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Miller field of the North Sea has had high concentrations of natural CO2 for ~70 Ma. It is an ideal analog for the long-term fate of CO2 during engineered storage, particularly for formation of carbonate minerals that permanently lock up CO2 in solid form. The Brae Formation reservoir sandstone contains an unusually high quantity of calcite concretions; however, C and O stable isotopic signatures suggest that these are not related to the present-day CO2 charge. Margins of the concretions are corroded, probably because of reduced pH due to CO2 influx. Dispersed calcite cements are also present, some of which postdate the CO2 charge and, therefore, are the products of mineral trapping. It is calculated that only a minority of the reservoired CO2 in Miller (6–24%) has been sequestrated in carbonates, even after 70 Ma of CO2 emplacement. Most of the CO2 accumulation is dissolved in pore fluids. Therefore, in a reservoir similar to the Brae Formation, engineered CO2 storage must rely on physical retention mechanisms because mineral trapping is both incomplete and slow.

Geochemical models of CO2 injection into reservoir sandstones often predict the growth of minerals that will permanently store the CO2 in solid form, and injection experiments record significant fluctuations in porewater chemistry on a short time scale. Yet the proportion of CO2 reaction may be small, even over geological time scales. A southern North Sea (UK) gas accumulation with a high natural CO2 content (c. 50%) forms a natural analogue to engineered storage, and provides a calibration point for geochemical models of CO2–rock reaction. In the analogue site, the carbonate mineral dawsonite has formed in only trace amounts (0.4 ± 0.3% solid volume) despite exposure to high levels of CO2 for 50 Myr or more. It is calculated that only 2.4 (± 0.9)% of the CO2 present within the structure is currently locked up as dawsonite, and a similar quantity in solution in the porewaters. Comparison of stable O and C isotopes with a neighboring field with low CO2 content gas suggests that up to 0.7 (± 2)% solid volume dolomite cement is associated with the CO2 charge, equivalent to 0–25% of the total CO2. The remaining 70–95% of the CO2 is present as a free phase, after tens of millions of years. Consequently, geological storage of anthropogenic CO2 in reservoirs similar to the Rotliegend Group must rely on physical containment and not mineral sequestration. The Rotliegend Group is still an excellent candidate for a CO2 storage reservoir, though using physical trapping mechanisms and not chemical ones.

The ability of mudrock seals to prevent CO 2 leakage is a major concern for geological storage of anthropogenic CO 2 . The long-term performance of a mudrock seal, which provides a natural analogue, in the North Sea Miller oil fi eld has been evaluated. This mudrock seal is immediately above a natural CO 2 -rich reservoir. The paper reports the stable isotopes of carbon from carbonate minerals in the mudrock that have precipitated in contact with CO 2 during 4 km of burial. A well-defi ned linear trend of upward-decreasing δ 13 C traces the progressive penetration of free-phase CO 2 causing dissolution and reprecipitation of carbonate minerals. The CO 2 was emplaced ca. 70‐80 Ma, and has only penetrated 12 m vertically in this case. The infi ltration rate is estimated as ~9.8 ◊ 10 ‐7 g cm ‐2 yr ‐1

The paramagnetic clay mineral illite can have important controls on fluid permeability and microporosity in sedimentary rocks. Increases in illite content of a few percent can reduce permeability by several orders of magnitude. Traditional X-ray diffraction (XRD) techniques for quantifying illite content can be very time consuming, requiring significant sample preparation, and generally examine only a relatively small sample volume. In contrast, a technique based on magnetic susceptibility described here is very rapid, cheap, sensitive, nondestructive, and requires no extra preparation of the sample. It is representative of a much larger sample volume, and in the present paper utilizes standard 1 inch diameter cylindrical core plugs, but it can be applied to other sample shapes and volumes. The magnetic-susceptibility-derived estimates of illite content from core plugs correspond well with XRD results from small powdered samples of the same material, in terms of both absolute values and overall trends with depth. However, neither the magnetic method nor XRD should be regarded as definitive. Each technique has merits and limitations, and these help to explain some observed differences in the illite determinations by each method. XRD may underestimate the illite content, particularly in muddy rocks, whereas the magnetic method theoretically provides an upper limit for the illite content. The magnetic method is also capable of simultaneously estimating the quartz content in the present samples. Changes in the concentration and distribution of illite during core cleaning or core flooding experiments can also be potentially quantified by the magnetic technique. The development of portable field sensors potentially allows the magnetic method to provide high-resolution illite profiles on slabbed cores, outcrops, and unconsolidated samples without the need to cut core plugs. The method could also be applied to whole-core measurements and to downhole magnetic susceptibility data. The processing of the magnetic susceptibility signal can also be extended to quantify other minerals in simple systems.

Reconstruction of the hydrocarbon filling history of a reservoir is important for prediction of field-scale porosity and permeability, and for the calibration of basin models. Published histories of the Brent Group show only a single phase of hydrocarbon filling, which occurred after the diagenetic reactions had run their course. In contrast, diagenetic minerals preserve evidence of multiple episodes of hydrocarbon charging. In the Cormorant Field, UK North Sea, authigenic blocky kaolin shows a systematic change in oxygen isotopic composition with depth, and the trend displays a conspicuous shift at the Mid-Ness Shale, a regionally extensive permeability barrier. The isotopic data are most readily explained if kaolin recrystallisation (from earlier vermiform kaolin) was synchronous with hydrocarbon charging, with two hydrocarbon pools, one above and one below the Mid-Ness Shale. The filling history begins with a relatively early, slow, filling phase (45–70 °C; 80–50 Ma) that formed an oil column with the oil–water contact substantially below the present-day position. This first hydrocarbon subsequently leaked off, allowing renewed diagenetic activity, including the formation of quartz overgrowths with aqueous inclusions. Emplacement of the present-day hydrocarbon charge was the last event in the history of the reservoir (90–100 °C; 10–0 Ma).

Oil can fill pores in reservoir sandstones at any burial depth by long or short distance migration. There has been a debate since 1920 concerning the effect of oil charge. We have made detailed local measurements of cements and porosity from individual fields and regional compilations from the North Sea. We consider that oil charge can be dated by K Ar ages of fibrous illite, and oil stops quartz cementation in sands containing less than 20% water. Consequently, abnormally large porosity values can be preserved from 2 km down to more than 4 km of burial. Many oilfields are anomalously porous. K Ar dates within fibrous illite and quartz cement abundance can now be used to calibrate basin models back though geological time.

Oxygen isotope microanalyses of authigenic quartz, in combination with temperatures of quartz precipitation constrained by fluid inclusion microthermometry and burial history modelling, are employed to trace the origin and evolution of pore waters in three distinct reservoirs of the Brae Formation in the Miller and Kingfisher Fields (North Sea). Oxygen isotope ratios of quartz cements were measured in situ in nine sandstone thin sections with a Cameca ims-4f ion microprobe. In conjunction with quartz cement paragenesis in the reservoirs, constrained from textural and cathodoluminescence (CL) microscopy studies, pore water evolution was reconstructed from the time of deposition of the sandstones in the Upper Jurassic until the present. CL photomicrographs of quartz overgrowths in the Brae Formation sandstones show three cement zones (A, B and C) which can be related to different oxygen isotope compositions: (1) the earliest, and thinnest, zone A (homogeneous CL pattern with probable δ 18 O values between +23‰ and +26‰—direct measurements were not possible) precipitated in the sandstones at temperatures δ 18 O values between +15‰ and +18‰) precipitated in the sandstones most likely between 70 and 90 °C; (3) the third zone C (homogeneous CL pattern and directly measured δ 18 O values between +16‰ and +22‰) precipitated in the sandstones most likely at temperatures >90 °C. Calculated oxygen isotope compositions of pore waters show that zone A quartz cements, and enclosing concretionary calcite, precipitated from a meteoric-type fluid (∼−7‰) during shallow burial ( δ 18 O −7‰) to a more 18 O-enriched fluid ( δ 18 O −4‰) as burial continued to ∼3.0 km. Data from zone C quartz cements are consistent with further fluid evolution from δ 18 O −4‰ to basinal-type fluids with δ 18 O similar to the present-day formation water oxygen isotope composition (+0.6‰ at 4.0 km burial). A similar pore water evolution can be derived for all three reservoirs studied, indicating that hydrogeologic evolution was similar across sandstones of the whole Brae Formation. The quartz cement zones observed in the Brae Formation sandstones, and the pore water history derived for the area studied, is analogous to published petrographic and pore water evolution data from the nearby Brent Group reservoirs and from reservoirs located in the Haltenbanken area on the Atlantic margin offshore Norway. Considering quartz cement is a major porosity-occluding phase in many reservoir sandstones, and because pore waters both dissolve quartz and carry the dissolved silica to cementation sites, the data presented are valuable for improving the understanding and prediction of reservoir quality development in sandstones globally.

K–Ar ages of Gulf Coast shales do not correspond to diagenetic ‘events’, but are a combination of inherited detrital ages and authigenic overprints. The key to understanding K–Ar ages of shales is the fate of the 40 Ar when the host minerals dissolve or react during the illitisation reaction: is 40 Ar incorporated within the illite, or is it lost to the porewaters? Ages are modelled as due to a single illitisation reaction, assuming a uniform precursor sediment and open-system import of K. Both whole-rock and clay mineral fraction data can be reproduced by modelling. The 40 Ar incorporated into illite is a surprisingly high proportion (75–100%) of that produced within detrital phases (smectite, K-feldspar, mica), perhaps implying that the illitisation reaction is a solid-state transformation, or that 40 Ar is imported into the shales along with incoming K. The model does not provide a good fit to the data below 4000 m, when the illitisation reaction is apparently complete, but shales lose 40 Ar by solid-state diffusion or recrystallisation.

Systematic patterns of detrital K-feldspar decline with increasing burial depth are revealed during deep burial (1.5–4.5 km) of sandstones in diverse sedimentary basins from three rift and two passive margin settings in the North Sea and USA Gulf Coast. K-feldspar destruction is predominantly by dissolution, though in-situ alteration to albite also occurs. Dissolution can remove up to 15% solid volume of the rock by 4.5 km, so that primary arkosic sandstones become diagenetic quartzarenites at depth, with increased secondary:primary porosity ratios. The quantity of K-feldspar destroyed during temperature increase is very variable: 0.05–1.0% solid volume/°C. Compaction continues during deep burial and petrographic textures diagnostic of secondary porosity are destroyed (so that many sandstones preserve Exceptions to this scenario of detrital K-feldspar dissolution are noted. Proximity of K-rich evaporites, high SiO 2 and K + activity due to the breakdown of volcanic clasts or very low P CO 2 can stabilize both K-feldspar and a suite of K-rich cements.

Upper Jurassic Fulmar Formation shelf sandstones of the high-temperature, high-pressure Franklin and Elgin Fields (South Central Graben, North Sea) contain abundant disseminated and concretionary ankerite. In contrast, most Jurassic North Sea reservoirs contain only minor amounts of dispersed ankerite. Disseminated ankerite cement in the Franklin and Elgin Fields has a fairly uniform isotopic composition (18O -10 to -12.5o/oo PDB, 13C -3 to -5o/oo PDB). Ankerite concretions have 18O values similar to disseminated cements but a wider range of 13C values (+1 to -5.5o/oo PDB). They also have highly variable intergranular volumes, which (together with the 13C data) are interpreted as a combination of pore-filling cementation and in situ replacement of comminuted bioclastic debris by ankerite. Fluid-inclusion, 18O, and paragenetic evidence suggests that ankerite formed during deep burial (c. 3.5 to 4.5 km, 140-170°C), after the onset of overpressuring, but before hydrocarbon emplacement in the reservoirs. The regionally consistent 18O data suggest that ankerite formed via a temperature-influenced mechanism, and the relatively uniform 13C cement value indicates that organic matter and marine bioclastic carbonate contributed to the dissolved carbon reservoir in constant proportion. This can be explained by calcite dissolution in response to pH decrease during thermal breakdown of organic acids. Such acids were derived from adjacent mudrocks undergoing hydrocarbon maturation and clay-mineral transformations, and are likely to have been transported in pore fluids with Mg2+ and Fe2+. The presence of these cations in solution upon thermal decarboxylation is inferred to have stabilized ankerite at the expense of calcite. A relative paucity of ankerite in other Fulmar Formation reservoirs may reflect different sedimentological compositions (less bioclastic debris) and/or lower burial temperatures (less advanced decarboxylation).

Sellafield in West Cumbria was a potential site for the location of the UK's first underground repository for radioactive, intermediate level waste (ILW). The repository was to lie around 650 m beneath the ground surface within rocks of the Borrowdale volcanic group (BVG), a thick suite of SW dipping, fractured, folded and metamorphosed Ordovician meta-andesites and ignimbrites. These are overlain by an onlapping sequence of Carboniferous and Permo-Triassic sediments. In situ borehole measurements showed that upward trending fluid pressure gradients exist in the area of the potential repository site, and that there are three distinct fluid types in the subsurface; fresh, saline and brine (at depth, to the west of the site). Simulations of fluid flow in the Sellafield region were undertaken with a 2D, steady-state, coupled fluid and heat flow simulation code (OILGEN). In both simplified and geologically complex models, topographically driven flow dominated the regional hydrogeology. Fluids trended persistently upwards through the potential repository site. The dense brine to the west of the site promoted upward deflection of topographically driven groundwaters. The inclusion in hydrogeological models of faults and variably saline sub-surface fluids was essential to the accurate reproduction of regional hydraulic head variations. Sensitivity analyses of geological variables showed that the rate of groundwater flow through the potential repository site was dependent upon the hydraulic conductivity of the BVG, and was unaffected by the hydraulic conductivity of other hydrostratigraphic units. Calibration of the model was achieved by matching simulated subsurface pressures to those measured in situ. Simulations performed with BVG hydraulic conductivity 100 times the base case median value provided the “best-fit” comparison between the calculated equivalent freshwater head and that measured in situ, regardless of the hydraulic conductivity of other hydrostratigraphic units. Transient mass transport simulations utilising the hydraulic conductivities of this “best fit” simulation showed that fluids passing through the potential repository site could reach the surface in 15 000 years. Simple safety case implications drawn from the results of the study showed that the measured BVG hydraulic conductivity must be less than 0.03 m year −1 to be simply declared safe. Recent BVG hydraulic conductivity measurements showed that the maximum BVG hydraulic conductivity is around 1000 times this safety limit.

Petrographical observations from a sandstone buried to between 3.2 and 5.7 km in the North Sea Central Graben suggest a significant change in bulk chemistry during burial. The sand contained 35% or more feldspar upon deposition, but due to extensive dissolution, only 5% remains in the deep sandstones. This has released much more Al than is currently contained as authigenic minerals: up to 1460 μ mol cm −3 of Al have been exported. This deep secondary porosity is economically important, but its recognition by petrographic textures is unreliable, as are bulk-chemical provenance studies of deeply buried sedimentary and metamorphic strata.

Oil emplacement retarded the rate of quartz cementation in the Brae Formation deep-water sandstone reservoirs of the Miller and Kingfisher fields (United Kingdom North Sea), thus preserving porosity despite the rocks' being buried to depths of 4 km and 120 degreesC. Quartz precipitation rates were reduced by at least two orders of magnitude in the oil legs relative to the water legs. Important contrasts in quartz cement abundances and porosities have emerged between the oil and water legs where reservoirs have filled with hydrocarbons gradually over a prolonged period of time (>15 m.y.). The earlier the hydrocarbon fill, the greater is the degree of porosity preservation. Failure to consider this phenomenon during field development could lead to overestimation of porosity and permeability in the water leg, potentially leading in turn to poor decisions about the need for and placement of downflank water injectors. During exploration, the retarding effect of oil on quartz cementation could lead to the presence of viable reservoirs situated deeper than the perceived regional economic basement.

The Fulmar oil field lies at the faulted western margin of the Central Graben in the central North Sea. Chalcedony and quartz, and later dolomite, occur as diagenetic cements in veins within the Upper Jurassic Fulmar Formation. Quartz in one particular vein contains primary aqueous fluid inclusions with homogenization temperatures ranging from 85 degrees to 140 degrees C, with a mean of 117 degrees C. The measured d18O of the authigenic quartz ranges from +25.1 to +27.1o/oo SMOW. The d18O of the water from which most of the quartz precipitated is calculated to have been around +8o/oo, which is significantly more positive than the measured present-day formation water d18O of +4o/oo. Quartz and chalcedony cements in the vein are postdated texturally by dolomite that has a d18O of +23.5o/oo SMOW. High salinities are recorded in the fluid inclusions in the quartz, the majority ranging from 15 to 19 wt. % NaCl equivalent. Also, the quartz contains inclusions of anhydrite. High-salinity aqueous inclusions, combined with anhydrite inclusions in the quartz and the 18O-enriched calculated water composition, suggest that the quartz precipitated from an evaporite-influenced fluid. The same fluid contributed Mg for dolomite precipitation. More deeply buried sections, including the Permian Zechstein evaporites underlying the Fulmar field, are the most obvious source of an 18O-enriched saline fluid. Mineralization of the vein resulted from cross-formational flow of warm fluids from depth through the Zechstein and into the Jurassic succession over vertical distances of 600-2000 m. Fluids most probably migrated up faults and fractures during episodes of salt movement or overpressure release.

Diagenetic ferroan carbonates grew in the Upper Jurassic reservoir sandstones of the Magnus oilfield in porewaters which differed in composition across the field. These porewaters remained compositionally different and stratified for at least 35 M.y. Variations in carbonate chemistry across the field are attributable to these porewater variations, which resulted from displacement of marine depositional water from the crest of the field by meteoric water during late Cimmerian subaerial exposure. Original depositional facies and detrital mineralogy strongly influenced diagenetic carbonate distribution. Rare diagenetic calcite occurs as discrete rhombic crystals. Diagenetically late magnesian siderites have developed throughout the reservoir sandstone and are commonly intimately associated with altered detrital biotite grains. Poikilotopic ankerite cement postdates calcite and siderite and occurs only adjacent to mudstones and in thin sandstones within mudstones. Three compositional growth zones in siderite crystals are observed across the field from crest to flank. In all three wells studied, a similar trend of compositional evolution through time is observed in both two and three zoned rhombs. First-formed siderite is relatively magnesian, intermediate zones are more ferroan, and outer zones are at least as magnesian as the first stage. These individual grain variations overprint a fieldwide variation where siderite is more ferroan in the crestal samples (up to 87 mol % Fe + Mn) and more magnesian downdip (up to 58 mol % Mg). This reflects the greater influence of relatively Fe-rich meteoric-derived water in the crest and the greater influence of marine-derived Mg-rich porewater downdip. Ankerite shows a similar variation in Fe and Mg abunda ces across the field (crest and flank maximum 25 and 17 mol % Fe + Mn respectively, 27 and 43 tool % Mg) and developed due to release of Mg, Fe, Ca and HCO3- ions from mudstones into adjacent sandstones following dissolution of detrital minerals and organic decarboxylation reactions. Both siderite and ankerite have lower 18O at the crest of the oilfield than downdip (respectively siderite 16.0 and 17.6 SMOW; ankerite 17.7 and 21.4). These differences in 18O reflect the retention during burial diagenesis of a larger component of meteoric water in the crest of the field below the unconformity, whereas downdip porefluid contained a larger marine-derived component. Strong organic influence on 13C (-8.0 to -14.6 PDB for magnesian siderite; -7.7 to -13.6 for ankerite), closed system 34S values (up to 15.7 for late cubic pyrite), and stratified 18O from crest to flank of the field argue against large scale porewater movements. Diagenetic porewater stratification is strongly supported by the parallel, but distinct, geochemical fingerprints of siderite and ankerite cements from the crest to the flank of the field.

Two preparation methods used for determining the 18O of quartz overgrowths are compared from the sandstones of the Middle Jurassic Brent Group, Northern North Sea. Method one uses dilute HF to preferentially attack interfacial materials present between the overgrowth and the detrital core, with subsequent ultrasonic agitation used to break the overgrowths off at the weakened contacts. Size fractionation by sieving should concentrate overgrowths into the smaller grain size separates. In the Brent sands studied, however, little interfacial material is present between overgrowth and core, and the overgrowths are thinly developed (average width 20-50 microns). Concentration of the diagenetic overgrowths is therefore difficult. This difficulty is compounded by large amounts of detrital quartz in the size fractions, as a result of internal fracturing of the grains. The 18O (SMOW) values for the finest size fraction (supposedly overgrowth rich), ranging from 9 to 13.3, are unusually low for diagenetic quartz overgrowths, and result from the high detrital component. The second method derives the 18O values for the overgrowths by the difference in 18O for the quartz grains plus overgrowths and quartz grains from which the overgrowths have been removed by HF dissolution. The mass balance used requires the overgrowth abundance to be known from point counting. The calculated 18O values obtained are typical for authigenic quartz overgrowths and range from 17.1 to 20.9. Direct isolation of quartz overgrowths for 18O measurement is suitable where interfacial material exists between the detrital grain and the overgrowth and where overgrowths are well developed. Otherwise, preferential HF dissolution of the overgrowths and a 18O mass balance calculation is the preferred method.

When quartz-rich sands are buried and heated, pore space is gradually filled by precipitation of quartz cement from aqueous formation fluids. Here we examine whether the presence of oil in the pore space can retard or halt this loss of porosity by slowing or stopping quartz cementation. The effect of oil fill on quartz cementation is examined by using the distribution of quartz cement in the Brae Formation deep-water sandstone reservoir of the Miller oil field (North Sea). Petrographic data demonstrate that sandstones from the oil zone have much less quartz cement, and more porosity, than sandstones from the water zone. Sandstones in both oil and water zones are compositionally and texturally identical and have been affected by a similar burial history. Kinetic modeling of the cementation process suggests that progressive oil charging has slowed quartz-cement growth rates by at least two orders of magnitude, halting it completely in the most extreme cases. Our data demonstrate that early oil charging in the crestal part of an anticline can preserve porosity in deeply buried sandstones. This knowledge is especially relevant to porosity prediction for petroleum exploration in deeply buried sandstones.

Isotopic analyses of carbon and strontium in diagenetic dolomite and ankerite present in the eolian Lower Permian Rotliegend Sandstone, southern North Sea, show an excellent linear correlation between {delta}{sup 13}C and {sup 87}Sr/{sup 86}Sr (r = 0.93), suggesting that the sources of these elements were interlinked. The isotopic data indicate that Late Permian Zechstein seas flooded the basin and displaced the interstitial meteoric Rotliegent pore fluids, producing early dolomite with predominantly marine bicarbonate ({delta}{sup 13}C = {minus}15{per thousand}) and strontium ({sup 87}Sr/{sup 86}Sr = 0.7076) isotopic signatures. The isotopic values of this dolomite form a trend to merge with the diagenetically later ankerite, which has lower {sup 13}C/{sup 12}C ({delta}{sup 13}C = {minus}4{per thousand}) and more radiogenic strontium ({sup 87}Sr/{sup 86}Sr = 0.7112). The obvious source of low-{delta}{sup 13}C carbon was thermal decarboxylation of organic matter in the underlying Upper Carboniferous mudstones. The authors also suggest that the increasing quantities of radiogenic strontium ({sup 87}Sr/{sup 86}Sr = 0.720) were released when organic acids dissolved silicates (feldspar ) in the same underlying mudstones. The upward movement of fluid or diffusion of ions across formation boundaries, therefore, progressively became the dominant control on carbonate precipitation in the Rotliegend Sandstone.