The relationship between porosity and burial depth of sandstone reservoirs is complex. It depends on primary porosity, which is related to grain-size and depositional process and also on the porosity gradient (the change in porosity with depth) which, in most cases, is linear.
This relationship may be expressed as: φd =φp − G.D, where φd is the porosity at depth D below surface, φp is the primary depositional porosity, and G is the porosity gradient in % porosity per 1000 ft (305 m). Porosity gradients decrease with increasing quartz content, abnormal pressures, and in the presence of hydrocarbons. They increase with increasing geothermal gradient. There are at present insufficient data to quantify these relationships.
Empirically derived porosity gradients for North Sea Rotliegendes, Jurassic and Palaeocene reservoir sandstones are 2.2, 2.6 and 2.9 per cent per 1000 ft (305 m) respectively.
When calibrated with the porosity spectra of the various facies, graphs can be drawn showing the depth: porosity windows within which economic reservoirs may be found.