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Metre-scale lithologic cycles, visible in core and on logs from Maastrichtian chalks of the Dan Field, were examined to determine their mechanisms of deposition and relation to hydrocarbon production. The lower parts of cycles consist of porous, cream-coloured, largely non-stylolitic, commonly laminated chalk with limited bioturbation (mainly escape burrows). Cycles are capped by thinner intervals of white to grey, hard, stylolitic chalk with concentrations of bioclastic material, intense burrowing and few preserved primary sedimentary structures. The cycle caps contain nearly twice as much Mg as compared to the more porous parts of cycles and also have slightly larger d 18 O values (A4á1& for the caps; A4á4& for porous zones). There is a signi®cant reduction of average cycle thickness, as well as total thickness of the Maastrichtian chalk section, from SW to NE across the Dan Field. The cycle thinning largely results from a reduced thickness of porous chalks from the lower parts of cycles and thus is re¯ected in lower average porosity and permeability on the NE side of the ®eld. These data indicate that episodic winnowing removed ®ne-grained constituents from highstanding northeastern areas. Porous cycle bases were deposited at relatively high rates that precluded complete bioturbation; preserved laminae, coupled with escape burrows, re¯ect episodic sediment in¯ux in areas that¯ank the sea¯oor highs. Cycle tops apparently accumulated more slowly (throughout the region, but especially on sea¯oor highs), perhaps because of reduced productivity of planktic organisms. Slower sedimentation allowed more complete bioturbation and destruction of sedimentary structures, and also led to incipient high-magnesium calcite sea¯oor cementation (suf®cient to yield ®rmer sediment and enhanced burrow preservation, but not to form true hardgrounds). Thus, the elevated magnesium contents and reduced porosity of the cycle caps re¯ect very early diagenetic processes that were only partially modi®ed by burial diagenesis.Rates of chalk deposition, as inferred from physical and geochemical evidence, appear to be a signi®cant control on reservoir characteristics in North Sea chalks. The highest average porosities and permeabilities are found in areas with the highest sediment accumulation rates where sea¯oor diagenesis is minimized. Topographic depressions at the time of sedimentation can thus be expected to have the best production characteristics, and synsedimentary topographic highs should have the thinnest sections and the poorest petrophysical properties.

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