The Devonian-Carboniferous reservoir of the Clair Field contains a complex and variably abundant clay mineral assemblage. An abrupt vertical change in clay mineralogy has been observed in both wells studied (UKCS 206/8-7 and 206/8-8) from being rich in Mg-chlorite, chlorite- smectite (including dioctahedral corrensite), illite, illite-smectite and Fe-chlorite, to being smectite- rich. This change broadly coincides with the unconformable boundary between the Lower Clair Group and Upper Clair Group of the reservoir which possibly defines the Devonian-Carboniferous boundary. The clay minerals present reflect interaction between: (1) tectonic stability and climate that controlled the detrital composition of the sediment; (2) the pore-waters that developed; and (3) the result of diagenetic modification.

Matrix porosity and permeability is severely restricted where the pore-lining clays are abundant (>15%) and where the clays bridge or fill pores. The type of clay mineral species appears to have a minor influence on reservoir quality, although they may be extremely important during production. The best reservoir quality occurs in mature aeolian sediments with a thin illite and illite-smectite clay coating.

As plots of K-Ar ages vs. percentage of the detrital component of mineral mixtures containing a detrital and a diagenetic illite (or illite-smectite) have different curved shapes, the linear extrapolation method of extracting the end-member ages is not justified. The curvature depends on the difference in K2O contents; thus it can be minimized if the percentage of detrital illite is normalized to the sum of detrital illite plus illitic fraction of diagenetic illite-smectite.

The different rates of smectite illitization have been investigated in interbedded sandstone and mudstone samples from the Neogene to Lower Pleistocene stratigraphy of the Niigata sedimentary basin. For samples of approximately the same age and under the same temperature and pressure conditions, the analytical results of XRD and EDS show that differences in the transformation from R = 1 I-S to R≥3 I-S at a temperature of ~30–40°C and 1000 m in depth are determined by the initial smectite composition and the relative ion concentration of pore-fluid in the sedimentary rock system. The ordered I-S and illite in the mudstone have inherited compositional characteristics from the initial detrital smectite and illite. In the sandstone, the I-S composition was changed by interaction with pore-fluids and by precipitation and crystal growth during diagenesis.

In the Maastrichtian-Danian chalk in the North Sea, discrete intervals, appearing as normal white chalk, contain up to 60% α-quartz <2 μm in size. Atomic force microscopy (AFM) reveals that the particles are of nm size, appearing as spherical particles and aggregates. Similar particles consisting of opal-CT were found in surface exposures of chalk in Denmark. Two new abiogenic pathways of silica formation in chalk are proposed. The first model proposes that SiO2 nano-size particles and aggregates precipitated and flocculated in the free-water phase as opal and were diagenetically transformed from opal-CT at low temperature to α-quartz at elevated temperature. In the second model, the dominance of radiolarians in the deep-water environment of the North Sea resulted in low dissolution supply with subsequent precipitation and flocculation of nano-size α-quartz particles. In the shallower water of the shelf environment of the present onshore chalk, the abundance of sponges and their dissolution supplied enough Si to precipitate opal-CT in the free-water phase.

Petrographic, mineralogical and geochemical core analysis of Palaeocene turbiditic sandstones in the Sleipner East gas-condensate reservoirs show the importance of diagenetic clay distribution on porosity, permeability, and water saturation. An observed ‘high resistivity zone’ (HRZ) corresponds to intervals with low water saturation, a more restricted distribution of diagenetic clay (mainly chlorite), and up to 5% quartz cement. The underlying ‘low resistivity zone’ (LRZ) corresponds to intervals with more widely distributed diagenetic clay, which have lower degrees of quartz cementation, higher porosity, and variably reduced permeability. Crosscutting relationships of the HRZ/LRZ with mapped sedimentary depositional units, as well as fluid inclusion analysis data, suggest that the distribution of diagenetic clay was affected by an earlier (late Miocene?) oil charge, and more extensive chlorite formation in a palaeo-water zone. Recent gas condensate charge and structuring of these sandstones resulted in LRZ reservoirs with substantially higher water saturations than those in the HRZ.

In the Miller Field, diagenetic quartz abundance, isotopic compositions and salinities of quartz-cementing fluids display a distinct pattern which is related to the structural depth of the reservoir sandstones. Quartz cement volumes increase from the crest of the field (average 6.0±1.5%) towards the flanks of the field (average 13.2±2.1%) and directly reduce reservoir porosity. By integrating petrographic observations with results of fluid inclusion measurements and O isotope analyses of diagenetic quartz, the pattern of quartz cementation is seen to be related to the reservoir filling history. Oil filled the crest of the reservoir first and prevented extensive quartz cementation. At greater depth in the reservoir oil zone, quartz overgrowths continued to precipitate until inhibited by the developing oil column. Oxygen isotope compositions of diagenetic quartz imply that quartz cement continued to precipitate in the water zone of the reservoir up to the present day.

Silicification and illitization due to diagenesis result in the modification of porosity and permeability in many North Sea reservoirs, including Greater Alwyn. A good estimation of the mineral composition is required as input/output data for geochemical modelling modes intended to reproduce these effects. Using sedimentological studies to determine laterally equivalent facies, three facies were chosen. ‘Initial states’ for modelling were thus defined as the present-day composition of less-evolved laterally-equivalent facies, ‘final states’ as the present-day composition in the most evolved part of the basin. Precise actual mineral compositions were calculated using a reconciliation program, DATREC. Average compositions calculated for each facies in each field provided data useful in constraining input/output values used in modelling, and gave insights into depositional variation. Arithmetic tests assuming isochemical transformations, constraining the chemical compositions of ‘initial states’ to fit actual mineralogical compositions of ‘final states’, were run using this software. They show that, from an arithmetical point of view, if the assumption of equivalent facies holds, isochemical reactions can occur in some, but not in all facies, thereby demonstrating the importance of checking equivalence of facies and diagenesis hypotheses by quantitative calculations.

The Tarbert Formation on a north±south oriented structure along the eastern flank of the Viking Graben has been studied. The reservoir in the two wells studied is buried ~100 m deeper in the northern than in the southern well. The present reservoir temperature is ~130°C. The reservoir quality is good, but due to extensive illitization of kaolin, the northern well shows lower permeability values than the southern well, for similar porosity values. The initial composition of the analysed samples in the two wells is very similar. There are no significant differences in total clay content and both wells contain K-feldspars and kaolin. However, the diagenetic alteration is more advanced in the deeper well. Total porosity is about the same in both wells but, due to illitization, the ratio of micro- vs. macro-porosity is higher in the deeper well. Petroleum emplacement in the structure occurred in two phases: oil emplacement predates, whereas gas emplacement postdates, most of the quartz cementation. In the deeper well, illitization occurred after gas emplacement.

Rocks above and below the Precambrian-Palaeozoic unconformity at the base of many Palaeozoic sedimentary basins of mid-continental North America are distinguished by secondary K-feldspar, chlorite and illite. Thrusting of the Appalachian foreland is generally considered responsible for the fluid migrations which caused this alteration. We have examined secondary chlorite and illite from Cambro-Ordovician rocks which overlie the unconformity in southern Ontario, Canada. This area is situated along the interface between the Michigan and Appalachian basins. Water involved in chloritic alteration at ~150°C has an O isotope composition compatible with a basinal brine evolved from seawater. Brine migration was probably triggered by the Taconic Orogeny to the east. In comparison, the secondary illite (365–321 Ma) largely postdates the Acadian Orogeny, and the illite-forming fluids have stable isotopic compositions typical of tepid (~40–55°C) meteoric water. We suggest that basement arches beneath southern Ontario were reactivated by this orogeny, which facilitated introduction of meteoric water. Similar alteration in underlying Precambrian rocks shows that fluid flow was focused along the unconformity during the regional Ordovician brine migration, as well as during the localized Mississippian introduction of fresh water.

Grain-coating chlorites in clastic quartz-rich sandstones have long been recognized as an important porosity-preserving constituent in medium- to deep-burial diagenesis. As little is known about the occurrence and origin of chlorite coatings, chlorite synthesis experiments were performed to study how grain-coating chlorites form in certain sandstones during burial. The starting material was naturally-occuring sandstones from the Oseberg and the Veslefrikk fields offshore Norway, where the same sandstone formation is buried to different depths due to faulting. Grain-coating chlorites exist below ~3000 m burial depth only. At shallower burial (2400 m), an X-ray amorphous iron containing thin clay coating is present.

The samples were heated to 200 and 250°C (at water vapour pressure) in a hydrothermal bomb for 2–4 weeks. Both starting material and end-products were studied (electron-) optically in both scanning and transmission microscopes. The TEM showed the Fe-rich precursor material to consist of a fine-grained berthierine-dominated mixed-layer. The neoformed grain coatings in the reacted samples were similar in appearance to naturally-occurring chlorite coatings. The TEM analyses of individual grains documented an Fe-rich chloritic phase with an average composition of Mg0.41Fe3.52Mn0.10Al1.51(Al0.58Si3.42)O10(OH)8. The reacted waters were found to be close to saturation with the newly formed chlorites.

Grain-coating chlorite thus appears to form in the natural environment from Fe-rich berthierine precursors at a burial depth corresponding to a temperature around 90°C.

Several types of authigenic illite differing in their distribution and morphology are described in Rotliegend (Permian) gas-bearing sandstones of northern Germany. The microtextural environment seems to control the K-Ar age variations of illite-fractions of different sizes. Thin, uncompacted, and radial coatings are 200 Ma old, whereas uncompacted fibrous laths filling enlarged secondary porosity are 180 Ma old, and compacted or recrystallized tangential particles yield intermediate or even younger ages. Such Liassic ages relate to a thermal event, the effects of which on the clay material have not been erased subsequently, despite present-day burial depth of 4500 m and temperatures of 150°C.

Neutron diffraction experiments can provide an extremely high-resolution structural picture of clay-fluid systems. Here we describe the application of time-of-flight neutron scattering to hydrated clays, including discussion of issues such as isotopic labelling, sample containment, and data analysis. Recent studies of hydrated vermiculites under ambient conditions are used as an example. We then describe a new high-pressure/high-temperature sample environment that is being used to study clay-fluid interactions, in situ under hydrostatic sedimentary basin conditions. This environment enables us to approximate conditions encountered during burial, at depths of up to 10 km.

X-ray diffraction is used widely for quantitative analysis of geological samples but studies which document the accuracy of the methods employed are not numerous. Synthetic sandstones of known composition are used to compare a ‘routine application’ of a Rietveld and a reference intensity ratio (RIR) method of quantitative phase analysis. Both methods give similar results accurate to within ~±3 wt.% at the 95% confidence level. The high degree of accuracy obtained is believed to depend to a large extent on the spray-drying method of sample preparation used to eliminate preferred orientation.

Sulphur isotope ratio measurements (δ34S) of diagenetic pyrite are commonly used to identify S sources and mechanisms of sulphide formation in basinal sediments. This study reports such data for a diagenetic pyrite nodule from the Brent Group sandstones of the northern North Sea at three sampling scales: 50 cm (core subsample), 500 μm (laser microprobe) and 50 μm (ion microprobe). Similar δ34S variations are found by the laser and ion microprobe techniques. There is a very wide range in δ34S (<−10% to >+50%) within the nodule and isotopically heavy S (δ34S >+20%) is common at all scales. The nodule δ34S distribution does not fit a Rayleigh fractionation pattern. The laser microprobe sampling at 100–500 μm scales seems to be adequate to characterize S isotope variations in this material.

The stable isotopic compositions of diagenetic minerals can provide valuable constraints on the sources, precipitation temperatures and relative timing of cements in reservoir rocks. This type of information is essential when trying to understand and predict the distribution of cements in the subsurface, and their impact on reservoir quality. Conventional isotope methods contribute to answers to many diagenetic problems, but where core or time are scarce, or where good mineral separation is unobtainable, laser-based stable isotope methods offer several advantages. These include the ability to analyse carbonates, sulphides and anhydrite in situ with 50–100 μm resolution, simple and clear sample and analysis viewing optics, savings on sample preparation time and greatly reduced sample size requirements.

Diagenetic silicates such as quartz and clay cements cannot be analysed in situ by laser but, where in situ analysis of quartz δ18O is demanded, ion microprobe analysis can provide very high resolution (20–30 μm) capability with a precision of ±1%.