HYDRAULIC PROPERTIES OF THE CRYSTALLINE BASEMENT

Abstract

Hydraulic tests in boreholes, up to 4.5 km deep, drilled into continental crystalline basement revealed hydraulic conductivity (K) values that range over nine log-units from 10(-13)-10(-4) m s(-1). However, K values for fractured basement to about 1 km depth are typically restricted to the range from 10(-8) to 10(-6) m s(-1). New data from an extended injection test at the KTB research site (part of the Continental Deep Drilling Program in Germany) at 4 km depth provide K=5 10(-8) m s(-1). The summarized K-data show a very strong dependence on lithology and on the local deformation history of a particular area. In highly fractured regions, granite tends to be more pervious than gneiss. The fracture porosity is generally saturated with Na-Cl or Ca-Na-Cl type waters with salinities ranging from < 1 to > 100 g L-1. The basement permeability is well within the conditions for advective fluid and heat transport. Consequently, fluid pressure is hydrostatic and a Darcy flow mechanism is possible to a great depth. Topography-related hydraulic gradients in moderately conductive basement may result in characteristic advective flow rates of up to 100 L a(-1) m(-2) and lead to significant advective heat and solute transfer in the upper brittle crust.