In groundwater hydrology, how are shales classified with respect to porosity and permeability? Consider the typical microstructure of shale (very fine-grained, laminated clayey rock) and state which combination best describes its ability to store and transmit water.

Introduction / Context:
In hydrogeology and water-well engineering, understanding the difference between porosity (ability to store water) and permeability (ability to transmit water) is essential for identifying aquifers and aquitards. Shale, a very fine-grained sedimentary rock, commonly appears in stratigraphic sequences that control groundwater movement.

Given Data / Assumptions:

• Shales are composed largely of clay minerals and silt-sized particles.
• They typically have a laminated structure and very small pore throats.
• No secondary fracturing is assumed unless stated (i.e., intact shale).

Concept / Approach:
Porosity refers to the fraction of void space in a material. Permeability refers to the connectivity and size of pathways that allow flow. Materials with abundant but isolated or extremely small pores can have appreciable porosity but very low permeability. Intact shale is a classic example: water can be stored in microscopic pores but cannot move through easily due to tiny, tortuous, low-conductivity pathways.

Step-by-Step Solution:
Recognize shale’s texture: fine clay-sized particles lead to very small pores.Porosity: present due to voids between particles and within clay platelets.Permeability: extremely low because pore throats are minute and poorly connected.Conclusion: shale is porous but effectively impermeable in intact conditions.

Verification / Alternative check:
Field practice categorizes shale layers as aquitards or confining beds, causing perched water or confining artesian aquifers. Laboratory tests show very low hydraulic conductivity values (often orders of magnitude lower than sands and gravels).

Why Other Options Are Wrong:

• Porous only / Permeable only: each describes one property; shale exhibits porosity but not meaningful permeability.
• Porous and permeable: applies to sands/gravels, not intact shale.
• Neither porous nor permeable: contradicts the presence of micro-porosity in shales.

Common Pitfalls:

• Confusing fractured shale (which can be locally permeable) with intact shale.
• Assuming high porosity always implies high yield; permeability controls yield.

Final Answer:
porous but not permeable