R. Stuart Haszeldine

Pressure cells and pressure seals in the UK Central Graben

The Central Graben of the North Sea is characterised by high levels of overpressure (up to 40 MPa overpressure at 4500 m depth). We present pressure data for Cenozoic and Mesozoic reservoirs. Palaeocene sandstones control pressures in Tertiary mudstones and Cretaceous Chalk by acting as a regional ‘drain’. We divide the Jurassic into 18 pressure cells. The rift structure of the Graben controls the magnitude of pressure in each cell. Lateral hydraulic communication exists over 10 km distance between deeply-buried terraces (> 5000 m depth) and shallow structural highs (< 4500 m depth). Lateral communication increases pressure in the structurally-elevated sandstones to the minimum stress. This dynamic process produces zones of vertical fluid flow on the Forties-Montrose High, termed Leak Points. Vertical flow at Leak Points produces a 20 MWm−2 heat flow anomaly and controls hydrocarbon retention. Leak Points are water-wet, while deep terraces in hydraulic communication with Leak Points are condensate-bearing. The Kimmeridge Clay Fm. forms the pressure seal in deep terraces.

Distribution, chemistry, isotopic composition and origin of diagenetic carbonates; Magnus Sandstone, North Sea

ABSTRACT 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.

Linear coupling of carbon and strontium isotopes in Rotliegend Sandstone, North Sea: Evidence for cross-formational fluid flow

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.

Aluminium loss during sandstone diagenesis

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.

Cryptic structural trends in basement revealed by Patterson diagrams: examples from the Scottish and Irish Caledonian orogen

Recent studies have shown a link between occurrences of ore deposits and plutons with deep crustal structural trends. A type of graphical autocorrelation analysis, the Patterson diagram, provides a simple visual method by which possible alignments may be amplified and more easily recognized. The Patterson diagram, unlike the distribution of data in geographical space, has a centre of symmetry and is amenable to statistical analysis for the identifi cation of alignments of features which are nonrandom. Examples are given of the application of Patterson diagrams to Carboniferous-age gold and SEDEX base-metal deposits in Ireland and Scotland and to historical earthquake epicentres in NW Scotland, which highlight alignments related to Caledonian and other trends.