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Table 1 Factors influencing porosity and permeability in the Bunter Sandstone and Gassum formations

From: Pre-drilling assessments of average porosity and permeability in the geothermal reservoirs of the Danish area

Factors Depth <2500 m Depth >2500 m
Depth (max. burial depth) Bunter Sandstone Fm Mechanical compaction. Core data represent a narrow depth interval
Gassum Fm Mechanical compaction. The porosity reduction with depth is comparable to the porosity estimated from a mechanical compaction curve. Depth is an important factor, when dealing with permeabilities of shallowly buried sandstones
Bunter Sandstone Fm No core data
Gassum Fm Chemical compaction. The porosity reduction with depth may be higher than indicated by a mechanical compaction curve. Permeability is not that depth dependent
Grain size Bunter Sandstone Fm Porosity reduction is highest for very fine-grained sandstones and less for coarser-grained sandstones. Increasing grain size leads to higher permeabilities
Gassum Fm Limited influence on porosity. Increasing grain size leads to higher permeabilities
Gassum Fm The porosity reduction is highest for very fine-grained sandstones and less for coarser-grained sandstones
With respect to permeability, grain size has less influence than at shallow depths. Increasing grain size still leads to higher permeabilities, however
Detrital clay Bunter Sandstone Fm Presence of inter-granular clay and clay clasts reduce porosity, but not substantially since much microporosity is present within the clays. Even small amounts of inter-granular clay reduce permeability considerably, whereas larger amounts of clay clasts are needed to produce a similar reduction in permeability
Gassum Fm Presence of inter-granular clay and clay clasts reduce porosity. Detrital clays and/or clay clasts are often present. Clays and clay laminae lower the permeability, since some of the pore throats are very narrow
Gassum Fm The amount of clay increases with depth, but it has only minor effect on porosity, since the clay grows on the expense of other minerals. In addition, clay clasts increase the effect of compaction
High amounts of detrital clays result in reduced permeability
Cement Bunter Sandstone Fm Pervasive carbonate, anhydrite or halite cement may reduce porosity significantly, whereas patchy carbonate cement does not have a notable effect on porosity. Pervasive carbonate, anhydrite or halite cement occludes pores and thus prevents fluid flow. However, most commonly the cement is patchy and has only limited effect on fluid flow
Gassum Fm Siderite and calcite cement occasionally result in larger porosity reduction than mechanical compaction. Presence of siderite cement leads to a marked reduction in permeability
Gassum Fm Pronounced porosity reduction with depth due to the presence of quartz and/or ankerite cement
The permeability is markedly reduced where authigenic illite is present. Kaolinite has limited reducing effect on permeability. The permeability is primarily reduced by quartz and ankerite cement
Coatings Bunter Sandstone Fm Sandstones with thick iron-oxide/hydroxide coatings are characterized by high permeability. The coatings may also preserve porosity
Gassum Fm Sandstones with chlorite coatings have high permeability, unless other cement types are present
Gassum Fm Chlorite coatings may preserve porosity and permeability
  1. Overall, the porosity distribution depends on the depositional environment and the maximum burial depth. Similarly, the permeability is strongly related to the depositional environment, which controls the distribution of grain sizes, the abundance of detrital clays, and the amount/type of cementing minerals etc. Based on information from Olivarius et al. (2015a) and Weibel et al. (submitted)