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Refining the Chalk Group Stratigraphy of the Danish Central Graben

Abstract

After more than forty-five years of hydrocarbon exploration and production, the Cretaceous Chalk Group in the Danish North Sea is still an attractive target. Most of the production is from four-way dip closures in the Danian Ekofisk Formation and the Maastrichtian Tor Formation. On a field-scale their stratigraphy is well understood. The Chalk Group, however, consists of more than these two high-flyers and it is in this remainder of the Chalk where additional exploration targets may be found. To help find these targets the stratigraphic framework for the Chalk Group has been updated and is being refined. We used chemostratigraphy and detailed SEM petrography of cuttings and core from eight wells and integrated this with a recently established seismic stratigraphic framework. In this framework, which spans a large part of the Danish Central Graben, the eight wells provide a reference section. Using seismic morphology as context, our observations from cores further confirm that chalk sedimentation was dynamic. Large facies variability including coccolith-debris dominated chalk, lithoclastic chalk and foram-packstones indicate that besides gentle pelagic settling, mass-wasting and strong bottom currents were common. Silica-altered hardgrounds with sponge borings further suggest the existence of current swept areas where marine cementation stabilized the substrate. This variability in depositional facies and early diagenetic alteration affected reservoir and seal properties as is indicated by the variability in poretypes found in different facies. Geochemistry further refines the stratigraphic framework beyond seismic and biostratigraphic resolution. The carbon isotopic signal appears to be well preserved and can be correlated with published on-shore U.K. reference sections. Major and minor trace elements provide detail of detrital input of K- and Al rich clays. Si-elemental variations, in lieu of these detrital markers, indicate the importance of silica-diagenesis in the chalk. SEM petrography indicates that this silica is commonly present as sub-micron scale spherical aggregates, which precipitated during or soon after deposition of the chalk. Based on the integration of multi-scale data we are refining the Chalk stratigraphy. With this framework we are defining new play concepts which will help in unlocking the remaining hydrocarbon potential of the Danish Central Graben and the North Sea Basin.