Formation and diagenesis of bedding cycles in uppermost Cretaceous chalks of the Dan Field, Danish North Sea

Abstract

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.

Prokaryotic Community Structure and Sulfate Reducer Activity in Water from High-Temperature Oil Reservoirs with and without Nitrate Treatment

“…The crude oil contained in the Dan and Halfdan formations is situated about 2,000 m below the surface. Oil production at Dan commenced in 1972, and oil and natural gas are produced from low-permeability chalks of the latest Cretaceous as well as Paleocene (46). The Halfdan gas and oil accumulation was discovered in 1998 by drilling an ϳ9-km-long horizontal well from Dan to Halfdan (26).…”

How depositional texture and diagenesis control petrophysical and elastic properties of samples from five North Sea chalk fields

“…The depth scale is in km below mean sea-level (msl). Andersen (1995), Kristensen et al (1995), Scholle et al (1998), Damholt & Surlyk (2004). b Doyle & Conlin (1990).…”

“…The apparent lamination seen in Figure 3d is interpreted to reflect low-angle fractures; however, other researchers have interpreted presumably similar structures differently. Scholle et al (1998) They interpreted winnowing processes to have caused the lamination together with periods of rapid deposition as evidenced by the presence of escape burrows. Damholt & Surlyk (2004) interpreted the biogenic structures as central parts of Zoophycos burrows rather than escape burrows.…”

Geostatistical Scaling Laws Applied to Core and Log Data

“…The section shows cyclic porosity variations probably caused by climatic variations during deposition of the pelagic chalk material 11 . The core measurements represent a volume of about 5x2x2 cm (vertical resolution 0.02 m).…”