Worldwide growth in multiple uses of the subsurface, as well as environmental concerns related to these activities have increased the need for better understanding of the subsurface, including its hydraulic properties and groundwater flow conditions, at greater depths than those conventionally studied for water supply purposes. Clay-rich sedimentary sequences may be barriers for fluid flow, and as such of key importance for evaluating their sealing capacity for e.g. geological storage of water, energy and wastes, and for hydrocarbon accumulations at these greater depths. In the Netherlands, data and information from oil and gas wells and petroleum-related studies at these greater depths provide direct measurements of properties of reservoirs and fluids therein (porosity, permeability, pore pressures), but not of clay-rich sequences.
There are a number of indirect local and regional data and methods available, though, that can be used to estimate pore pressures or hydraulic heads in clay-rich sequences, such as pressures in adjacent or enclosed permeable units, and methods using mud weights, wireline logs, seismic interval velocities, and (basin)modelling. The integrated application of these methods to estimate pore pressures will be illustrated with case studies of Paleogene clay-rich sequences in onshore and offshore Netherlands. Basin modelling and knowledge on the regional geological setting are used to interpret the assessed variation in pore pressure distribution and the role of the clay-rich sequences in the regional groundwater flow system.
The focus of an onshore Dutch case study concerns the assessment of the spatial variation in porosity and permeability of the Oligocene Rupel Clay Member (~ Boom Clay). The Rupel Clay is present across the country at burial depths from near surface to 1500 m. Porosity and permeability or hydraulic conductivity measurements of the Rupel Clay are limited to measurements at shallow depths of about tens of meters below surface. Grain sizes measured in samples of the Rupel Clay from different locations spread across the country were used to generate new porosity and vertical permeability data at greater burial depths. Effective stress and clay content are important parameters in the applied grain-size based calculations. The spatial variation in lithology, heterogeneity, and also burial depth, is apparent in the variation of the calculated permeability. The samples in the north of the country consist almost entirely of muds with vertical permeability of less than 8.3E-19 m2 (8.3E-12 m/s). The vertical variation in permeability in the more heterogeneous Rupel Clay Member in the southern and east-southeastern part of the country can reach several orders of magnitude due to increased permeability of the intercalated coarser grained sandy-mud and muddy sand layers. The calculated porosity and permeability values were used as input in large-scale groundwater flow modelling to study groundwater flow velocities across the Rupel Clay.