Piper and Tartan Fields are Upper Jurassic reservoirs located in fault blocks on either side of the Witch Ground Graben. The reservoirs are divided into a series of structural units, each successively downthrown towards the graben, covering a total depth interval of 8500–15 000 ft. Mudrocks interbedded with and overlying the reservoir sequence show a progressive increase in illite content with depth from ordered I/S clays with around 40% smectite to almost pure illites with low expandibility. K-Ar analyses on 13 <0·5 µm illite separates from mudrocks covering this depth interval give ages that range from 153 to 68 Ma. Shallowest samples give the oldest ages which are considered to reflect clay neoformation in Jurassic weathering profiles. In the 40Ar/K Harper diagram this data set falls on a mixing line which reflects the amount of smectite present. Deepest samples give the youngest ages and show a major reduction of 40Ar* with increasing K content indicating formation of new illite. These data are considered to document that ‘illitization’ proceeds via a two-step reaction: initial destruction of smectite followed by neoformation of illite. According to decompacted burial curves, ‘illitization’ took place 60–50 Ma ago. Reservoir sandstones have undergone a complex diagenetic history, involving multiple illite authigenesis. K-Ar analyses of 21 <0·5 µm illite separates selected across the oil-water contact in each of the main structural units show progressively younger ages in the water zone and with increasing depth of burial. Oldest ages in the oil zone (144–120 Ma) reflect eogenetic illite authigenesis whilst progressively younger ages in the water zone result from successive mesogenetic illite generations. The youngest age of 29 Ma was obtained from an illite-rich zone at the present oil-water contact in the Tartan downthrown block. Assuming illite authigenesis in the Tartan downthrown block took place shortly after oil accumulation, ∼29–35 Ma can be considered as a maximum age for the present oil accumulation. A minimum age for oil accumulation is calculated as 22 Ma on the basis of contamination of the 29-Ma sample with up to 10% of 93-Ma authigenic illite.

Wells 13/28-2, 13/28-3 and 13/29-1, located in the Inner Moray Firth, penetrated a predominantly arenaceous marine to non-marine sedimentary complex of Oxfordian age banked up against an argillized Caledonian granite. These sediments exhibit a close relationship between the pattern of mineral diagenesis and the physico-chemical conditions of the original depositional environment. Both marine and non-marine lithofacies have undergone a complex alternation of mineral cementation and grain dissolution. The marine lithofacies is dominated by silicate cements. The earliest cements consist of chert, phosphate and pyrite; these were followed, sequentially, by fine-grained quartz and minor mica, quartz overgrowth, dolomite (non-ferroan to ferroan), and calcite (non-ferroan to ferroan). The non-marine lithofacies is dominated by clay mineral cements. The earliest cement is kaolinite, followed by quartz overgrowth cement, then by mica cement and, finally, by a late phase of kaolinite cement. The porewaters responsible for the non-marine mica cement have caused extensive argillization of the granite occurring adjacent to the non-marine sediments in Well 13/29-1. K-Ar radioisotope dating suggests that the precipitation of the mica cement and the granite argillization both took place sometime during the late Early Cretaceous (95–115 Ma, late Aptian to Albian). The close correlation between style of mineral diagenesis and depositional environment in this sedimentary complex developed during burial within an essentially sealed and overpressured reservoir in which the different facies stewed in their own porewaters. At between 4000 and 6000 ft of burial, minor tectonic perturbations caused the limited invasion of the marine sediments by non-marine porewaters and of non-marine sediments by marine porewaters. These were responsible for the presence locally of (i) allochthonous non-marine cements (mica, late kaolinite) within the marine lithofacies, and (ii) allochthonous marine cement (ferroan calcite) within the non-marine lithofacies. Prior to the entry of hydrocarbons during the Early Tertiary at a burial depth of between 6000 and 9000 ft, the sandstones experienced a major loss of pore-fluid pressure, localized collapse, and the precipitation of allochthonous baryte cement in both the marine and non-marine lithofacies.

The NW Hutton Field lies within the Brent Province of the East Shetland Basin and is located in Block 211/27, 130 km NE of the Shetland Islands. Oil of 37° API, the source of which is the Upper Jurassic Kimmeridge Clay Formation, is reservoired in Brent Group sandstones in a complex, highly faulted and tilted fault-block structure at an average depth of 12000 ft subsea. The Brent Group Reservoir shows a complex diagenetic sequence. Early calcite concretions showing partial replacement by siderite occur in the marine-facies sandstones of the Broom and Rannoch Formations. The fluvio-deltaic and marine-facies sandstones of the Etive, Ness and Tarbert Formations show a different diagenetic sequence (post-dating carbonate cementation) of secondary porosity formation by K-feldspar dissolution, followed by quartz overgrowth cementation and kaolinite and illite authigenesis. Development of kaolinite was concentrated in the crestal areas of the structure while both illite and quartz cements increase with depth in the downflank areas. The marine sandstones of the Broom Formation are the exception, being kaolinite-rich with little illite even below the oil-water contact. An early phase of oil migration into the structure appears to have been concurrent with the later stages of quartz overgrowth and clay mineral cementation, the later main phase of migration causing diagenesis to cease in the crestal part of the field. In the down-flank areas illite cementation appears to have ceased soon after the main migration of hydrocarbon fluids into the structure.

A study of the ion-exchange properties of montmorillonite has been performed in order to facilitate computer predictions of the chemical properties of natural fluids and mineral assemblies. Clay/electrolyte interactions can be described using a technique based on the concept of hypothetical surface complex formation. This technique, which is compatible with ion-association models such as GEOCHEM, can be used to simulate simultaneous ion-exchange, hydrolysis of clay edges and anion adsorption on clay surfaces. Effects such as variable cation-exchange capacity and compositionally dependent exchange constants, normally indicating non-ideal behaviour, can be simulated using different combinations of ideal reactions involving charged surfaces and complexing groups representing clay edges. The modelling procedures are flexible and thermodynamically self-consistent. The techniques were applied to data on the ion-exchange characteristics of Wyoming bentonite to yield thermodynamic data for the reactivity of this clay with alkali metals, alkaline earths and a range of first-row transition metals at 25°C.

The shale from the Witch Ground Graben in the North Sea consists mostly of clay and calcite with relatively small amounts of quartz and plagioclase. The clay mineralogy is dominated by smectite and illite with varying amounts of chlorite and kaolinite. The ion composition was determined by ion chromatography, the porewater anions and cations being removed by leaching the shale with water, while the exchange cations were removed by reacting the shale with a large excess of tetramethylammonium ions. The clay mineralogy from XRD is consistent with the measured values of CEC. The water content and wireline log conductivity of the shale is controlled largely by the CEC (i.e. clay mineralogy). The cation content of the shale section is dominated by Na, with only small concentrations of K, Mg and Ca ions. The total anion concentration, which is dominated by chloride, shows an inverse relation to the concentration of exchange sites in the shale, suggesting that the compaction of the shale is to some extent controlled by a Donnan or salt-exclusion mechanism. Application of the Donnan equilibrium led to the reasonable conclusion that the shale is in compactional equilibrium with an external reservoir of about four times the anion content of seawater. The extent of salt exclusion in the shale is relatively low—a consequence of the high salt concentration in the external formation and the relatively low concentration of exchange sites in the shale.

Existing data on interatomic potentials have been used to study the interactions between an uncharged clay sheet and a water molecule. Calculations show that most of the clay surface is relatively hydrophobic, with binding energies for a water molecule in the range 1·0–4·5 kcal mol−1. There is, however, a low-energy site for an oriented water molecule above the layer OH group and within the ring of six SiO4 tetrahedra. Using two different models for the interactions, the binding energy in this position is found to be either 13·2 or 21·8 kcal mol−1. The existence of the low-energy site accounts for the formation of the hydrated ‘10 Å’ phase of talc, which is known from high-pressure experiments. Data on the PT stability of this phase can be used to estimate its energy of dehydration. This quantity is shown to be consistent with the value of 21·8 kcal mol−1 for the binding energy of a water molecule and the energy associated with the expansion of the layers from the 9·35 Å phase.

Late Triassic sandstones from Quadrant 22 in the North Sea contain two generations of chlorite cement. Grain-coating chlorites consisting of curved, irregular crystal plates formed very early in diagenesis and are largely confined to marginal marine beds. Later diagenetic pore-filling chlorites, consisting of euhedral platy crystals, occur throughout the sequence in fluvial as well as marginal marine beds. Both generations of cement show similar XRD patterns; the 001 basal reflection of 14·0–14·06 Å is broad and of low intensity, whereas the 002 reflection at 7·05 Å is sharp and intense. Slight contraction of the 14 Å basal spacing to 13·6 Å occurs after heating to 600°C. Electron microprobe analyses show the cements to be Fe-rich clinochlores with Fe/(Fe + Mg) ratios varying between 0·41 and 0·50. They are more Mg- and Si-enriched than most other reported North Sea authigenic chlorites. The grain-coating chlorites have an average composition of (Fe3·06Al3·93Mg3·32) [(Si6·76Al1·24)O20] (OH)16. The pore-filling chlorites tend to show more variable compositions. The high Si values probably reflect both minor interstratification with smectite or vermiculite and contamination of the analyses by a closely associated illite/smectite clay. An origin related to alteration of a precursor swelling chlorite or corrensite is attractive on chemical and morphological grounds for the early grain-coating chlorites. However, direct precipitation from porewaters is the favoured origin for the later pore-filling chlorites, with the required ions being supplied by mineral decomposition reactions involving detrital silicate minerals and dolomite cements.

Authigenic illite samples have been isolated from preserved reservoir core and characterized by XRD, XRF, SEM and ATEM. Reactivity towards aqueous solutions containing acids, alkalis, complexing agents and reducing agents has been evaluated using both static (flasks) and dynamic (flow-cell) experiments at 80°C and atmospheric pressure. In flask experiments with dilute reagents, reaction appears to be simple stoichiometric dissolution. The same pattern extends to higher concentrations (molar), except for alkalis where Si is preferentially leached leaving an Al-rich residual phase. Under the more vigorous conditions of flow (continuous leaching by fresh reactant solutions), stronger acid and alkaline solutions both cause substantial dissolution although by different mechanisms. Acids displace K+ and cause illite to swell. In many of these experiments, gels were seen to precipitate downstream from the reaction site. These reactions are rapid, taking place in hours at 80°C. Similar reactions can be expected to take place in consequence of a range of drilling, stimulation and production procedures. Clay dissolution, modification and, possibly, migration would be anticipated close to the well-bore and damage from scaling (gels) somewhat further away.