As the utility of gravity and magnetic methods for oil exploration expands, so does the need for more awareness on how these methods can be used to enhance geological interpretations, according to Calgary-based consultant Henry Lyatsky.
Lyatsky advises oil and gas explorers and occasionally teaches industry short courses and lectures at the University of Calgary on the optimal use of geophysical techniques. He is both a geophysicist and a geologist, and considers that there is not enough direct communication between the two disciplines. As a result, the usefulness of gravity and magnetics in oil and gas exploration is still too often overlooked, he says. But it’s not for lack of technological advances or effectiveness in the field.
Ready role for gravity and magnetics
With both gradiometry and conventional potential-field data, even in mature basins with hundreds of thousands of wells, such as the Alberta Basin, there is a ready role for gravity and magnetics to assist with exploration and delineation. “You still need your basement faults, you still want your fault network,” says Lyatsky, “and that’s where the value comes in.”
Gravity and magnetics methods are also proving useful in British Columbia. A number of basins in the central part of the province have been conventionally explored to various degrees and with limited success, but with the shale gas revolution growing exponentially, they’re now being looked at afresh for unconventional oil and gas deposits.
When revisiting previously explored basins, the use of more accurate tools such as gradiometry, and gravity and magnetic methods in general, can open up new opportunity. “The more basins you look at, the more you need tools like gravity and magnetics,” Lyatsky says. “In central British Columbia there are several basins in the Intermontane Belt that can still be considered semi frontier environments, and this is where gravity and magnetics would be most useful.”
On the west coast of British Columbia, Lyatsky has also made extensive use of conventional gravity and magnetic data, in conjunction with seismic data and geological information, to help understand the geology of that region’s offshore basins and evaluate their hydrocarbon potential. His well-publicized study of those basins concluded that the best petroleum prospects probably exist in the southwestern part of the Queen Charlotte Basin.
Gravity gradiometry
“One recent trend has been the expansion of gravity gradiometry,” Lyatsky notes, adding that “there are multiple providers of all types of gradiometers.” An advantage of this trend is better accuracy. “This is an excellent advance and I am delighted to see it happen,” he says. “Gravity gradiometry is a high-precision delineation tool which has its uses in mining and exploration as well as in the delineation of oil and gas.”
Improved gradiometry products can also be helpful farther afield, such as in the delineation of offshore subsalt deposits. When used together with traditional gravity and seismic methods, these tools can sometimes assist in assessing the geometry, structure and gravitational effect of a salt dome and what’s around it. “Again, gradiometry is an essential tool,” he notes, “not by itself but as an additional, complementary technique.” “It’s hard to carry out seismic surveys in offshore subsalt environments, especially in deep-water conditions,” says Lyatsky, who has used seismic and conventional gravity techniques on subsalt projects in the North Sea. “Salt targets are difficult at every level as they are irregular, high-velocity and low-density. The more techniques you can use to help visualize them, the better.”
Gravity and magnetic inversion
Inversion is now being more broadly applied in the oil and gas sector. It has been used for depth determination with some success, especially where adequate geological constraints are present.
Lyatsky cautions that there usually aren’t enough geological constraints to invert uniquely. “For example, if you want to know the depth to source, it really helps to know the shape of the source. If you want to know the shape, it really helps to know the depth. If you don’t know either, it becomes tricky to come up with an inversion that is unique.” And yet even when there are not enough constraints, cautiously executed inversion can be a useful tool for reconnaissance and exploration, particularly when gravity and magnetics are used together.
New tools bring new value, however they don’t replace what’s tried and true. “Just because you have a scalpel doesn’t mean you no longer need an axe,” Lyatsky quips. “The more tools in the tool box, the better, and we should not forget the value of the old ones. Everything has its use in different circumstances.”
Enhancing geological interpretations
Determining which tools to consider, in any exploration project, begins with a good understanding of your geological objective. “You need to start by thinking like a geologist,” says Lyatsky. “Be sure of what it is you are trying to find, that is to say, what geological information you are trying to extract. Know your target. And don’t go looking for the silver bullet, because it doesn’t exist.”
If you have a good sense of the geological exploration model you are trying to test, says Lyatsky, you can then ask yourself: How can seismic data help refine this model? How can magnetics and gravity help me refine this model?
“Ultimately any exploration project is a geological project, and the geologist must be convinced his tools are useful. The mistake we make as geophysicists is that we don’t talk enough with geologists. If I were to make a recommendation, it would be to reverse that situation.”
At the university and around the oilpatch, Lyatsky does his best to ensure young professionals don’t overlook gravity and magnetics applications. “Where we fall short in educating students is that we don’t teach them to think comprehensively about their geological model, and we don’t teach them about applying geophysical tools for a particular set of circumstances or a particular model,” he states. “Half the time you get geophysicists and geologists talking at cross-purposes. Seismic and gravity experts talk past each other because they are too narrowly specialized.”
“It’s important to know the traditional methods, and investigate new developments such as gradiometry and inversion” recommends Lyatsky. “Understand what they can do and what their limitations are. But above all, remember that everything relates to the geology.”