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%.