Jurassic Mudrocks Research Articles

Classic Scottish sandstone concretions formed from hot water

Large calcite‐cemented sandstone concretions found in Middle Jurassic sandstones from the isles of Eigg and Skye are a striking feature of the Jurassic geology of the Inner Hebrides. Recent clumped isotope data unequivocally give concretionary calcite cementation temperatures above 50°C and up to 98°C. The high temperatures contrast with the mild thermal history of the enclosing less permeable Jurassic mudrocks. The clumped isotope data also enable calculation of the oxygen isotopic composition of the precipitating fluids: these included percolating rainwater that had interacted with Paleocene volcanics; then, later hot fluids of deeper, basinal origin, squeezed out of compacting mudrocks at depth as the Paleocene lava pile loaded the crust. There is thus direct connection between ‘Jurassic’ sedimentary features and the complex Paleocene volcanic history of the region.

Radish concretions grown in mud during compaction

AbstractRadish concretions exhibit a typical columnar to pear‐shaped, stipe downward geometry. In Middle Jurassic mudrock in south‐west Germany, radish concretions started to form around an iron‐sulphide lined tube by pervasive cementation constituting an ellipsoidal parent domain in uncompacted sediment at burial depths of ≤5 to 8 m as recorded by 75 to 80% minus‐cement porosity. Thereafter, the concretions grew vertically in compacting sediment as evidenced by laminae within the concretions being increasingly inclined towards the tips, and concomitantly decreasing minus‐cement porosity. During early diagenesis, prior to septarian crack formation, bicarbonate generating the microbial cement originated within the sulphate reduction zone chiefly from anaerobic oxidation of methane and to a lesser degree of organoclastic material. Later, at 50 to 70 m burial depth, septarian cracks formed as evaluated by sedimentation–compaction analysis based on minus‐cement porosity data and compressibility of similarly composed sediments. The outward‐narrowing septarian cracks indicate that they formed when the concretions were still in a plastic state but already cemented sufficiently to be resistant against compaction. In this stage, up to one‐quarter of the pore volume of the concretions was still open as suggested by shrinkage experiments. This pore volume, and the septarian cracks, were filled with cement termed late diagenetic. In the study area, the decompacted net‐sedimentation rate was low, about 2 to 3 cm kyr−1, for ca 2.5 Myr, allowing the concretions to reside for a long time within the sulphate reduction zone and to grow. Radish concretions formed within the transition zone from thick, rapidly deposited to long‐term, slowly accumulating sediment.

Geotechnical characterization of sulfur species in UK Jurassic mudrocks

Weathering of pyritic mudrocks results in the oxidation of sulfides and generation of sulfates in concentrations potentially hazardous to civil engineering works. Geochemical and petrographic analyses undertaken on samples from sites underlain by the Kimmeridge Clay Formation and Oxford Clay Formation proved three distinct zones: an upper sulfur-leached zone, an intermediate weathered zone with sulfate-rich horizons and a lower relatively unweathered zone of low-sulfate, high-sulfide material. It was found that water-soluble sulfate and total potential sulfate may vary by up to 1500 mg SO 4 2− l −1 and 8% SO 4 2− respectively, over a depth interval of only 0.2 m. A targeted sampling strategy and ground aggressivity assessment approach can be applied to these formations based upon the sulfur species zones identified.

On modelling of consolidation processes in geological materials

Low-permeablity materials may be seen as natural geological barriers for radioactive waste repositories. However, to ensure their safe performance, a good understanding of their mechanical properties is required. Although the standard Biot's poroelastic model is widely used to estimate the key properties of these materials, experimental observations differ from this mathematical formulation and suggest that a more complex rock deformation behaviour to include a creep effect is needed. In this study, the Biot's differential equations are modified to include a rheological skeleton. In comparison with other existing models, here we propose a formulation with a minimal parametric uncertainty: we show that with just one additional physically-based parameter, the experimental creep behaviour is properly described. This enhanced model is implemented within a finite element framework and employed in a fitting algorithm to extract the hydro-mechanical properties from experimental data. To illustrate its generality, we analyse laboratory tests performed on three different types of materials: (a) an unlithified lower Oligocene clay from Belgium (Boom Clay), (b) an indurated Jurassic mudrock (Callovo-Oxfordian mudstone) and (c) a Triassic siltstone (Mercia Mudstone Formation). Numerical fits to the data support the validity of this approach and demonstrate its applicability to a range of low-permeability materials regardless of mineralogy or burial history.

Sulphide Sinking in Magma Conduits: Evidence from Mafic–Ultramafic Plugs on Rum and the Wider North Atlantic Igneous Province

AbstractNi–Cu–PGE (platinum group element) sulphide mineralization is commonly found in magmatic conduit systems. In many cases the trigger for formation of an immiscible sulphide liquid involves assimilation of S-bearing crustal rocks. Conceptually, the fluid dynamics of sulphide liquid droplets within such conduits is essentially a balance between gravitational sinking and upwards entrainment. Thus, crustal contamination signatures may be present in sulphides preserved both up- and down-flow from the point of interaction with the contaminant. We examine a suite of ultramafic volcanic plugs on the Isle of Rum, Scotland, to decipher controls on sulphide accumulation in near-surface magma conduits intruded into a variable sedimentary stratigraphy. The whole-rock compositions of the plugs broadly overlap with the compositions of ultramafic units within the Rum Layered Complex, although subtle differences between each plug highlight their individuality. Interstitial base metal sulphide minerals occur in all ultramafic plugs on Rum. Sulphide minerals have magmatic δ34S (ranging from –1·3 to +2·1‰) and S/Se ratios (mean = 2299), and demonstrate that the conduit magmas were already S-saturated. However, two plugs in NW Rum contain substantially coarser (sometimes net-textured) sulphides with unusually light δ34S (–14·7 to +0·3‰) and elevated S/Se ratios (mean = 4457), not represented by the immediate host-rocks. Based on the Hebrides Basin sedimentary stratigraphy, it is likely that the volcanic con duits would have intruded through a package of Jurassic mudrocks with characteristically light δ34S (–33·8 to –14·7‰). We propose that a secondary crustal S contamination event took place at a level above that currently exposed, and that these sulphides sank back to their present position. Modelling suggests that upon the cessation of active magma transport, sulphide liquids could have sunk back through the conduit over a distance of several hundreds of metres, over a period of a few days. This sulphide ‘withdrawal’ process may be observed in other vertical or steeply inclined magma conduits globally; for example, in the macrodykes of East Greenland. Sulphide liquid sinking within a non-active conduit or during magma ‘suck-back’ may help to explain crustal S-isotopic compositions in magma conduits that appear to lack appropriate lithologies to support this contamination, either locally or deeper in the system.

Contrasting mechanisms for crustal sulphur contamination of mafic magma: evidence from dyke and sill complexes from the British Palaeogene Igneous Province

The addition of crustal sulphur to magma can trigger sulphide saturation, a process fundamental to the development of some Ni–Cu–PGE deposits. In the British Palaeogene Igneous Province, mafic and ultramafic magmas intrude a thick sedimentary sequence offering opportunities to elucidate mechanisms of magma–crust interaction in a setting with heterogeneous S isotope signatures. We present S-isotopic data from sills and dykes on the Isle of Skye. Sharp contrasts exist between variably light δ 34 S in Jurassic sedimentary sulphide (−35‰ to −10‰) and a local pristine magmatic δ 34 S signature of −2.3 ± 1.5‰. Flat-lying sills have restricted δ 34 S (−5‰ to 0‰) whereas steeply dipping dykes are more variable (−0‰ to −2‰). We suggest that the mechanism by which magma is intruded exerts a fundamental control on the degree of crustal contamination by volatile elements. Turbulent flow within narrow, steep magma conduits, discordant to sediments, and developed by brittle extension or dilation have maximum contamination potential. In contrast, sill-like conduits emplaced concordantly to sediments show little contamination by crustal S. The province is prospective for Ni–Cu–PGE mineralization analogous to the sill-hosted Noril’sk deposit, and Cu/Pd ratios of sills and dykes on Skye indicate that magmas had already reached S-saturation before reaching the present exposure level. Supplementary material: Details of the whole-rock chemical sulphur extraction method; simplified geological maps of the Isle of Skye, a geological cross-section of part of southern Skye, and a stratigraphic log of the Mesoproterozoic and Mesozoic sediments of western Scotland; trace element diagrams of sills and dykes used in this study; and tables for QA/QC of S-isotope results, sample location information, and whole-rock major and trace element results for sill and dyke samples and for Jurassic mudrocks are available at http://www.geolsoc.org.uk/SUP18834 .

Assessment of pyritic Lower Lias mudrocks for earthworks

The weathering of pyritic mudrocks can have serious consequences for construction, and in particular for earthworks. Prediction of the likely behaviour of pyritic mudrocks involved in earthworks tends to be overlooked, with only the determination of aggressivity to steel and cementitious materials carried out. British Jurassic mudrocks tend to be soil-like overconsolidated deposits, typically with significant calcite and pyrite content. They may have appropriate engineering properties for earthworks under certain conditions, but they are susceptible to rapid atmospheric deterioration in the presence of both oxygen and water. The rate and products of pyrite oxidisation are dependent upon the mineralogy and fabric of the host material, its hydrological setting, and its immediate environment following exposure. This ultimately controls the rate and impact on construction. This paper outlines the construction through an area of Lower Lias mudrocks of Jurassic age for the A46 Newark to Widmerpool Improvement Scheme. The approach adopted allowed an understanding of the likely reactions and implications for construction at a site-specific level. This provided confidence for the client, designer and contractor that the adopted methodology was appropriate for long-term performance.

Zircon Compositions of Lower Mesozoic Redbeds of the Tethyan Margins, West-Central Mediterranean Area

Typologic study and chemistry of zircon populations from Middle Triassic-Lower Liassic arenitic continental redbeds of the Internal Domains successions of the Mediterranean chains, from the Gibraltar Arc to the Calabria-Peloritani Arc, allow assessment of whether this type of study may be used to obtain more comprehensive inferences on the sedimentary evolution of ancient clastic rocks. Calabria-Peloritani Arc zircons are primarily euhedral, with subordinate rounded and subrounded grains. Rifian Maghrebides samples contain abundant euhedral zircons, with minor rounded and ovoid zircons. Betic Cordillera zircons are rounded or subrounded, with minor euhedral elongate grains; rounded zircons of these samples cannot be typologically classified. Electron-microprobe analyses (Si, Zr, Hf, P, and HREE) indicate homogeneous compositions, suggestive of common source areas characterized by granitoid rocks, whereas typology suggests the existence of two distinct groups. The first group includes arenites from the Calabria-Peloritani Arc and the Rifian Maghrebides. The second group includes arenites from the Betic Cordillera. Differences between the two zircon groups may be related to recycling effects, in agreement with a geochemical study of Triassic to lowermost Jurassic mudrock redbeds. Recycling likely was more significant for the Betic Cordillera samples. These results suggest that zircon typology and mudrock geochemistry can be useful tools for provenance, especially in complex sedimentary contexts where arenite-mudrock couplets occur.

Rapid heating of carbonaceous matter by igneous intrusions in carbon-rich shale, Isle of Skye, Scotland: an analogue for heating of carbon in impact craters?

The response of organic matter to high-temperature events is important to astrobiology, as it governs the survival of carbon during several processes that may be critical to the origin and spread of life. Impact cratering is a widespread high-temperature process. The behaviour of carbon during impact events is not well understood. But there is the potential to examine other examples of the response of organic matter to high-temperature processes in the terrestrial geological record. In this study, we report on the interaction of Tertiary igneous intrusions (dolerite sills) and carbon-rich Jurassic mudrocks on the Isle of Skye, Scotland. Despite the high temperatures of the igneous intrusion, carbon has been preserved at the dolerite–shale contact and in shale enclaves where partial melting of the shale has occurred. Even though the temperatures achieved by igneous intrusion are much lower than during impact events, it is a valuable analogue, because it represents a rapid introduction of high temperatures to a carbon-rich rock.

Sedimentary recycling, provenance and paleoweathering from chemistry and mineralogy of Mesozoic continental redbed mudrocks, Peloritani mountains, southern Italy

The Triassic to Lowermost Jurassic mudrocks from continental redbeds of the Longi-Taormina Unit (Calabria-Peloritani Arc; southern Italy) have been mineralogically, chemically and petrographically analyzed, in order to reveal their complex history, which record an important phase of the geological evolution of the Mediterranean area.The Late Triassic corresponds with a low first-order sea level stand and a time of high continental emergence whereas the Early Jurassic was the time of the initiation of the first-order sea level rise in the mid-Mesozoic, generally marked by a transgressive trend.The mineralogical assemblage, dominated by the occurrence of illite and illite/smectite mixed layers, coupled to the CIA index and to the A-CN-K plot, indicates post-depositional K-enrichments.Palaeoweathering indices (CIW and PIA ratios) suggest that the source experienced intense weathering and that they likely record a recycling effect from their metasedimentary basement rocks. A recycling effect is also suggested by the distribution of Al2O3, TiO2, and Zr. In the Zr/Sc vs. the Th/Sc plot the redbeds are not clustered along the primary compositional trend but fall along a trend involving zircon addition and thus sediment recycling. Recycling could significantly affect the weathering indices which likely monitor a cumulative effect including a first cycle of weathering at the source rocks. Weathering occurred under hot, episodically humid climate with a prolonged dry season. Wet-humid conditions favored the formation of stream channels that eroded the soil profiles, whereas the dry season promoted the sedimentation. The climate alternation in the Early Jurassic favored recycling.An additional provenance terrane occurring in the basement, including metavolcanic rocks having mafic composition cannot be excluded, since the Eu anomaly is slightly higher than the PAAS value. Although the effects of recycling on REE distribution are uncertain, the Eu/Eu* should increase, as more feldspar is destroyed during weathering and diagenesis. This involves that the Eu/Eu* ratio could monitor a supply of low Eu/Eu* mafic detritus which compensate for the recycling effect by reducing Eu/Eu*. Eu released during the dissolution of feldspar could be retained by clay minerals contributing to minimize the recycling induced increase of the Eu-anomaly. This may involve that the recycling effect on the Eu-anomaly was minor and that the low Eu/Eu* mafic detritus supply was also minor. The subordinate importance of a mafic supply is confirmed also by other provenance proxies including the La-Th-Sc plot and the Cr/V and Y/Ni ratios.The proportions of illitic layers in I/S mixed layers coupled with the illite crystallinity values, expressed as Kübler Index (KI), suggesting an estimated temperature experienced by the Longi Units in the range of 100-150°C. Starting from this range the diagenetic/tectonic evolution should correspond to a lithostatic/tectonic loading of about 4-5 km.

Localized Quaternary uplift of south–central England

The Early Quaternary of south–central England is characterized by uplift, as is indicated by the river gravels that extend intermittently from Oxfordshire, through Northamptonshire, to Lincolnshire. The earliest of these gravels contain abundant quartz and quartzite clasts derived from Lower Triassic conglomerates in the West Midlands, but later deposits contain progressively larger amounts of locally derived Jurassic sediments. This compositional change is associated with the incision of the rivers that flowed eastwards from the West Midlands. Subsequently, the headwaters of some of these rivers were diverted towards the Bristol Channel and their suspended material was deposited on the outer parts of the UK continental shelf and slope. The area of incised drainage (Gloucestershire to Lincolnshire) did not extend to East Anglia (in the east) or to Somerset (in the west). Both these coastal regions contain several marine horizons showing that the area of uplift did not include the whole of southern England. The uplift cannot therefore be attributed to eustatic changes in sea level. The area of uplift is centred on the broad topographic depression of the English Midlands and corresponds approximately to the outcrop of thick ( c . 1 km) Upper Triassic and Lower Jurassic mudrocks. We have used an Ordnance Survey 50 m × 50 m topographic ‘grid’ to estimate the amount of mudrocks that has been removed from the depression and 2D flexural unloading models to calculate the tectonic uplift that resulted, for various assumptions of the effective elastic thickness (which is determined by the flexural rigidity) of the lithosphere. The amount of material removed is in accord with what is known of the volume of Quaternary deposits offshore, and the uplift accounts for the incised plateau surfaces of the flanking Cotswold Hills and the high ground of Northamptonshire to the east, and the Forest of Dean and the hills of Herefordshire to the west.

The White Stone Band of the Kimmeridge Clay Formation, an integrated high-resolution approach to understanding environmental change

The Kimmeridge Clay is a Jurassic mudrock succession that shows Milankovitch Band climatic cyclicity. A key issue is to determine how the subtle changes that define this cyclicity result from climatic change. Using material from the Natural Environment Research Council Rapid Global Geological Events (RGGE) Kimmeridge Drilling Project boreholes, the White Stone Band was investigated at the lamination scale using backscattered electron imagery and quantitative palynofacies. Fabric analysis shows the lamination to represent successive deposition of coccolith-rich and organic-matter-rich layers. Individual laminae contain unsorted palynological debris with a consistent ratio of marine and terrestrial components. Such mixed organic matter input is interpreted as the result of storm transport. Linking water column processes to laminae deposition suggests seasonal input with a coccolith bloom followed by a more diverse assemblage including dinoflagellates and photosynthetic chlorobiacean bacteria. As the photic zone extended into the euxinic water column organic matter export to the sea bed underwent minimal cycling through oxidation and subsequently became preserved through sulphurization with greatly increased sequestration of carbon. This was significantly increased by late season storm-driven mixing of euxinic water into the photic zone. Increased frequency of storm systems would therefore dilute the coccolith input to give an oil shale. Hence climatically induced changes in storm frequency would progressively vary the organic content of the sediment and generate the climate cycle signal.

Environmental controls on clay mineralogy of an Early Jurassic mudrock (Blue Lias Formation, southern England)

We present a dataset including clay mineralogy, gamma-ray spectrometry, organic matter content and magnetic susceptibility of the Hettangian to lowest Sinemurian successions of Dorset and Somerset, southern UK (Blue Lias Formation, Bristol Channel and Wessex basins). In both areas, the clay assemblages comprise predominantly detrital illite, kaolinite and illite/smectite mixed layers. Clays probably originated from the erosion of the Hercynian massifs, the relative proportions of kaolinite and illite being modulated by arid–humid climatic fluctuations. The organic matter (OM) content (types II to IV) ranges up to 12% in both areas. A clear stratigraphical trend in clay mineral assemblages is apparent in Somerset, whereas in Dorset sharp contrasts between adjacent horizons and a greater dilution by carbonate mask the long-term evolution. Correlations between both areas based on similar vertical trends in clay mineral abundance support the suggestion of a hiatus within the angulata Zone of the Dorset succession. As expected, the kaolinite/illite ratio correlates with the Th/K ratio deduced from gamma-ray spectrometry. However, significant departures from the correlation occur in OM-rich intervals, suggesting that Th may be partly adsorbed on to OM particles. Surprisingly, high magnetic susceptibility correlates with abundant kaolinite, not with Fe-rich clays, indicating either that kaolinite is accompanied by a soil-inherited magnetisable phase (possibly iron oxide) or that illite-rich rocks are more strongly diluted by carbonate than are kaolinite-rich strata.

Quartz silt in mudrocks as a key to sequence stratigraphy (Kimmeridge Clay Formation, Late Jurassic, Wessex Basin, UK)

Sequence‐stratigraphic interpretation of mudrocks is often carried out using proxy indicators for grain size or by consideration of other aspects of lithology thought to relate to sea‐level change, such as organic‐matter content. An alternative stratigraphic analysis, based on direct estimation of quartz‐silt content, was carried out on a major Late Jurassic mudrock (and oil source rock), the Kimmeridge Clay Formation of the Wessex Basin, Dorset, UK. The new data, generated by backscatter SEM, X‐ray and image analysis, show decametre‐scale stratigraphic patterns that are incompatible with many previous sequence‐stratigraphic interpretations based on gamma‐ray logs or visual lithofacies and biofacies description. Correlation with a basin‐margin section in the Boulonnais, northern France, indicates that silt‐rich intervals in basinal facies are coeval, within the limits of biostratigraphic resolution, with shallow‐water sand‐rich packages on the margin. Variation in silt content in the Kimmeridge Clay therefore appears to be a record of relative sea‐level change of at least regional extent. It is suggested that analysis of silt content offers the most reliable basis for generation of a regional sequence stratigraphy in basinal mudrocks. A revised relative sea‐level curve for the Wessex Basin Kimmeridgian and early Tithonian is presented based on this premise.

Enhanced lithological description of a Jurassic mudrock sequence using geophysical wireline logs

Methods to combine individual wireline logs such as resistivity, sonic, bulk density, neutron porosity and natural gamma have been developed to enhance lithological interpretation, particularly of mudrock sequences. Multiple log data presentation to generate a synthetic, pseudo-lithology log is achieved by combining downhole geophysical log records using a colour cube. Well-documented lithological descriptions of borehole cores from the Kimmeridge Clay Formation (Jurassic) of Britain are compared with the newly processed combination geophysical log data. The colour cube logs confirm existing lithological descriptions and, importantly, enable considerable refinement of general lithological descriptions of ‘undifferentiated’ mudstone.

Glacial erosion of the Wash and Fen basin and the deposition of the chalky till of eastern England

It is widely accepted that the low-lying area of eastern England comprising the Wash and the Fens was excavated by the Anglian ice sheet. Moving first from the northwest and then from the north, the ice moved rapidly across the deformable bed on the Jurassic mudrocks and rode up over the Chalk escarpment, eroding the scarp face and setting back the lowered escarpment by several kilometres. The eroded material was deposited beyond the basin as the Chalky till of the southeast Midlands and East Anglia. In this paper reconstruction of the form of the preglacial landscape of the Jurassic and Cretaceous outcrops allows estimation of the excavated volume of both mudrock (312 km 3) and of chalk (243 km 3), a total of about 555 km 3. This volume, and the lithological mix, matches closely the reconstructed volume (estimate 363 km 3) of the now-dissected chalky till around the margins of the glacial lobe. The amount of erosion achieved by the Anglian ice sheet amounts to 3–4000 Bubnoffs (B) — i.e. mm ka −1, even if we allow 20 ka for a complex glaciation; this far exceeds rates of erosion recorded anywhere with the British Isles today: Even if averaged over the 440,000 yr since that event the rate is 167 B, far greater than the erosion achieved on similar rocks within the adjacent unglaciated Thames basin, estimated at 45 B.

Astronomical calibration of the Jurassic time-scale from cyclostratigraphy in British mudrock formations

Three British Jurassic mudrock formations have been investigated, via timeseries analysis, for evidence of sedimentary cyclicity related to orbitalclimatic (Milankovitch) cyclicity: the Blue Lias, ...

An Experimental Study of the Swelling Behaviour of Mudrocks In the Presence of Water-based Mud Systems

Abstract Swelling behaviour of three different mudrocks (London Clay, Oxford Clay and Fuller's Earth) of varying composition, geological ages, degree of lithification, organic matter and. expandable clay contents have been investigated in the presence of water-based mud systems (KCIICMC, Polyamine, Polyamine, + glycol and PHPA + glycol) plus water to identify the factors controlling the swelling behaviour of shales. The results show, that the degree of swelling is not directly correlatable with the proportion of swelling clay minerals in the mudrocks. Based on the experimental results, the swelling behaviour of the mudrocks depends on the composition, geological history and degree of, lithification of the mudrocks. Well lithified smectite-rich mudrock was found to show less swelling than less well lithified mudrocks with lesser amounts of swelling clays. Organic matter rich mudrock showed a greater degree of swelling than those with little organic matter, suggesting that organic matter may also influence the swelling behaviour. This investigation pro' vi des a better understanding of the swelling behaviour of mudrocks by introducing the concept of the combined action of physico-chemical and mechanical swelling processes. Introduction Swelling behaviour of different rock-fluid combinations is very important in predicting different drilling problems and determining the proper mud system to stabilize the troublesome shaley borehole environment. This characteristic of rocks is a complex function of the rock type and the mud systems and is one of the main causes of sloughing, heaving, caving and progressive hole enlargement problems in the shaley section of a borehole environment, 3). Swelling causes easy detachment of clays or shale particles both from the borehole wall and the cuttings, incorporating into the mud system as additional solid materials, and thus produces considerable changes in the rheological properties of the mud, increasing risk of differential sticking, induced fracturing and mud treatment cost. It is widely believed that smectite clay content of the mudrocks is the main culprit for excessive swelling of shale materials. In the context of drilling, this criterion of shale swelling is often unable to explain the unusual swelling behaviour of some mudrocks having little or no swelling clays. On the other hand it was found that some smectite' rich shale formations(2) have very little swelling potential compared to shales having low expandable clay content. his illustrates the role of other factors in controlling the swelling behaviour of mudrocks. Bearing this in mind an investigation was carried out to determine the factors governing the swelling behaviour of mudrocks. TABLE 1: Composition of the clay fraction <2 micron) and TOC of the mudrocks. Three different mudrocks and four different drilling mud systems plus fresh water were used for the study. The mudrocks are London Clay representing the Upper Tertiary mudrock, Oxford Clay representing the Upper Jurassic mudrock and Fuller's Earth representing the Lower Tertiary mudrock encountered in the North Sea. Some commonly used water-based muds such as KCI/CMC, Polyamine, Polyamine + glycol, water and a newly designed water-based mud system were used to assess the swelling behaviour and factors governing the behaviour of the mudrocks.

Nature and causes of overpressuring in mudrocks of the Gullfaks Area, North Sea

AbstractPore-fluid pressure gradients in Lower Tertiary to Upper Jurassic mudrocks in the Gullfaks area show significant variations, both between and within individual structural compartments. Detection and quantification of abnormal pore-pressures, based on drilling parameters and wire line logs, are significantly influenced by petrological variations in the mudrocks and do not provide adequate direct pressure indicators. Empirical models show that the degree of overpressuring in mudrocks above the Gullfaks reservoirs cannot be fully explained by processes such as shale dehydration, compaction disequilibrium, aquathermal effects or in situ hydrocarbon generation, and other processes, such as caprock failure and subsequent migration of hydrocarbons (mainly gas) into the overlying mudrocks, appear to be influential in generating the observed pattern of overpressure.

JURASSIC MUDROCK FORMATIONS OF SOUTHERN ENGLAND: LITHOLOGY, SEDIMENTATION RATES AND ORGANIC CARBON CONTENT

The Jurassic mudrock formations of Southern England, especially the Lower Lias, Oxford Clay and Kimmeridge Clay, show great variations in lithology, organic content and rates of sedimentation. It is here suggested that the varying sedimentation rates of these formations have been a major contributory factor, resulting in the differences in the organic richness observed between these fine‐grained Jurassic rock units. Variations in lithology have only a minor effect on the organic richness of these sediments, compared with the marked influence of sedimentation rate