The Geophysical Corner is a regular column in the EXPLORER, edited by Bob A. Hardage, senior research scientist at the Bureau of Economic Geology, the University of Texas at Austin.
Take Your Pick: Skeptic or Proponent
Intriguing seismic examples are being developed in multi-component seismic research at the Bureau of Economic Geology, specifically examples documenting which one of the S-wave seismic modes images a key geologic feature better than does the P-wave mode -- the only seismic mode many explorationists have ever used.
One of those examples is illustrated here.
The prospect is a carbonate Strawn play in West Texas. Traditional P-wave seismic data in the area are good quality, produce accurate structure maps and sometimes, but not consistently, provide reservoir-sensitive attributes across Strawn targets.
At this particular prospect, the Strawn play is stratigraphic, not structural, and traditional P-wave data were having limited success in predicting optimal drill sites. A modest-size 3C3D seismic survey was acquired to determine the value of multi-component seismic data for prospect evaluation.
Figure 1 shows time-structure maps at the reservoir level created from the PP (compressional) and PS (converted shear) modes provided by the 3C3D data. In making these maps, we depth registered the PS data to the PP data before interpreting the PS data volume.
The equivalence of the structural geometry shown by these two maps suggests that the PS data processing has been done well and that the PP-to-PS depth registration is reasonably accurate across the reservoir interval.
Superimposed on the maps are existing wells, both producers and non-producers, showing that there is no obvious relationship between structure and producing facies. Some producers are lower on the structure than are non-producers.
Drilling targets are thus controlled by stratigraphic conditions, not by structure.
Figure 2 shows one amplitude attribute (rms amplitude) extracted from narrow windows spanning the reservoir interval across the PP and PS data volumes.
In other areas, PP amplitude attributes have successfully delineated productive carbonate reservoirs in the Strawn; here, PP reflectivity is not definitive, as an inspection of figure 2a shows.
PP reflection amplitudes are random in nature, and amplitudes at non-producer wells look like PP amplitudes at producing wells. In contrast, PS reflection amplitude appears to react to productive and non-productive reservoir conditions (figure 2b). The PS data show a sinuous, high-amplitude anomaly (yellow/red) that reasonably segregates producing wells from non-producers.
This reservoir facies is a low-porosity carbonate unit; porosity ranges from 1 to 7 percent across the prospect, and minimum productive porosity is 4 percent. Detecting the narrow porosity range between non-productive facies (1 to 3 p.u.) and productive facies (4 to 7 p.u.) is beyond seismic sensitivity for both the PP mode and the PS mode.
Rather than using seismic data to segregate areas of productive porosity from areas of nonproductive porosity, interpreters try instead to use seismic attributes to find maximum reservoir thickness. They then cross their fingers and hope that zones of favorable porosity will be found across intervals where there is maximum unit thickness.
In this instance, the predictive value of PS reflection amplitude was tested by drilling well AL-1, labeled on the PS map (figure 2b). This well found the thickest reservoir facies (122 feet) of all the wells shown on the maps. In other wells, the reservoir interval ranged from 80 to 111 feet.
From the standpoint of reservoir thickness, this project supports the use of multi-component seismic technology for carbonate strat-trap exploration, because the PS data defined a maximum-thickness reservoir interval when conventional PP seismic data could not.
However, even though well AL-1 penetrated a maximum-thickness reservoir unit, insufficient productive porosity occurred across the interval to make the well commercial. This AL-1 well falls into that famous category many call “technical success but economic failure.”
Skeptics can say that multi-component seismic data did not yield a productive well. Proponents can say that the PS mode delivered exactly what was needed -- a definition of the maximum thickness of the reservoir.
Take your pick: skeptic or proponent.
The real message is that at this prospect, the PS mode provided vital reservoir information that the PP mode could not.
Acknowledgment: This research was funded by DOE/NETL.