse occurred around the Eocene–Oligocene boundary coinciding with the Pyrenean tectonic event. We reconstructed the stratigraphic and structural development of the area during the Palaeogene in detail. Basic data are two 2D-seismic surveys, supported by 74 geophysical wireline logs, on which newly constructed depth and thickness maps are based. The reconstruction is aided by a quantitative subsidence analysis. Cessation of compression resulted in relaxation and tensional reactivation of faults caused differential subsidence. Inheritance of geological structures is a key element in the tectonic activity in the Broad Fourteens Basin during the Palaeogene; tectonic movement during the Palaeogene was always accommodated by reactivation of Palaeozoic–Mesozoic faults. The intensity of inversion in the southern North Sea Basin during the Pyrenean phase appears much larger than previously noticed. Moreover, the area affected by the inversion is much wider than during previous phases. Minor differences in the direction of the local stress field between the tectonic phases and by the deposition of a thick sedimentary succession during the periods of tectonic quiescence between the inversion phases can account for this.

Fluid flow in the northern Broad Fourteens Basin during Late Cretaceous inversion

AbstractA basin-scale hydrogeological study of the inverted northern Broad Fourteens Basin, Netherlands offshore, has resulted in a reconstruction of geological evolution, an estimate of Late Cretaceous topography and model scenarios of syn-inversion meteoric water infiltration. This study was performed in the scope of a basin-scale analysis of the hydrogeological setting and hydrodynamic evolution of the Broad Fourteens Basin. This analysis is aimed at obtaining quantitative knowledge of depositional history and hydrogeological parameters, and qualitative knowledge of hydrodynamic evolution of the Broad Fourteens Basin from Carboniferous to present-day. We present an overview of the tectonic and depositional history, the most likely hydrogeological setting and model scenarios of Late Cretaceous meteoric water infiltration in the northern Broad Fourteens area.We constructed a detailed south-west north-east geological cross-section of the present-day northern Broad Fourteens Basin, and reconstructed Late Cretaceous basin geometry and topography. Using this geometry in a numerical model of density-dependent topography-driven fluid flow, we modelled several scenarios of meteoric water infiltration with estimated ranges of basin-scale permeabilities and water table head. Results indicate that a deep freshwater lens was developed during Late Cretaceous inversion, if the basin-scale hydraulic conductivity of the Rijnland and Altena Groups was at least 1⋅10-9 to 1⋅10-10 m/s, which is in general the highest value for claystones.

History of petroleum systems in the southern part of the Broad Fourteens Basin

Abstract2D Basin modelling was used to evaluate the response of source rock maturation, and of petroleum expulsion, migration, accumulation and preservation to the evolution of the southern part of the inverted Broad Fourteens Basin. Modelling results show that the temperature, maturation and petroleum generation history as well as migration characteristics of both the Jurassic oil systems and the Carboniferous gas systems vary over short distances relative to the differences in burial history of the basin. Model results indicate that no major gas accumulations are preserved in the Slochteren Formation along the cross-section at present-day. Gas accumulations are predicted in sandstone-dominated Triassic units in the southern part of the section. Present-day oil accumulations predicted in the Vlieland Sandstone Formation sealed by the Vlieland Claystone Formation (in P9 and Q1 crestal structures) are in accordance with known oil accumulations. Additional oil accumulations are predicted in the sandstone-dominated Middle Werkendam Member, and in sandstones of the Delfland Subgroup.The modelling offers an explanation for the different geochemical compositions of the accumulated oils in the P9 and Q1 areas. Modelling implies, that the oils in the Q1 oil field were sourced by remigrated oils expelled over time, from early mature to mature source rocks of the Posidonia Shale Formation. The biodegraded and water-washed nature of the Q1 oil is explained by the concentrated topography-induced groundwater flow through the Vlieland Sandstone Formation during the Late Cretaceous inversion of the basin. The oils accumulated in the P9 area were sourced from an early mature part of the Posidonia Shale Formation and were probably not affected by water washing and biodegradation because of post-inversion charging of the reservoir.

Development of a freshwater lens in the inverted Broad Fourteens Basin, Netherlands offshore

The Mesozoic Broad Fourteens Basin is a northwest-southeast trending structural element, situated in the southern North-Sea,Netherlands offshore. Biodegraded and water-washed oils in the southern Broad Fourteens Basin indicate topography-driven meteoric water flow during Late Cretaceous inversion. Density-driven groundwater flow models support the development of a freshwater lens in the northern Broad Fourteens Basin during Late Cretaceous inversion. Three model scenarios with basin-scale permeabilities and water table heads within the range of most likely values show the possible development of a freshwater lens in the northern Broad Fourteens Basin. The freshwater-saltwater interface is located at a depth of 200-1200 m below mean sea level. Near steady-state flow conditions are reached within 1.5–4 Myr.

Geodynamic and hydrodynamic evolution of the Broad Fourteens Basin (The Netherlands) in relation to its petroleum systems

Data analysis in combination with 2D basin modelling is used to study the hydrogeologic and hydrodynamic response of the basin fill of the Broad Fourteens Basin to its geodynamic evolution and the significance of this response for the evolution of the petroleum systems in the basin. Data analysis resulted in the identification of the major mechanical processes involved in overpressure and fluid flow evolution in the basin (sedimentary loading and unloading, tectonic compression and extension, topography of the groundwater table) and provided a timeframework for the important permeability alterations in the basin and the distinct phases of fluid flow. Observed present-day diagenetic features and fracture characteristics in the basin indicate permeability alterations and paleo fluid flow during distinct periods of tectonic activity in the basin, e.g. during Late Jurassic rifting and Late Cretaceous inversion. The basin boundary fault zones were repeatedly reactivated during the periods of tectonic activity. The 2D basin modelling included sedimentary loading and unloading, topography of the water table and petroleum generation as pressure and fluid flow influencing mechanisms. In addition, the influence of time-dependent permeability of fault zones on the overpressure history and the evolution of petroleum systems was modelled and evaluated. The favoured modelling scenario, including a time-dependent permeability of faults, shows that syn-rift and post-rift sedimentation in combination with gas generation induced overpressured conditions in restricted parts of the basin, that is mainly in the deeper low permeable Carboniferous shales and Zechstein evaporites. The calculated pre-inversion overpressured conditions disappear during subsequent inversion of the basin. Predicted present-day pressures in the reservoir horizons are near-hydrostatic and reasonably in accordance with observed pressures. The favoured scenario also explains the known location of oil fields, and the location and saturation characteristics of the gas occurrences. The model results further show that the difference in timing of oil charging of the P9 and Q1 Lower Cretaceous reservoirs explains the observed differences in geochemical composition of the accumulated oils. The biodegraded and water-washed nature of the Late Cretaceous charged Q1 oil reservoir is consistent with the concentrated topography-induced groundwater flow through Lower Cretaceous reservoir units during the Late Cretaceous inversion period. Late charging of the Lower Cretaceous oil reservoir in the P9 area explains the non-biodegraded character of the accumulated oils. In general, this paper shows the importance of using data analysis to obtain an a priori characterization of the basin geofluid evolution and shows that paleohydrodynamic conditions and time-dependent permeability of the hydrogeological framework have influenced significantly the petroleum systems in the basin.

Tertiary sedimentary development of the Broad Fourteens area, the Netherlands

AbstractThe Broad Fourteens Basin is a NW-SE trending Mesozoic structural element that was affected by inversion movements during the Late Cretaceous (Subhercynian Phase) and Early Tertiary (Laramide Phase). Since the overlying Tertiary section of this area is relatively little studied, a multi-disciplinary investigation was initiated using both seismic and well data. The present study is the first phase of that project and deals exclusively with the stratigraphical framework and sedimentary development based on well logs from approximately forty wells.The Tertiary sedimentary section in this area (maximum thickness up to approximately 800 m) consists of siliciclastic rocks that generally correlate well with the established lithostratigraphical framework of the Netherlands. However, the highly variable thickness of these lithological units, illustrated by isopach maps for all units, reflect the intricate pattern of sea level movements, differential vertical movements and the ensuing erosion, that took place in this tectonically active area.Well P06-02, one of the most complete sections in the center of the area, was selected for detailed biostratigraphical analysis to support further stratigraphical subdivision and correlation. This yielded information on both the distribution pattern of various lithological units and the influence of large, partly global, events during basin development. It is concluded that application of detailed sequence stratigraphy in the study area is very problematic and only 2ndand some 3rdorder cycles could be recognized.

Geodynamic and hydrodynamic evolution of the Broad Fourteens Basin and the development of its petroleum systems: an integrated 2D basin modelling approach

Results of integrated 2D basin modelling indicate that both sedimentary loading and gas generation play an important role in the overpressure history of the Broad Fourteens Basin. We analysed the influence of time-dependent permeability of faults on the overpressure history and evolution of petroleum systems. We favour time-dependent permeability of faults as the most likely option to explain the present-day near-hydrostatic conditions in the basin, the known location of oil fields, and the location and characteristics of the gas occurrences. The model results further show that the difference in timing of oil charging of the P9 and Q1 Lower Cretaceous oil reservoirs, explains the observed differences in geochemical composition of the accumulated oils. The biodegraded and water-washed nature of the pre-inversion charged Q1 oil reservoir is consistent with the concentrated topography-induced groundwater flow through the Lower Cretaceous reservoir units during the Late Cretaceous inversion period. Late charging of the oil reservoir in the P9 area explains the non-biodegraded character of the accumulated oils. Present-day conditions in the Broad Fourteens Basin are near-hydrostatic. In general, this paper shows that paleohydrodynamic conditions and time-dependent permeability of the hydrogeological framework have influenced significantly the petroleum systems in the basin.

Lithosphere rheology during intraplate basin extension and inversion: Inferences from automated modeling of four basins in western Europe

Forward reconstructions of the (mainly) Mesozoic and younger rheological evolution have been made for four basins (Broad Fourteens Basin, Sole Pit Basin, Brittany Basin and the Iberian Basin) in a very consistent way by backstripping and automatic forward modeling of subsidence data, including potentially important effects of heat production, sediment infill and sedimentary blanketing. For default compositional and thermal parameters, the modeling results show strengthening in all basins, and in particular during inversion, with strength increases up to about 2 TN m −1 (20%) relative to their initial values. Given predominantly relatively constant intraplate stresses in continental lithosphere, this is in disagreement with repeated localization of basin deformation. In a thorough sensitivity analysis we explore the possibilities that permissible variations in tectonic history, compositional, rheological and thermal parameters can, in a particular combination, result in slight weakening of the basin, in agreement with reactivation during inversion. However, such a combination reflects an extreme scenario, which is not founded by geological evidence and, statistically, is very unlikely to apply for all basins. A far more likely explanation for relative and permanent weakening of the basins is the presence of pre-existing weak zones, deviating from standard rheological assumptions. At (upper) crustal levels, weakening can be attributed to pre-existing marked faults by a reduced friction angle. This weakening has a pronounced influence on lithospheric strength provided that the reduction in friction angle of pre-existing faults can be extrapolated to the upper mantle. Alternatively, weakening of the upper mantle can be attributed to (1) ductile localization mechanisms, as reflected by the occurrence of upper mantle shear zones, or (2) the occurrence of rheologically weak material, as indicated by upper mantle reflectors.

Fracture networks in Rotliegend gas reservoirs of the Dutch offshore: implications for reservoir behaviour

AbstractFracture systems of Rotliegend gas fields in and at the margins of the northern Broad Fourteens Basin in the Dutch offshore are described in terms of orientation, frequency, origin and type, and in relation to larger-scale structures. First, fracture data collected from core and image logs have been corrected to account for the bias related to the 1-D sampling. Second, these results were integrated with data on fracture cements and diagenesis in order to assess the timing of the fracture network development.On the basis of their regional extent three phases of fracturing and four orientation trends can be distinguished in the basin: (1)at Triassic times and related to early diagenesis and burial, NW-SE to NNW-SSE and NE-SW to ESE-WNW particulate-shear fractures developed;(2)during the Mid-Kimmerian and related to the main burial stage, shear-related and dilational-shear-fault-related fracturing occurred parallel with larger-scale faults;(3)during the Cretaceous and related to uplift, NW-SE and NE-SW joints propagated; a regional joint system developed outside the Jurassic rift basin, preferentially oriented E-W to ESE-WNW; these joints have not been dated accurately.The fault-related shear fractures tend to compartmentalise the reservoirs, whereas the regional joints tend to enhance reservoir flow properties. These fracture systems are thought to play a negative or positive role, respectively, but only in fields with poor reservoir quality. Consequently, in such cases small-scale fractures should be taken into account in field development planning.

THE TIMING OF CEMENTATION IN THE ROTLIEGEND SANDSTONES OF THE SOUTHERN NORTH SEA: A PETROLOGICAL AND FLUID INCLUSION STUDY OF CEMENTS

We report here on fracture and pore‐filling cements in the Rotliegend Sandstones of the Sole Pit area, UK Southern North Sea. Standard thin‐section petrography and scanning electron microscopy have been complemented by fluid inclusion microthermometry. The main objectives of this paper are to establish the stratigraphy of the sandstones' cements and to describe the cements' characteristics. In addition, the combined use of petrography and fluid inclusion microthermometry is used to define the timing of cementation and gas migration in the Southern North Sea.The geological history of the Sole Pit area is similar to that of other parts of the Southern North Sea. A period of aeolian deposition resulting in the Upper Permian Rotliegend Sandstones was ended by the Zechstein marine transgression. Rapid subsidence followed until Cretaceous times, when a phase of uplift affected the Sole Pit area as well as, for example, the Broad Fourteens Basin. This uplift continued through the Cretaceous and, following a minor phase of subsidence, uplift again took place in the Late Cretaceous and into the Tertiary.When plotted on burial‐thermal history diagrams, the fluid‐inclusion and petrographic results show that quartz cement precipitated over a range of burial depths. Quartz mineralisation was, however, more common at deep burial levels and also during the uplift period in the Sole Pit area, which caused extensive fracturing of the Rotliegend Sandstones. Dolomite precipitation occurred during deep burial diagenesis and ceased largely with the onset of fracturing. Sulphates (anhydrite and barite) were mainly precipitated during the inversion phase in the Sole Pit area. As both the anhydrite fluid inclusions, and possibly also the quartz fluid inclusions within some of these cements contain hydrocarbon gases, the migration of gas took place during the inversion period, using the fractures as migration pathways.

Illitization of Diagenetic Kaolinite-to-dickite Conversion Series: Late-stage Diagenesis of the Lower Permian Rotliegend Sandstone Reservoir, Offshore of the Netherlands

ABSTRACT This paper describes the diagenetic evolution of clay minerals in the Rotliegend sandstone reservoir under contrasting burial histories in the Broad Fourteens basin (Dutch sector, Southern North Sea). The diagenetic modifications affecting the crystal structure of clay minerals (both kaolin-group and illitic minerals) were studied using X-ray diffraction (XRD). The XRD study includes oriented and random mounts of various size fractions, numerical processing (decomposition) of XRD profiles, and simulation of one-dimensional and three-dimensional XRD patterns. Petrographic observations, differential thermal analysis, K/Ar geochronology, and geochemical considerations complement the XRD study and allow determination of the sequence of mineral crystallization and the morphological evoluti n of clay minerals and place further constraints on the absolute timing of diagenetic events and on the nature of the fluids responsible for clay-mineral crystallization. From deposition time ( 275 Ma) to the Kimmerian orogeny ( 155 Ma), crystallization of kaolinite at the expense of K-feldspars was favored by acid fluids from the underlying Carboniferous Coal Measures source rocks; kaolinite crystallization is followed by a steady kaolinite-to-dickite transformation affecting both the structure and the morphology of kaolin-group minerals. The structural characteristics of kaolin-group minerals are related to the burial history of the sediments prior to the Kimmerian orogeny. During the Kimmerian orogeny, rapid illitization of kaolin-group minerals was favored both by increased heat flow in the sedimentary pile and by widespread presence of faults, which permitted significant fluid flow probably from the Zechstein Formation. The morphological and structural characteristics of illitic minerals, i.e., illite content of illite/smectite mixed layer (I/S), ratio of illite to I/S, and three-dimensional structure of illitic minerals, do not represent the progress of a smectite-to-illite transformation, but these characteristics clearly reflect the temperature during illitization of kaolin.

Inverstion of the Broad Fourteens Basin (offshore Netherlands), a small-scale model investigation : T. Nalpas, S. Le Douaran, J.-P. Brun, P. Unternehr & J.-P. Richert, Sedimentary Geology, 95(3–4), 1995, pp 237–250

Inverstion of the Broad Fourteens Basin (offshore Netherlands), a small-scale model investigation : T. Nalpas, S. Le Douaran, J.-P. Brun, P. Unternehr & J.-P. Richert, Sedimentary Geology, 95(3–4), 1995, pp 237–250

Molecular nitrogen from coal pyrolysis: Kinetic modelling

It has been shown that molecular nitrogen (N 2) in natural gas accumulations can result from thermal maturation of coal and other forms of sedimentary organic matter. The present paper investigates the loss of coal nitrogen in the late stage of coalification and the genesis of N 2 from coal nitrogen in studying N 2 evolution during programmed pyrolysis of a coal series ranking until meta-anthracite. The work confirms the decrease of nitrogen content already observed in the late stage of coalification. This late decrease is correlated with a relative increase of the N 2 content in the total gas emitted by pyrolysis. N 2 emission occurs above 600°C in two broad peaks. Assuming that the rate of denitrogenation is described by single first-order reactions with a single pre-exponential factor, a discrete distribution of activation energies is computed by data processing of the first nitrogen peak of a representative semi-anthracite coal (R o 2.3%). Activation energies are distributed between 65 and 80 kcal/mol with a mode at 72 kcal/mol and a pre-exponential factor of 7.71 × 10 12 s −1. Similar values have been obtained by other authors for the thermal decomposition of pyridine. These kinetic parameters are used to predict the N 2 genesis in the Centre Broad Fourteens Basin (southern North Sea). The model indicates that Carboniferous anthracites in this basin could have released up to 0.575 m 3 of N 2 per ton of carbon.