Introduction to May Focus: Rock Physics

Practitioners of Rock Physics can sometimes become quite Emo when a bunch of Punks call us Geeks because we Breakcore and Grindcore. But, if you need to make a piece of Experimental Rock, particularly if it is from a Hard-core, you sometimes have to Thrash it in your Garage. Okay, okay, enough with the puns, but this arises from being teased by my colleagues about what my group really does. So, thanks to Wikipedia’s list of rock genres for those ideas...

Rock Physics is actually pretty serious stuff and, to some degree, forms the basis of what we as geophysicists do. Traditionally, we search for geophysical anomalies that have different physical properties compared to the surrounding rock units within the Earth. These distinctions can be made by changes in density, magnetic susceptibility, electrical conductivity, dielectric permittivity, and seismic wave speeds. More recently, we are carrying out repeat geophysical surveys without understanding how the rock properties change within a reservoir leaving much of the value of such surveys wasted. Consequently, the study of Rock Physics has grown substantially in the last few years with new developments from laboratory experimentation, advanced computer modelling, and even field measurements. That said, it is still a relatively small community compared to, say, those working on topics in migration and inversion, which adds a lot to the fun of working in this topic.

Part of the fun of Rock Physics comes from the fact that we as a community really are just taking our first steps. Sorry to disappoint, but Gassmann fluid substitution is just the beginning. There are still some pretty important fundamental questions to ask regarding seismic wave propagation in fluid saturated rocks particular with regards to wave speed dispersion and attenuation. We have learned a lot about anisotropy, but no one, to our knowledge, has even come close to describing the elasticity of rocks containing clays. Rock, too, is NOT a linearly elastic medium even though we always assume it is and incorporating it may give us some new ways to analyze our data. New experimental tools that allow us to make seismic frequency band measurements together with new tools of imaging in 3D at the micron scale and advanced computational resources will help to solve some of these more basic issues. On the other hand, there will be a great deal of work to do to assimilate these findings into our day-to-day geophysical analyses particularly as our capabilities in field data acquisition continue to grow.

This issue kicks off the topic of Rock Physics with three papers, but there will be more appearing in upcoming issues. The first article by Fred Mayer, Carmen Dumitrescu, and Glenn Larson looks at the challenging problem of estimating both density and electrical resistivity from 3-C seismic surface data in the context of the oil sands. Density is important because it can discriminate shale from sand. Electrical resistivity is also key to being able to separate brine from bitumen saturated oil sands. They employ a strategy that includes calibration from well logs, inversion of PP-PS data, and neural network analysis to find water saturated zones to avoid and highlight the best bitumen bearing sands. This study serves as an excellent example in the use of multiple types of data to come up with a solution.

The second paper, by Xiwei Chen, James Kessler, Jim Evans, Randy Kofman and one of us (Schmitt), veers more towards the linkages between geomechanics and rock physics. This is a preliminary report of an extensive series of laboratory measurements of VP and VS under in situ pressures on low permeability basalts from the International Continental Drilling Program Mountain Home borehole, Idaho. The same samples were then crushed to obtain the unconfined compressive strength (UCS). The study found reasonably good linear relationship between UCS and porosity and a quadratic relationship with VP and VS. These results will allow us to make predictions as to the state of stress from interpretation of the image logs.

The third contribution, by Nam (Oliver) Ong, Jaime Melendez-Martinez and again, Kofman and Schmitt, details a technique to measure the full set of elastic properties on the unconventional Duvernay ‘shale’. These rocks display unusually high levels of anisotropy. Both the P-wave ε and S-wave γ anisotropy are in the neighbourhood of 35% even at a high confining pressure of 90 MPa (about the overburden stress at 3.5 km). This has implications for both passive and active seismic investigations of such formations.

Page restrictions did not allow all of the papers to be published, but in upcoming issues there will appear contributions on modelling of reflectivity at the contact between anisotropic layers. We hope we can nudge others of our colleagues in the coming year in order to highlight the range of Rock Physics studies taking place across Canada.