| Abstract |
A key parameter in assessing subsurface geothermal potential is the flow rate of hot water at depth. Flow rates are difficult to predict but have a straight-forward relationship to permeability and porosity. Though porosity variations are obtained from well logs, other methods have to be utilised for estimating the permeability, and commonly porosity−permeabi-lity correlations based on conventional core analysis are applied. However, porosity−permeability plots generally show large scatter, and deviations from a well-defined porosity−permeability trend are often observed. It is such deviations that we aim to understand in terms of texture, facies variations and diagenetic processes. The Upper Triassic – Lower Jurassic Gassum Forma-tion in Denmark has been chosen for the investi-gation of the porosity and permeability relationships. The Gassum Formation forms the most extensive reservoir for geothermal exploitation in Denmark. Furthermore, the formation is the primary reservoir for the most recent geothermal wells. It is dominated by extensive shoreface sandstones with minor intervals of estuarine and fluvial sandstones, marine and lacustrine mudstones. For the Gassum Formation, the overall porosity−-permeability relationship can be expressed by a power function. More well-defined trend lines appear, however when grouping the sediment according to grain size. Parameters such as grain size, sorting, grain shape, mineral composition, clay content and diagenetic cements all influence the permeability, and the aim is to determine to what degree they can explain the deviations from the overall porosity−permeability trend. Multivariate analyses are applied to identify the parameters that have the greatest influence on porosity and permeability. The outliers are investigated petrographically in order to find explanations for the deviation from the typical porosity−permeability trend. Micro-scale anisotropy is one factor that is found to have an effect on permeability. The permeability difference between horizontal and vertical plugs is typically due to lower permeability perpendicular to the bedding, for example due to lamination in the sediment. Micro-scale anisotropy, probably due to enlargements of small depositional variations by cementation along relatively minor fluid boundaries, also seems to have a strong influence on permeability in horizontal direction |