Geochemistry of the Upper Jurassic Heno Formation in the Karl-1 well, Danish North Sea

This report is a continuation of a previous investigation on the chemostratigraphy and mineral-chemical fingerprinting in the Upper Jurassic Heno Formation from the Danish North Sea. Geochemical analysis of cuttings samples from the Karl-1 well are compared with results from core and cuttings samples from the Hejre-2 well, and with results from core samples in all other investigated wells from the Danish Central Graben: Diamant-1, Gert-1, Gert-2, Gert-4, Hejre-1, Jeppe-1 and Rita-1. In all 45 cuttings samples from the Karl-1 well have been geochemically analysed for major elements by XRF (X-ray fluorescence) and for trace elements by ICP-MS (inductively coupled plasma - mass spectrometry).

The cuttings samples from the Ravn and Gert Members of the Heno Formation in the Karl-1 well can generally be distinguished from each other by the same parameters as core samples from the other previously investigated wells. The distinction, though, is less clear for the cuttings samples in the Karl-1 well than for the core samples in the other investigated wells. Furthermore, the Cr content cannot be used, as it is high for all cuttings samples probably due to steel chip impurities rather than sediment differences. The Gert Member in the Karl-1 well generally has lower Th / TiO₂ (and REE / TiO₂), Nb / TiO₂, P₂O₅ / Al₂O₃, Na₂O / Al₂O₃ and higher V / P₂O₅ ratios than the Ravn Member. Several possible explanations can account for the observed differences between the Ravn and Gert Members. Less altered material could have been brought into the system during deposition of the Gert Member, whereas intensively altered material dominated during deposition of the Ravn Member. Different source areas could have been exposed during deposition of the Gert Member than when the Ravn Member was deposited. The Mid North Sea High seems a likely source for the high Th, REE and P₂O₅ content characteristic of the Ravn Member. The Gert Member in the Karl-1 and Hejre-2 wells is not characterised by as high peak amounts of TiO₂ as Gert Member in other wells. The local source that supplied high Cr and TiO₂ during deposition of the Gert Member seems to be less important or more distal in the areas of the Karl-1 and Hejre-2 wells. The depositional environment may also have played a major role in the variation of Mg, as the Ravn Member, in the more distally placed wells, generally has a much higher MgO content than the Ravn Member from the wells characterised by shoreface conglomerate.

The Karl-1 well, as well as the Hejre-2 well, have volcanic material underlying the Upper Jurassic sediments. The volcanic material in the Karl-1 well is Permian and consists of alternating horizons of dominating sodic and potassic compositions. The volcanic material in the Hejre-2 well is characterised by high amounts of K-feldspar, which also has influenced the overlying Upper Jurassic sediments. A volcanic sediment source may also have played an important role in the Karl-1 well, but would have been dominated by alternating sodic and potassic composition rather than the dominating potassic composition found in the Hejre-2 and Jeppe-1 wells. Future petrographic investigations could verify if sodium incorporation in feldspars and clays are correct; and explain whether potassium is related to detrital K-feldspar or clays in the Karl-1 well.