FORMATION AND DIAGENESIS OF BEDDING CYCLES IN UPPERMOST CRETACEOUS CHALKS OF THE DAN FIELD, DANISH NORTH-SEA

Metre-scale lithologic cycles, visible in core and on logs from Maastr ichtian chalks of the Dan Field, were examined to determine their mech anisms of deposition and relation to hydrocarbon production. The lower parts of cycles consist of porous, cream-coloured, largely non-stylol itic, commonly laminated chalk with limited bioturbation (mainly escap e burrows). Cycles are capped by thinner intervals of white to grey, h ard, stylolitic chalk with concentrations of bioclastic material, inte nse burrowing and few preserved primary sedimentary structures. The cy cle caps contain nearly twice as much Mg as compared to the more porou s parts of cycles and also have slightly larger delta(18)O values (-4. 1 parts per thousand for the caps; -4.4 parts per thousand for porous zones). There is a significant reduction of average cycle thickness, a s 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 reflected in lower average porosity and permeability on the NE side of the field. These data indicate that episodic winnowing remove d fine-grained constituents from highstanding northeastern areas. Poro us cycle bases were deposited at relatively high rates that precluded complete bioturbation; preserved laminae, coupled with escape burrows, reflect episodic sediment influx in areas that flank the seafloor hig hs. Cycle tops apparently accumulated more slowly (throughout the regi on, but especially on seafloor highs), perhaps because of reduced prod uctivity of planktic organisms. Slower sedimentation allowed more comp lete bioturbation and destruction of sedimentary structures, and also led to incipient high-magnesium calcite seafloor cementation (sufficie nt to yield firmer sediment and enhanced burrow preservation, but not to form true hardgrounds). Thus, the elevated magnesium contents and r educed porosity of the cycle caps reflect very early diagenetic proces ses that were only partially modified by burial diagenesis. Rates of c halk deposition, as inferred from physical and geochemical evidence, a ppear to be a significant control on reservoir characteristics in Nort h Sea chalks. The highest average porosities and permeabilities are fo und in areas with the highest sediment accumulation rates where seaflo or diagenesis is minimized. Topographic depressions at the time of sed imentation can thus be expected to have the best production characteri stics, and synsedimentary topographic highs should have the thinnest s ections and the poorest petrophysical properties.