ABSTRACT

Cores of chalks from the Ekofisk field in the North Sea have been compared with outcrop samples of the “Upper Chalk” in southern England, Yorkshire, and Northern Ireland. Techniques used included petrography, scanning electron microscopy, and isotope and trace-element geochemistry. Although all of the chalks appear to have shared a similar initial composition, subsequent variation in degree and type of diagensis have yielded a remarkable range of ultimate lithologies. The Chalk of Northern Ireland is extremely hard (porosities of 1-10%), has oxygen isotopic values averaging -5.60‰, has low Sr concentrations and shows equant, blocky calcite micrite. Samples from Yorkshire have porosities of 18%, oxygen isotopes of about -4.00‰, and moderately extensive blocky calcite. Chalks from southern England (Dover, Thanet, Brighton) are very soft (porosities of about 43%), yield oxygen isotope values around -2.88‰, and show relatively slight recrystallization to blocky calcite. The Ekofisk chalk has an average porosity of 30% with oxygen isotope values averaging -0.42‰, high Sr concentration, traces of dolomitization, and common rounded crystal shapes.

These diagenetic variations appear related to the extent of freshwater diagenesis. Areas of continuing subsidence (North Sea basin) were not exposed to fresh water; cementation there was a function of burial depth and associated pressure solution-reprecipitation of calcite in marine or brine waters. In areas of increasing uplift (generally greatest near basin margins), progressively greater freshwater input brought about increasing diagenetic alteration of lithologic and petrophysical characteristics. These diagenetic facies are petrographically, as well as geochemically, recognizable.

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