Is the Bright Spot Gas?
Learning objectives
- State the DHI question as a risk problem: a bright reflection is necessary for gas but nowhere near sufficient
- Anchor the numbers on the soft sand at porosity 0.25: brine impedance 6.47, and a fizz-water case with only 10 percent gas that drops the impedance 19.2 percent while full gas drops it 26.9 percent
- Read the Wood-collapse lesson: a whisper of gas makes almost the whole bright spot, so amplitude cannot separate fizz from pay, and even the velocity ratio barely splits them (1.55 against 1.48)
- Assemble the interpreter's checklist: what else brightens a reflection and what independent evidence corroborates a genuine hydrocarbon fill
The Reflection That Started an Industry
A bright spot is a locally strong seismic reflection, and for fifty years it has been read as a possible direct hydrocarbon indicator: gas in the pores softens a sand, drops its impedance, and lights up the top-reservoir reflection against the shale above. The physics is real. Take the standard soft quartz sand of this course at porosity 0.25 and 20 MPa, brine-filled: it carries an acoustic impedance of about and a velocity ratio near 1.91. Fill those same pores with gas and the impedance falls to 4.74, a 26.9 percent drop, and the ratio crashes to 1.48. That is a genuine, large, gas-driven amplitude change, and drilling it can find a field.
It can also find nothing worth producing. The whole discipline of DHI risking exists because the bright spot is a necessary consequence of gas that is very far from a sufficient proof of it. This section runs the rock physics toolkit at the reflection to show exactly why the amplitude alone cannot be trusted, and what has to stand beside it before the prospect is drilled.
The Fizz-Water Trap
The sharpest failure is fizz water: a sand carrying only a low, uneconomic gas saturation, a few percent to a tenth. Intuition says a little gas should give a little dimming. The rock says otherwise. Mix 10 percent gas uniformly into the brine and the pore fluid modulus is not a 90-10 blend of the two; it collapses toward the gas value along the Wood average of Part 3.5, because the soft gas dominates the reciprocal mixing. Feed that fizz fluid to the sand and the impedance drops to 5.23, a 19.2 percent fall. A tenth of the pore space holding gas has already produced two-thirds of the amplitude change that a full gas fill would give. The bright spot is almost entirely built by the first whisper of gas.
This is the trap in one number. Amplitude cannot tell fizz from pay, because both look bright: 19.2 percent dimming for worthless residual gas against 26.9 percent for a commercial column is not a distinction a seismic amplitude, riddled with tuning and overburden and processing effects, can be counted on to resolve. Worse, the velocity ratio barely helps. The fizz sand reads and the full gas sand 1.48; both plunge from the brine value of 1.91, and the gap between them is small. Saturation discrimination is intrinsically weak in the rock, not merely hard in the data. That is the physical reason DHIs are risked, not trusted.
The Interpreter's Checklist
Because the amplitude is ambiguous at both ends, a bright spot is worked as a risk case with a checklist, not a verdict. First, ask what else brightens a reflection: a hard streak such as a tight carbonate or a volcanic sill, a lithology contrast, tuning where two interfaces interfere constructively at a particular thickness, or exactly the low-saturation fizz gas above. Any of these can counterfeit the pay signature. Then look for the independent evidence that a real trapped column leaves and fizz does not: does the amplitude conform to structure, brightening down to a common contact and stopping there; is there a flat spot, a horizontal fluid-contact reflection cutting across the dipping bedding; and does the amplitude-versus-offset behaviour match the expected AVO class for that depth and rock. No single item is proof. A bright spot that conforms to structure, shows a flat spot, and carries the right AVO response is a strong lead; one that does none of these is a hard streak or a fizz until proven otherwise. The next section takes a different disguise, a sand that looks tight on velocity yet holds a third of its volume as porosity, and shows how a single transform can screen out real pay.
References
- Han, D., & Batzle, M. L. (2004). Gassmann's equation and fluid-saturation effects on seismic velocities. Geophysics, 69(2), 398-405.
- Avseth, P., Mukerji, T., & Mavko, G. (2005). Quantitative Seismic Interpretation. Cambridge University Press.