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: 1800-4000 m/s, the fastest being old, well-compacted shales.
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 ( $\rho$ , 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: $\rho \approx 0.23\,V_{\mathrm P}^{0.25}$ with $V_{\mathrm P}$ in ft/s and $\rho$ in g/cm³, which is $\rho \approx 0.31\,V_{\mathrm P}^{0.25}$ with $V_{\mathrm P}$ in m/s. It is an average trend, not a law: brine sands and carbonates sit close to it, while salt, coal and gas sands are far lighter than it predicts. Used with care, 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

$Z = \rho\,V_{\mathrm P}$

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 Section 1.2, but the short version is: for a wave arriving straight down on a flat contact (normal incidence), the reflection coefficient is

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

Here $Z_1$ is the impedance above the contact and $Z_2$ the impedance below it. The sign of $R$ matters. Positive $R$ (impedance increases downward across the contact) means the reflection keeps the polarity of the source pulse, a peak on SEG normal polarity. Negative $R$ (impedance drops, e.g., a gas sand beneath a shale) flips it into a trough. That sign is what lets interpreters distinguish a "hard" reflector (carbonate below shale) from a "soft" one (gas sand below shale).

In Figure 0.3, put one rock of the book’s catalog above a flat contact and another below it, and read how the change in impedance sets the size and the sign of the reflection.

Every rock this book uses is on the crossplot in (a), and three things are worth reading off it. The families overlap: medium shale sits among the sandstones, and its impedance, 7105 (m/s)(g/cm³), falls between oil sand’s 6820 and brine sand’s 7590, which is why a rock’s name does not decide the reflection. The cloud trends up and to the right along Gardner’s grey curve, though halite, coal and gas sand sit well above and to the left of it: they are lighter than their velocity predicts. And the dashed curves are contours of constant impedance, so two rocks on the same contour give no reflection whatever their names: dolomite over anhydrite, a carbonate on an evaporite, returns $R$ = +0.002 (exercise 2). Now keep medium shale on top and change rock 2 from brine sand to gas sand: the same sandstone with a different pore fluid turns a weak peak, $R$ = +0.033, into a trough three times as strong, $R$ = −0.106.

Fluids change everything

Two rocks that are lithologically identical can have very different impedances if one contains water and the other contains gas. Gas lowers both velocity and density, so a gas-filled sandstone is typically much softer (lower impedance) than the same sandstone filled with water. In the book’s catalog, gas takes the sandstone from 3300 to 2800 m/s and from 2.30 to 2.05 g/cm³, a 24% drop in impedance. 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.