Large scale hot water migration systems around salt diapirs in the Danish Central Trough and their impact on diagenesis of chalk reservoirs

Jørgen Jensenius, Niels Munksgaard

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Fracture filling calcite from the piercement fields of East Rosa, Skjold, Rolf, North Ame, Nils and Vagn (directly underlain by salt diapirs) and the dome field Dan (overlying a deep seated salt structure) have been studied by fluorescence and cathodoluminiscence microscopy, fluid inclusion microthermometry and carbon and oxygen isotopes. The carbon and oxygen isotope compositions of the reservoir matrix chalk have also been measured. The temperature estimates obtained from fluid inclusion data suggest that the fracture filling calcite of the piercement fields precipitated during hot water flushing of the reservoirs. The flushing system only existed around the salt diapirs and was probably related to expulsion of overpressured fluid from the surrounding sediments. The thermal anomaly and the faults associated with the diapirs probably were important factors in focusing the ascending water. By combining fluid inclusion thermometry with O isotope data for the fracture filling calcites ranges of O isotopic values of the flushing water are deduced: in Skjold, Rolf, North Ame, Nils and Vagn: d18O = - 1 to +7 per mil SMOW, values typical of sedimentary porewater of sea water origin; in East Rosa: d180 = -4 and +4 per mil SMOW, which combined with data on fluid inclusion salinities suggest that the flushing waters were responsible for substantial salt dissolution. Lower d180 values with shallower depth of burial suggest that the porosity of chalk in the shallower fields was more severely reduced by calcite cementation during the flushing event than in the deeper fields. In the case of North Ame, Nils, Vagn and
Dan, d13C values of the fracture filling calcites are similar to those of normal chalk (0.5 to 3.5 per mil). However, calcites from Rolf, Skjold and East Rosa are depleted by - 1.8, -6.2 and - 16.7 per mil, respectively. Only in East Rosa is the matrix chalk itself depleted in 13C (range: -2.0 to +0.7 per mil). The low d13C values are interpreted as the result of biodegration. Sulphate originating from the underlying diapirs was flushed into the reservoir and used by bacteria to degrade present hydrocarbons. This process produced 13C depleted bicarbonate which was incorporated into the fracture-filling calcites.
Original languageEnglish
Pages (from-to)79-88
Number of pages10
JournalGeochimica et Cosmochimica Acta
Issue number1
Publication statusPublished - Jan 1989
Externally publishedYes


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