Accurate method of fold interpretation revealed

On October 7th, Rick Groshong, consultant and PetraSkills instructor, came to talk about fault-related fold interpretation as part of the Van Tuyl lecture series. After receiving his Ph.D. from Brown University, he worked in the oil industry for about 10 years. He then went on to teach at the University of Alabama for 20 years. He is now a “semi-retired consultant geologist”. “So this is what I do for fun,” Groshong laughs.

Groshong began his lecture by discussing the most common pitfall in measuring fault bends and folds. Traditional methods assume a constant line length, which produces inaccurate results. Area-depth-strain relationships are better for interpreting faults because “it makes fewer assumptions about how things work…the fewer things you have to assume, the better,” Groshong explained. “It allows us to make the interpretation without making assumptions about line length.”

To compare the differences between the two methods, Groshong applied them both to different models, including the Rosario oil field and the Pigeon Mountain outcrop. Each method yielded different results, but the area-depth-strain had numbers that made the most sense and that were closest to the true value. In his findings, Groshong was most surprised with “how dramatically wrong the conventional interpretation of the sandbox models is. I couldn’t even believe for quite a while.”

Fold interpretations can reveal interesting information about faults. In the Pigeon Mountain outcrop, something very unusual was found. “Usually you get layer-parallel shortening…but over here we have a layer-parallel extension,” said Groshong. “This means that what this is trying to do is lift an enormous thick overbearing. So it’s actually getting squeezed out horizontally relative to the vertical uplift…which is causing this extension to strain parallel to this.”

Fault-bend interpretation is important for “people who are trying to produce liquids out of reservoirs,” Groshong said. “Small faults can be discontinuities if you’re trying to get stuff out.” For instance, someone who is mining oil would want to know if there is a fault blocking the path between the oil and the well. Also, “if you’re trying to get rid of CO2, pumping it around, you don’t want it to hit a barrier right outside the well.” Using the correct tools for interpretation, the fault-bends and folds can be pinpointed, and barriers can be avoided.