Rocks 101: the three families we meet

Prerequisites

Learning objectives

Distinguish sandstone, shale, and carbonate as acoustic media
Understand how porosity and fluid fill change velocity and density
Compute acoustic impedance and predict which boundaries reflect strongly
Develop a first intuition for what a seismic reflection “looks like” for a given geology

Seismic interpretation is fundamentally a rock problem. What we call a reflector is a surface where the product of density and velocity changes — and that product, as we will see, depends entirely on what the rock is and what is in its pores.

The sedimentary-rock cast of characters

Nearly every exploration or production setting we will interpret is made of sedimentary rocks — rocks that formed by deposition and burial of sediment. Three families cover most of the ground:

Sandstone — grains of quartz (mostly) cemented together, with variable pore space between the grains. Porosities of 5–30% are common. Sandstones host most of the world’s conventional oil and gas because they can store fluid in their pores. Typical P-wave velocity: 2500–5000 m/s depending on burial depth and cementation.
Shale — fine-grained, clay-rich rock formed from mud. Usually has tiny pores but very low effective permeability (fluid cannot flow easily), so it is often a seal — the rock that traps hydrocarbons against escaping upward. Typical P-wave velocity: 1800–3500 m/s.
Carbonate — limestone (CaCO₃) and dolomite (CaMg(CO₃)₂). Often formed in warm shallow seas from reef-building organisms and chemical precipitation. Can be extremely porous (reefs, vuggy carbonates) or extremely tight (chalk, massive limestone). Typical P-wave velocity: 3500–6500 m/s.

These are ranges, not certainties. A young, shallowly-buried sandstone at 500 m depth might have a velocity of 2500 m/s; the same sandstone compacted at 4000 m depth might be at 4500 m/s. Depth and compaction matter as much as rock type. A useful working rule: deeper rock tends to be faster and denser.

Density follows velocity (roughly)

Density (ρ, in g/cm³) and P-wave velocity tend to correlate in sedimentary rocks — faster rocks are usually denser. A famous empirical relation is Gardner’s equation: $\rho \approx 0.23 \cdot V_p^{0.25}$ (with $V_p$ in ft/s and $\rho$ in g/cm³). It is not exact, but it gives you a density estimate from a velocity log when you lack a density log.

Now the concept that ties all of this to seismic: acoustic impedance.

Acoustic impedance = density × velocity

$Z = \rho \cdot V_p$

Acoustic impedance $Z$ is the property that matters at a boundary. A seismic reflection happens when $Z$ changes across a boundary; the magnitude of the reflection depends on how big that change is. Two rocks with identical impedance produce no reflection at all, even if they are completely different lithologies. Two rocks with very different impedance produce a bright reflection even if they are the same lithology on either side of (say) a fluid contact.

We will derive the reflection coefficient in §1.2, but the short version is: the reflection strength is approximately

R \approx \dfrac{Z_2 - Z_1}{Z_2 + Z_1}

The sign of R matters. Positive R (impedance goes up going into the reflector) means the reflection keeps the same polarity as the source pulse. Negative R (impedance drops — e.g., a gas sand beneath a shale) flips the polarity. That sign is what lets interpreters distinguish a "hard" reflector (carbonate below shale) from a "soft" one (gas sand below shale).

Fluids change everything

Two rocks that are lithologically identical can have very different impedances if one contains water and the other contains gas. Gas substantially lowers velocity and density — a gas-filled sandstone is typically much softer (lower impedance) than the same sandstone filled with water. This is the reason reflection amplitude can indicate the presence of hydrocarbons in certain conditions (a topic we will cover carefully in Part 5 under AVO and DHI — direct hydrocarbon indicators).

For now the takeaway is: the rock and what is in its pores together determine the impedance.

References

Mavko, G., Mukerji, T., & Dvorkin, J. (2009). The Rock Physics Handbook (2nd ed.). Cambridge University Press.
Castagna, J. P., Batzle, M. L., & Eastwood, R. L. (1985). Relationships between compressional-wave and shear-wave velocities in clastic silicate rocks. Geophysics, 50(4), 571–581.
Sheriff, R. E., & Geldart, L. P. (1995). Exploration Seismology (2nd ed.). Cambridge University Press.
Bacon, M., Simm, R., & Redshaw, T. (2003). 3-D Seismic Interpretation. Cambridge University Press.