Characterizing flow pathways in a sandstone aquifer: Tectonic vs sedimentary heterogeneities

Area of Science:

• Hydrogeology
• Sedimentary Geology
• Structural Geology

Background:

• Sandstone aquifers are often assumed to be porous media, but fractures can dominate fluid flow.
• Faults, bedding planes, and joints create preferential pathways for fluids and contaminants.
• Lithified sandstones can exhibit fracture-dominated flow, similar to limestone and crystalline aquifers.

Purpose of the Study:

• To investigate the influence of sedimentary and tectonic heterogeneities on fluid flow in sandstone aquifers.
• To characterize fracture flow in shallow (≤180m) and deep (180-400m) St Bees Sandstone aquifers.
• To compare the hydraulic behavior of the St Bees Sandstone aquifer with other sandstone types.

Main Methods:

• Well test and outcrop-scale studies.
• Hydro-geophysical characterization of the shallow aquifer using fluid temperature, conductivity, and flow-velocity logs.
• Analysis of fault zones, bedding-parallel fractures, and matrix flow.

Main Results:

• Shallow St Bees Sandstone aquifer flow is dominated by fractures, with normal faults and bed-parallel fractures being significant conduits.
• Fault-related open fractures account for approximately 50% of water flow in the shallow aquifer.
• Deeper aquifer sections exhibit lower transmissivities, with matrix flow becoming more dominant due to overburden pressure and mineral infill.

Conclusions:

• Fracture networks, particularly those associated with faults and bedding planes, are critical for understanding fluid and contaminant transport in shallow, lithified sandstone aquifers.
• Sedimentary heterogeneities and overburden pressure significantly influence flow dynamics, with fracture aperture reduction at depth.
• The St Bees Sandstone aquifer's hydraulic behavior contrasts with weaker sandstone types where faults act as flow barriers.