Mudstone compaction curves in basin modelling: a study of Mesozoic and Cenozoic Sediments in the northern North Sea

Basin modelling studies are carried out in order to understand the basin evolution and palaeotemperature history of sedimentary basins. The results of basin modelling are sensitive to changes in the physical properties of the rocks in the sedimentary sequences. The rate of basin subsidence depends, to a large extent, on the density of the sedimentary column, which is largely dependent on the porosity and therefore on the rate of compaction. This study has tested the sensitivity of varying porosity/depth curves and related thermal conductivities for the Cenozoic succession along a cross‐section in the northern North Sea basin, offshore Norway. End‐member porosity/depth curves, assuming clay with smectite and kaolinite properties, are compared with a standard compaction curve for shale normally applied to the North Sea. Using these alternate relationships, basin geometries of the Cenozoic succession may vary up to 15% from those predicted using the standard compaction curve. Isostatic subsidence along the cross‐section varies 2.3–4.6% between the two end‐member cases. This leads to a 3–8% difference in tectonic subsidence, with maximum values in the basin centre. Owing to this, the estimated stretching factors vary up to 7.8%, which further gives rise to a maximum difference in heat flow of more than 8.5% in the basin centre. The modelled temperatures for an Upper Jurassic source rock show a deviation of more than 20 °C at present dependent on the thermal conductivity properties in the post‐rift succession. This will influence the modelled hydrocarbon generation history of the basin, which is an essential output from basin modelling analysis. Results from the northern North Sea have shown that varying compaction trends in sediments with varying thermal properties are important parameters to constrain when analysing sedimentary basins.     

Mechanical compaction and ultrasonic velocity of sands with different texture and mineralogical composition

This study presents the results of experimental compaction while measuring ultrasonic velocities of sands with different grain size, shape, sorting and mineralogy. Uniaxial mechanical compaction tests up to a maximum of 50 MPa effective stress were performed on 29 dry sand aggregates derived from eight different sands to measure the rock properties. A good agreement was found between the Gassmann saturated bulk moduli of dry and brine saturated tests of selected sands. Sand samples with poor sorting showed low initial porosity while sands with high grain angularity had high initial porosity. The sand compaction tests showed that at a given stress well‐sorted, coarse‐grained sands were more compressible and had higher velocities (Vp and Vs) than fine‐grained sands when the mineralogy was similar. This can be attributed to grain crushing, where coarser grains lead to high compressibility and large grain‐to‐grain contact areas result in high velocities. At medium to high stresses the angular coarse to medium grained sands (both sorted sands and un‐sorted whole sands) showed high compaction and velocities (Vp and Vs). The small grain‐to‐grain contact areas promote higher deformation at grain contacts, more crushing and increased porosity loss resulting in high velocities. Compaction and velocities (Vp and Vs) increased with decreasing sorting in sands. However, at the same porosity, the velocities in whole sands were slightly lower than in the well‐sorted sands indicating the presence of loose smaller grains in‐between the framework grains. Quartz‐poor sands (containing less than 55% quartz) showed higher velocities (Vp and Vs) compared to that of quartz‐rich sands. This could be the result of sintering and enlargement of grain contacts of ductile mineral grains in the quartz‐poor sands increasing the effective bulk and shear stiffness. Tests both from wet measurements and Gassmann brine substitution showed a decreasing Vp/Vs ratio with increasing effective stress. The quartz‐rich sands separated out towards the higher side of the Vp/Vs range. The Gassmann brine substituted Vp and Vs plotted against effective stress provide a measure of the expected velocity range to be found in these and similar sands during mechanical compaction. Deviations of actual well log data from experimental data may indicate uplift, the presence of hydrocarbon, overpressure and/or cementation. Data from this study may help to model velocity‐depth trends and to improve the characterization of reservoir sands from well log data in a low temperature (<80–100^o C) zone where compaction of sands is mostly mechanical.     

Quartz Cement in Middle Jurassic Reservoir Sandstones in North Sea A Review. Part II： Duration and Silica Sources

The timing and duration of quartz cementation in sandstones have been mainly inferred from diagenetic texture, relationship between pore filling minerals, fluid inclusions and isotopic data. Fluid inclusion temperatures from North Sea reservoir sandstones indicate that most of the quartz cement forms at temperature exceeding 90℃ and is continually proceeding after oil emplacement, based on the fluid inclusion temperatures in quartz overgrowth which is approaching the bottom-hole temperatures. The duration of quartz cement after oil emplacement depends upon the saturation of porewater and the distribution of pore water film and the property of water-wet or oil-wet of the reactants. The leaching of K-feldspar by meteoric water requires pore water flow to move the released potassium and sodium and silica out the solution, which suggests the mechanism does not appear to be a major source of silica for quartz cementation. The quartz cementation coincidence with the compaction and pressure solution suggests the major source of silica. The alteration of feldspar by illitization of kaolinite may serve as another important source of silica at deep burial depth. External sources are not need to call on for illustrating the quartz cementation, because there is no evidences for large scale convection of pore water flow occurred in the burial history of reservoir sandstones of middle Jurassic in the North Sea.     

New Isotopic data from davidites and sulfides in the bidjovagge gold-copper deposit,Finnmark, Northern Norway

Summary The Bidjovagge gold-copper deposit (69°17'N, 22°29'E) occurs in the north-south trending Proterozoic Kautokeino greenstone belt. The greenstone belt consists of shallow marine sediments and mafic to ultramafic volcanites, which are intruded by diabase sills and granitoids. The sequence can be correlated with the 1930 to 2200 Ma Kiruna greenstone belt. Svecokarelian intrusions took place in two phases between 1880–1890 Ma and between 1860-1870 Ma.The deposit occurs in the stratigraphically lower part of the belt along a north-south trending anticlinal structure on the border between greenschist and amphibolite grade metamorphism within the Baltic-Bothnia megashear zone. The deposit is related to shear zones and hosted by albitic felsite, a metasomatic rock formed along the contact with the diabase sills.The only ore minerals of economic significance are native gold and chalcopyrite. Other common ore minerals are pyrite and pyrrhotite. Marcasite, magnetite, ilmenite, hematite, tellurides, rutile, sphalerite, galena, davidite and pentlandite occur in accessory amounts. A total of 10 samples of davidite, 14 sulfides and 4 samples of albitic felsite were analyzed for their isotopic compositions. Our best estimate for the age of the davidites (1885 ± 18 Ma) is based on the upper intersection of a U/Pb concordia diagram and is identified as the time of davidite (and gold) mineralization. This age is confirmed by a Sm/Nd date of 1886 ± 88 Ma on the same material. At 1339 ± 8 Ma, a metamorphic or metasomatic event disturbed the U/Pb system in some of the davidite samples and completely reset the U/Pb ratios in the albitic felsites. The sulfide minerals, containing trace U and Pb, were strongly disturbed during galena formation at about 500–550 Ma but apparently were initially formed at the same time as, or shortly after, the davidite. Based on an estimated time of formation of galena during the development of the early Paleozoic peneplain in the area, the Pb/Pb data are interpreted as a two-stage isochron with an initial age of 1876 ± 15 Ma.The relation between the mineralization and shear zones, the correspondence between the age of the mineralization and the Svecokarelian orogeny and the initial ratios of the radiogenic isotopes of the ore forming solutions, all support a metamorphic/metasomatic origin for the gold and copper mineralization.

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Neue Isotopendaten von Davidit und Sulfiden aus der Bidjovagge Gold-Lagerstätte, Finnmark, N.-Norwegen

Zusammenfassung Die Bidjovagge Gold-Kupfer-Lagerstätte (69°17'N, 22°29'E) liegt in dem N-S streichenden proterozoischen Kautokeino Grüngesteinsgürtel. Er besteht aus marinen Flachwassersedimenten, sowie mafischen und ultramafischen Vulkaniten, die von Diabas-Sills und von Granitoiden intrudiert werden. Diese Gesteinsabfolge kann mit dem 1930–2200 Ma alten Kiruna Grüngesteinsgürtel verglichen werden. Sveko-karelische Intrusionen erfolgten zweiphasig, zwischen 1880–1890 Ma und zwischen 1860 und 1870 Ma.Die Lagerstätte liegt in den stratigraphisch tieferen Anteilen diese Grüngesteinsgürtel entlang einer N-S streichenden Antiklinalstruktur im Übergangsbereich zwischen der Grünschiefer und Amphibolitfazies innerhalb der baltisch-bothnischen Mega-Scherzone.Sie ist an Scherzonen und an Albit-Felsite, einem metasomatisch am Kontakt mit den Diabas-Sills gebildeten Gestein, gebunden.Gediegen Gold und Kupferkies sind die wirtschaftlich wichtigsten Erzminerale. Pyrit und Magnetkies sind häufig. Markasit, Magnetit, Ilmenit, Hämatit, Telluride, Rutil, Zinkblende, Bleiglanz, Davidit und Pentlandit kommen akzessorisch vor. 10 Proben von Davidit, 14 Sulfide und 4 Proben von den Albit-Felsiten wurden auf ihre Isotopenzusammensetzung hin untersucht.Der obere Schnittpukt im U/Pb Concordiadiagramm ergab die beste Altersabschätzung für die Davidite (1885 ± 18 Ma). Dieses Alter wird als Zeitpunkt der Davidit (und Gold)-Mineralisation betrachtet. Ein Sm/Nd-Alter von 1886 ± 88 Ma bestätigt dieses. Ein metamorphes oder metasomatisches Ereignis, datiert mit 1339 ± 8 Ma störte das U/Pb System in einigen Daviditproben und führte zu einer vollständigen Reequilibrierung der U/Pb Verhältnisse in den Albit-Felsiten. Die Isotopenverhältnisse der Sulfide, die Spuren von U und Pb enthalten, wurden während der Bildung von Bleiglanz um etwa 500–550 Ma ebenfalls massiv gestört; die Sulfide wurden aber offensichtlich gleichzeitig, oder aber kurz nach Davidit, gebildet. Basierend auf einer Abschätzung des Bleiglanz-Bildungsalters im Zuge der Entwicklung einer altpaläozoischen Peneplain in diesem Gebiet, werden die Pb/Pb Daten als Zweistufen-Isochrone mit einem Ursprungsalter von 1876 ± 15 Ma interpretiert.Die Bindung der Mineralisation an Scherzonen, die Übereinstimmung des Mineralisationsalters mit dem der Sveco-karelischen Orogenese und die Isotopeninitiale der erzbildenden Lösungen belegen einen metamorph/metasomatischen Ursprung der Gold-und Kupfervererzungen.

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With 9 Figures     

Diagenesis in Mesozoic sandstones from Spitsbergen and the North Sea — A comparison

Mesozoic and Tertiary sandstones on Spitsbergen are generally tight and well cemented while Mesozoic and Tertiary sandstones from the North Sea include highly porous reservoir rocks.It is concluded that one of the most important controlling mechanisms for preservation of porosity is the build up of high pore pressures which effectively reduce pressure solution at grain contacts and the incidence of quartz overgrowth.High pore pressures are most commonly developed in onlapping sedimentary sequences with an effective shale seal like in the North Sea.Extensive diagenetic growth of kaolinite at the expence of clastic feldspar, which is commonly observed, requires a large flux of low salinity water. The most probable mechanism is fresh ground water drive through sandstones which have served as aquifers. During the late stages of burial (1–3 kms) the diagenetic reactions must have taken place within an isochemical semi-closed system. Calculations show that porewater expelled from compacting shales is insufficient to cause significant changes in the bulk chemistry of thick sandstones. Microprobe analyses carried out on Jurassic sandstones from the Statfjord field show that there are a continuous series of transitions from clastic mica to illite and kaolinite as indicated by a gradual loss of potassium. Evidence is presented to show that silica cementation is retarded in the presence of clastic mica while carbonate and feldspar cementation is favoured in this local high pH environment between sheets of mica. Electron probe analyses show that diagenetic feldspar overgrowths are pure potassium feldspar with a composition significantly different from that of clastic feldspar. During late diagenesis the porewater was most probably near equilibrium with the reactive phases.

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Zusammenfassung Die mesozoischen und tertiären Sandsteine von Spitzbergen sind im allgemeinen gut zementiert, während die gleichalten Sandsteine aus der Nordsee hochporöse Speichergesteine umfassen.Man erkennt, daß die Erhaltung der Porosität auf einen hohen Porenwasserdruck zurückzuführen ist. Dadurch wird Drucklösung an Kornkontakten sowie Weiterwachsen von Quarz verhindert.Hoher Porendruck entsteht gewöhnlich da, wo überlappende Sedimentserien mit einer Abdichtung durch Schiefer vorliegen—wie in der Nordsee.Ausgedehntes diagenetisches Wachstum von Kaolinit auf Kosten klastischer Feldspäte erfordert einen hohen Durchfluß von niedrigsalinem Wasser. Der wahrscheinlichste Mechanismus ist in einem Grundwasserstrom zu suchen, der durch die Sandsteine fließt, die als Wasserleiter dienen. Während der späten Stadien der Absenkung (1–3 km) müssen diese diagenetischen Reaktionen in einem halbgeschlossenem isochemischen System abgelaufen sein. Berechnungen aus Porenwasser, das von Schiefern gewonnen wurde, zeigen, daß dieses nicht ausreicht, bedeutende änderungen in der Gesamtchemie mächtiger Sandsteine zu bewirken.Mikrosondenanalysen, die an jurassischen Sandsteinen aus dem Statfjord-Feld vorgenommen wurden, zeigen, daß hier eine kontinuierliche Serie von übergängen aus klastischem Glimmer nach Illit und Kaolinit vorliegt. Eine stufenweise Abnahme von Kalium deutet dies an. Quarz-Zementation tritt zurück bei Anwesenheit von klastischem Glimmer, während Karbonat- und Feldspat-Zementation unter den Bedingungen eines hohen pH-Wertes zwischen den Glimmer-Blättern bevorzugt erscheint. Mikrosondenanalysen zeigen, daß der diagenetisch aufgewachsene Feldspat ein reiner Kali-Feldspat ist, der sich beträchtlich von den klastischen Feldspäten unterscheidet. Während der späten Diagenese stand das Porenwasser nahezu im Gleichgewicht mit den reagierenden Phasen.

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Résumé Les grès mésozoÏques et tertiaires du Spitzberg sont en général bien cimentés, tandis que les grès de mÊme âge de la Mer du Nord comprennent des roches-réservoirs très poreuses. On en conclut que l'un des plus importants mécanismes régissant le maintien de la porosité est la mise en jeu de fortes pressions dans les pores de la roche, ce qui effectievement réduit la dissolution par la voie de la pression s'exerÇant au contact des grains et l'intervention de la croissance des grains de quartz.De fortes pressions dans les pores sont le plus couramment développées dans les séquences sédimentaires isolées par un recouvrement argileux, comme dans la Mer du Nord.La croissance diagénétique à grande échélle de la kaolinite aux dépens du feldspath détritique, communément observée, requiert une grande circulation d'eau à faible salinité. Le mécanisme le plus probable est celui de la circulation d'eau douce dans des grès aquifères. Durant les derniers stades de l'enfouissement (1–3 km) les réactions diagénétiques se sont produites dans un système isochimique à moitié fermé. Des calculs montrent que l'eau poreuse chassée des argilites à la suite de leur compaction est insuffisante pour causer des changements de quelqu'importance dans la composition chimique des grès en masses épaisses. Des analyses à la microsonde sur des grès jurassiques de la région du Statfjord montrent qu'il existe une gamme continue de transitions depuis le mica détritique jusqu'à l'illite et la kaolinite, ainsi qu'il ressort de la perte graduelle du potassium. La preuve est donnée que la cimentation par la silice est retardée par la présence de mica détritique, tandis que la cimentation par le carbonate et par le feldspath est favorisée dans ce milieu restreint à pH élevé entre lamelles de mica. Des analyses électroniques montrent que les accroissements diagénétiques de feldspath sont faits de feldspath potassique pur d'une composition notablement différente de celle del feldspath détritique. A la fin de la diagénèse l'eau intersticielle était très probablement proche de l'équilibre avec les phases réagissantes.

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Noble gas and halogen evidence for the origin of Scandinavian sandstone-hosted Pb-Zn deposits

Fluid origins in the sandstone-hosted Pb-Zn class of ore deposit have been investigated in three deposits from Scandinavia; Laisvall, Vassbo and Osen. The deposits studied are hosted by autochthonous Cambrian sandstones that preserve a near original structural relationship to the underlying Precambrian basement, enabling the role of basement interaction to be assessed.Mineral samples have been collected from across the paragenetic sequence: sphalerite, galena, pyrite, fluorite and barite, of impregnation and related joint-hosted mineralization. Fluid-inclusion halogen (Cl, Br and I) and noble gas isotope (⁴⁰Ar, ³⁶Ar, ⁸⁴Kr) compositions were determined simultaneously by noble gas mass spectrometry of irradiated sample splits. Complementary He isotope analyses are obtained from nonirradiated splits of the same samples.³He/⁴He values at Laisvall and Osen are highly radiogenic, 0.02 Ra, and the ⁴He/⁴⁰Ar* ratio extends to values greater than the crustal production value of 5, characteristic of low-temperature crustal fluids. At Vassbo, a slightly elevated ³He/⁴He ratio of 0.1-0.3 Ra is compatible with a very minor mantle component (1%-4%) suggesting a distal source for the basinal brine-dominated fluid.Br/Cl molar ratios 3.2-8.2 × 10⁻³ are greater than the present seawater value of 1.54 × 10⁻³ and correspond with I/Cl molar ratios in the range 64-1600 × 10⁻⁶. The upper limits of both the I/Cl and Br/Cl values are amongst the highest measured in crustal fluids. Together, the data indicate acquisition of salinity by the evaporation of seawater beyond the point of halite saturation and subsequent fluid interaction with I-rich organic matter in the subsurface. The data are compatible with the independent transport of sulfate and sulfide and indicate that fluids responsible for joint-hosted mineralization were distinct to those responsible for impregnation mineralization.All three deposits preserve fluids with ⁴⁰Ar/³⁶Ar in the range of 6,000-10,000 and fluid inclusion ⁴⁰Ar* concentrations of >0.02-0.05 cm³cm⁻³. Fluid-inclusion ⁴He concentrations are also extremely elevated with maximum values of ∼0.1 cm³cm⁻³ in Laisvall fluorite and sphalerite. The high ⁴⁰Ar/³⁶Ar values, together with the high ⁴He and ⁴⁰Ar* concentrations, result from a very long premineralization crustal residence time on the order of 100-200 Ma.Together, the noble gas and halogen data are compatible with a Caledonian mineralization event (∼425 Ma) caused by mixing of two or more, long-lived, hydrothermal basinal brines and pore fluids at the sites of mineralization. The data suggest negligible recharge of the basinal brines by meteoric water and indicate extensive fluid-basement interaction before mineralization. The similar noble gas composition of each deposit, suggests that similar processes operated at all three deposits and favors a single-pass fluid-flow model for mineralization.     

Correlation of late precambrian and early Palaeozoic sequences by eustatic sea-level changes and the selection of the Precambrian-Cambrian boundary

The author argues for a pragmatic approach to the problem of defining the Precambrian-Cambrian boundary. A boundary at first occurrence of shelly fauna as recommended by the Precambrian-Cambrian working group will create severe difficulties in worldwide correlation. By placing the boundary stratigraphically somewhat higher (i.e. the Fallotaspis zone) with a more abundant fauna, there is a much better basis for biostratigraphic correlation.Eustatic sea-level changes may be another important tool for late Precambrian-early Palaeozoic correlation, and more detailed facies interpretation of such sequences should be carried out in order to recognise these eustatic sea-level changes.     

Microfabric and rock properties of experimentally compressed silt-clay mixtures

Oedometric mechanical compaction tests were performed on brine-saturated synthetic samples consisting of silt-clay mixtures to study changes in microfabric and rock properties as a function of effective stress. The silt consisted of crushed quartz (∼100%) with grain size range between 4 and 40 μm, whereas the clay consisted of 81% kaolinite, 14% mica/illite and 5% microcline of grain size between 0.4 and 30 μm. Five sample pairs ranging in composition from pure silt to pure clay were compacted to 5 and 50 MPa effective stress respectively. SEM studies were carried out to investigate microfabric changes in the mechanically compacted silt-clay mixtures. The degree of alignment of the different minerals present (quartz, mica/illite and kaolinite) were computed by using an image analysis software. Experimental compaction have measured the changes in the rock properties such as porosity and velocity as a function of effective stress for different mixtures of clay and silt. Clay-rich samples showed a higher degree of mineral orientation and lower porosity compared to silt-dominated samples as a function of effective stress. Pure clay sample had 11% porosity at 50 MPa effective stress whereas the pure silt sample retained about 29% porosity at the same effective stress. The experiments showed that low porosity down to 11% is possible by mechanical compaction only. A systematic increase in strain was observed in the silt-clay mixtures with increasing clay content but the porosity values found for the 50:50 silt-clay mixture were lower than that of 25:75 silt-clay mixture. No preferential mineral orientation is expected before compaction owing to the high initial porosity suggesting that the final fabric is a direct result of the effective stress. Both P- and S-wave velocities increased in all silt-clay mixtures with increasing effective stress. The maximum P- and S-wave velocities were observed in the 25:75 silt-clay mixture whereas the minimum Vp and Vs were recorded in the pure silt mixture. At 50 MPa effective stress P- wave velocities as high as 3 km/s resulted from experimental mechanical compaction alone. The results show that fine-grained sediment porosity and velocity are dependent on microfabric, which in turn is a function of grain size distribution, particle shape, sediment composition and stress. At 5 MPa effective stress, quartz orientation increased as a function of the amount of clay indicating that clay facilitate rotation of angular quartz grains. Adding clay from 25% to 75% in the silt-clay mixtures at 50 MPa effective stress decreased the quartz alignment. The clay mineral orientation increased by increasing both the amount of clay and the effective stress, the mica/illite fabric alignment being systematically higher than that of kaolinite. Even small amount of silt (25%) added to pure clay reduced the degree of clay alignment significantly. This study demonstrates that experimental compaction of well characterized synthetic mudstones can be a useful tool to understand microfabric and rock properties of shallow natural mudstones where mechanical compaction is the dominant process.