In general, the quality of conventional P-wave seismic data is poor when data are acquired across areas where high-velocity rocks (primarily carbonates and basalts) form the exposed, first-layer of the Earth.
Some basins that have high-velocity rocks exposed at the surface have deeper layers with good oil/gas potential. Examples would include:
- Large areas of Argentina, Paraguay and Brazil (basalt outcrops).
- The Val Verde Basin and other areas of West Texas (carbonate outcrops).
Numerous other carbonate-covered and basalt-covered exploration areas could be listed.
Explorationists working in these high-velocity outcrop areas are frustrated by their inability to acquire seismic data that have signal-to-noise character sufficient to see and map deeper hydrocarbon plays.
This month’s article is the first of a two-part series that will examine some principles of seismic imaging in areas where the seismic propagation velocity in the shallowest Earth layer is greater than the velocity in the layers immediately below the surface layer.
In this first article, we consider the question “Does the downgoing compressional (P) wave successfully penetrate a high-velocity surface layer and illuminate deeper targets?”
A generalized picture of the geology that needed to be imaged in one basalt-covered area is shown as figure 1.
The Earth surface here was covered by a thick basalt layer characterized by a fast seismic velocity, a rough surface and numerous large internal voids. Normal siliciclastic and carbonate rock layers existed below this exposed basalt. The seismic propagation velocities in these deeper shale, sandstone and carbonate rocks were less than the propagation velocity in the basalt.
Oil production had been established across this particular area by random drilling, without the aid of seismic data, because conventional P-wave seismic data were too noisy to define drilling targets.
Because random drilling is not an efficient or cost-effective option for developing a prospect, the operator decided to acquire offset-source VSP data in several wells to attempt to image across interwell spaces and to develop a better exploration model.
VSP data acquired in one well are displayed as figure 2 after considerable data processing has been done to isolate downgoing and upgoing P and S (shear) wave modes.
The seismic source was a vertical vibrator offset about one kilometer from the well – the same source used in several failed attempts to acquire usable surface-based P-wave data across the area.
These VSP data show several important facts, namely:
- A robust downgoing P wave (center panel), as well as a strong downgoing SV wave (left and right panels), travels through the deep, slower-velocity layers.
All doubts are removed about the possibility that the downgoing source wavelet does not penetrate the surface basalt layer and illuminate deeper geology. A good-quality illuminating wavelet reaches all target depths.
- Good-quality upgoing P-wave (left panel) and converted-shear (SV) reflections (center panel) are generated at several deep interfaces, including interfaces associated with critical reservoir intervals.
At this point we know that the deep geology has been illuminated and that reflection events from our primary targets head back toward the Earth’s surface. Yet these reflections cannot be recognized by surface-positioned receivers.
We appear to have isolated the imaging problem to something that occurs in the local vicinity of the surface receivers.
Why is the signal lost when it arrives at the surface?
Next month: A look at what appears to be the cause of this poor data quality – and one option for resolving the imaging dilemma.