Introduction

Geophysicists exploring for over-pressured tight gas sands (OP-TGS) in the Rocky Mountain region have not possessed the variety of seismic risk reduction tools that have been successfully utilized in higher porosity sandstones environments. Direct hydrocarbon indicators (DHIs) are one group of seismic tools used to reduce drilling risk in higher reservoir porosity environments. The literature is rich with theoretical treatises and case histories on DHIs. Residual Velocity Attribute (RVA) derived from seismic velocities has been used with varying degrees of success for natural gas exploration in the OP-TGS environments (Surdam et al, 2000, 2003). While RVA holds a promise of providing a risk reduction tool for OP-TGS exploration, literature coverage is sparse. A case history is presented on applying RVA to the exploration process in the Hoback Basin of western Wyoming. This case history emphasizes detailed well ties to the RVA.

Determination of fracture density and orientation through application of azimuthal velocity attributes such Vfast, Vfast-Vslow and Vfast azimuth is also discussed. These azimuthal velocity attributes are tied to oriented dipole sonic logs and bore hole breakout data. Top of pressure, top of gas and fracture orientation and density are inter-related prospect elements that must be considered together to understand the OP-TGS play system.

Detection of Natural Gas and Pressure in Hard Rocks: Where Biot-Gassman Falls Short, Fractures Take Over

Both AVO and conventional seismic velocity analysis for pressure are considered marginal tools from a theoretical standpoint in the harder rocks of the Rocky Mountain region. Many of the target zones have been either deeply buried or have undergone diagenesis and uplift. However, if the rock is “geophysically fractured”, the rules for fluid detection are changed from those requiring spherical grains and equant pores. A fractured rock becomes compliant to compressional waves across the fractures because incompressibility (k) becomes low, which drives the p-wave velocity down considerably. Gas in the pores, rather than an incompressible fluid (water), amplifies this effect allowing one to detect gas columns and thus indirectly pressure in fractured rocks.

The density and orientation of the fractures in the rock volume can change the p-wave velocity. The velocity change is azimuth dependent. When the seismic waves travel parallel to the open fractures, the seismic wave passes through the un-fractured rock, “seeing” the host rock velocity. The rock velocity is decreased when the seismic waves travel perpendicular to the open fractures. Again gas-filled fractures rather than water filled fractures amplify the observed effects.

The Residual Velocity Attribute exploits this decrease in velocity caused by gas filled fractures. In most basins the shale velocity increases with depth. This velocity increase reflects the normal compaction trend as the shales are buried. It has been observed in both unconsolidated and consolidated sediments that the shale velocity decreases from the normal compaction trend at the onset of overpressure. The shallow shale seismic interval velocities are calibrated to nearby well control. They are then projected to depth forming the shale regional compaction trend. The actual seismic interval velocities are subtracted from the regional trend resulting in the Residual Velocity Attribute (RVA). A normal compaction trend is present when the RVA is at or near zero. A potential over-pressured interval is indicated when the RVA is negative. The obvious caveat to using these data is the lithology effect overprint. Other large variables are fracture density and direction of fracturing with respect to the seismic acquisition direction. These variables may create confusion when using narrow azimuth data and 2-D data for analysis.

Seismic Data Sets

Approximately 120 miles of 2D and 430 mi.² of 3D data have been processed for RVA in the Hoback Basin. While the 2D data has proved to be useful for reconnaissance evaluation, it does not provide the same quality well ties as does the 3D data for the reasons discussed above. The 2D processing is relatively robust in relation to the acquisition far offset distance. Even though depth to target is in the 6,000 to 13,000 ft range, seismic data with 8,000 ft. maximum offset has given viable results. Older vintage data with a maximum offset of 3,000 ft. has not given reasonable results.

Azimuthal velocity processing shows that this area does exhibit fairly high anisotropy. While the 3D data was processed using azimuthal velocities, the RVA was calculated using an isotropic velocity field. The isotropic velocity field is essentially an average of the Vfast and Vslow derived from the anisotropic velocity analysis. The data density of 3D seismic helps to reduce the error range in calculating the RVA. Seismic velocities are calculated on a 3x3 super-bin every three CDPs. Vertical sampling is every 200 milliseconds. The dense spatial sampling and more accurate velocity picking leads to a higher resolution attribute than what has previously been available.

Top of Gas/Pressure from Mud Logs and Mud Weights

Due to difficulty in securing mud logs, mud weights are often used as an indicator for top of pressure. Mud weights are readily available from state records or drilling contractors. Onset of 9.5 lbs./gal. mud is normally used as the top of pressure indicator. However, a comparison of the 9.5 lbs./gal. mud onset depth and the sustained gas shows or flare depth reveals that there may not be good agreement. The onset of 9.5 lbs./gal. mud can be substantially above the top of sustained gas shows, especially for wildcat wells. This is due to the nature of drilling wildcat wells. For well control and safety reasons, the drilling mud can be weighted up early in anticipation of encountering pressure, even though pressure has not yet been reached. Therefore, the top of 9.5 lbs./gal. mud can be misleading and produce a poor correlation with the top of gas/pressure RVA indicator. A better tie with the RVA indicator is achieve by using the depth where the total gas curve on the mud log shows sustained gas shows in a 9.5 lbs./gal. mud environment.

Top of Gas Verses Top of Pressure

Regional comparison of the total gas curve with the RVA shows that there are two potential conditions at the top of a negative RVA anomaly. The first condition is the top of the gas transition zone. The transition zone is characterized by a negative RVA, gas shows where the strength of the show is dependant on the mud weight and mud weights are below 9.5 lbs./gal. In a basin centered gas environment, top of pressure is usually found in a relatively short distance below the top of the transition interval. This top of gas transition interval is associated with uneconomic rates of gas production. The second condition is top of gas and pressure. The top of gas/pressure is characterized by a negative RVA, sustained gas shows and flare that are difficult to suppress with increased mud weight and the mud weight is above 9.5 lbs./gal (see Figure 1). The RVA alone cannot discriminate between top of gas transition and top of gas/pressure. Other structural elements must be understood to identify which condition is present at a negative RVA anomaly.

Conclusions

The Residual Velocity Attribute (RVA) can provide a seismic risk reduction tool for tight gas sand exploration in an over-pressed environment The RVA is calculated by subtracting the regional compaction trend from the seismic interval velocity. While 2D and 3D seismic data can be used for calculating the RVA, 3D provides a better tie with the well control. Since top pf 9.5 lbs./gal. mud weight tends to show top of pressure shallower than actual occurrence, sustained gas shows from the mud log or sustained flare are used to calibrate the RVA. A negative RVA can be caused both by top of gas above the pressure cell and top gas in the pressure cell. Dr. R.J. Weimer (Weimer, 2003) suggested the challenge facing explorationists as they explore for new fields and plays is visualizing the trap that technology does not see. The Residual Velocity Attribute, in conjunction with understating other play elements, may be the technology that helps explorationists image the trap in over-pressured tight gas sands plays.

Acknowledgments

The authors wish to thank EOG Resources and Axis Geophysics for permission to give this talk. The authors also want to thank Veritas and SEI for giving permission to show their multi-client data.

References

Surdam, R. C., Z. S. Jiano, N. Boyd, “Anomalously Pressured Gas Compartments, Riverton Dome and Emigrant 3-D Areas, Wind River Basin, Wyoming”, paper presented at the 6th Annual RMAG/DGS 3D Symposium, Denver, CO (February 17, 2000).

Surdam, R. C., Z. S. Jiano, Y. Ganshin, “Rock-Fluid Systems Characteristics of the Rocky Mountain Laramide Basins: Wind River Basin, Wyoming”, paper presented at the RMAG Petroleum Systems and Reservoir of Southwest Wyoming Symposium, Denver, CO, (September 19, 2003).

Weimer, R. J., Keynote Address, presented at the RMAG Petroleum Systems and Reservoir of Southwest Wyoming Symposium, Denver, CO, (September 19, 2003).

Figure 1. Onset of negative RVA ties to top of continuous gas shows in a greater than 9.5 lbs./gal. mud weight environment (top of overpressure).

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