Geophysical Corner

Bob Hardage, senior research scientist at the Bureau of Economic Geology in the Jackson School of Geosciences at the University of Texas at Austin, writes the monthly Geophysical Corner column in the AAPG Explorer. Download his recent columns here:

Rocks-Physics Theory a Help (January 2008)
Rock-physics theory helps to understand why P-P and P-SV reflection images of deep geologic targets in the Gulf of Mexico (GOM) have spectacular differences, yet are still both correct. A key reason for the differences is variations in clay content in GOM sandstones. Differing P-P and P-SV sequences and facies provide a deeper and richer insight into rock physics and geology than do those produced by single-component seismic data. Explorationists working in areas having clay-dominated siliciclastic rock units should consider utilizing multicomponent seismic data to evaluate prospects rather than relying on P-wave data alone. | Download PDF

Curvature Can Be a Map to Clarity (December 2007)
Curvature attributes can be used for mapping channels, levees and other stratigraphic features– particularly in older rocks that have undergone differential compaction. Because of differential compaction and the presence of levees, the most-positive curvature defines the flanks of the channels, potential levees and overbank deposits. The most-negative curvature highlights the channel axes or thalwegs. In these cases, curvature can be a map to clarity. | Download PDF

Using Curvature to Map Faults, Fractures (November 2007)
Curvature can be a powerful tool for mapping faults, fractures and other geologic features useful in oil and gas exploration. The standard method of measuring curvature in 3D seismic data—horizon-based curvature estimation—has drawbacks, including susceptibility to noise. A significant advance has been the volumetric estimation of curvature. | Download PDF

Was That Survey Crew Sober? (October 2007)
In onshore 3-D seismic data acquisition, considerable time and effort can be expended in surveying the coordinates where source station and receiver station flags are placed, because these flags will later instruct field personnel exactly where to plant geophones and vibrator drivers exactly where to position their vehicles. That made perfect sense before GPS technology was available on vibrator trucks and on geophone-deployment crews. But is all the fuss still worth it? | Download PDF

No Reflection Signal Can Be Good (September 2007)
As seismic interpreters, most of us–including the author–have developed the mindset that robust reflection events are what we first try to associate with drilling targets. The association between robust reflection responses and drilling targets is successfully applied across many prospects and in several depositional environments. In this article, we look at the opposite principle and describe a drilling target for which the correct mindset is: “Drill where there is no reflection signal.” | Download PDF

Diving Into Gas Hydrate Systems (August 2007)
The need to understand deepwater gas hydrate systems is increasing, as several quarters of the geosciences world wants answers about: the use of hydrate as an energy resource; the role of hydrate in seafloor stability; hydrate linkage to shallow-water flow; and the nature of hydrate system architecture. However, a major problem that confronts geophysicists who attempt to use seismic attributes to infer hydrate concentration in deepwater systems is that no one knows with confidence how small unit-building blocks of hydrate are distributed within their host sediment. | Download PDF

How Can You See What’s Hidden? (July 2007)
Numerous oil and gas accumulations are beneath rock layers that effectively hide a reservoir by causing down going seismic ray paths to bend away from the geology that confines the hydrocarbons. How do we look into such hidden places? There are two requirements for analyzing a “hidden” reservoir with seismic technology. | Download PDF

Reflections on Class Two Reservoirs (June 2007)
Geophysicists define gas reservoirs as Class 1, 2, 3 or 4, depending on their P-P amplitude-versus-angle (AVA) response. Class 2 reservoirs are faint, low-amplitude P-P events and sometimes are almost invisible in P-P seismic data. Class 2 reservoir interpretation can be difficult in a conventional seismic stratigraphy study that uses only P-P seismic data, but can be on a more rigorous foundation when both P-wave and S-wave data are available and elastic wavefield stratigraphy is utilized for prospect evaluation. Download PDF

Gas Hydrate and LNG Tankers (May 2007)
Theoretical models have been developed at the Bureau of Economic Geology that relate formation velocity and resistivity to hydrate concentration (Cgh) in deepwater, near-seafloor sediments. Our studies indicate that in numerous targeted intervals across the Gulf of Mexico, Cgh is 0.5 to 0.6 of the available pore space in unconsolidated deepwater sediments. The reaction of most explorationists to this finding is “Too bad. That gas concentration is too low to be of interest.” This conclusion is logical for anyone whose experience has been only with conventional gas reservoirs . . . I t may not be a correct conclusion for gas hydrate reservoirs. | Download PDF

Which Seismic Wave Mode is Best? (April 2007)
Salt-sediment boundaries are common seismic imaging targets that exist at many depths across several basins. Some of these boundaries are salt-sand interfaces; others are salt-shale interfaces. In this column we consider the reflectivity behavior of P-P and P-SV wavefields at salt-sediment interfaces in marine environments, to determine if one seismic wave mode (P-P or P-SV) has an imaging advantage over the other for studying deep salt-related traps. | Download PDF

Why Do P-Wave Wipeout Zones Occur? (March 2007)
One hydrocarbon exploration application that has caused multicomponent seismic data to be acquired across several offshore areas is the ability of the converted-S mode to image geology inside broad, thick intervals of gas-charged sediment where P-P seismic data show no usable reflections. The term P-wave wipeout zone is often used to describe this imaging problem. . . . The rock physics cause of the P-P imaging problem usually is not discussed. | Download PDF

For more information about the Jackson School contact J.B. Bird at jbird@jsg.utexas.edu, 512-232-9623.