The New Frontier:
Exploring for Oil with
Gravity and Magnetics
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March 5, 2017
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January 19, 2017
By Carmela Burns
Since graduating from Purdue University in 1981, Geophysical consultant Mark Longacre has been dedicated to the field of Oil and Gas exploration, specializing in gravity and magnetics. He's had direct involvement in over 1,000,000 line kilometers of high-resolution aeromagnetic (HRAM) data acquisition, processing and interpretation.
For the past 20 years, Longacre and his geophysical consulting company MBL Inc. have been providing clients with maximum insight through fully-integrated geological and geophysical solutions to Oil and Gas exploration. The main objective: risk-reduction and prospect enhancement through a better understanding of the subsurface geology.
Longacre credits his success to the fact that he has managed to stay small, focused and closely connected to all aspects of his client's projects. "I do most of my projects on site in my client's offices. Working side by side with the seismologists, basin petroleum systems people and the structural geologists, I become a member of the team. That's one of the reasons I am as successful as I am."
In recent years, Longacre has seen an increased interest in the use of gravity and magnetic methods with an emphasis on an integrative approach to projects. It's an approach that fits well with his team philosophy. "From an exploration sense, I'm no longer a geophysicist working remotely on one specific piece – I'm part of a team contributing to a whole understanding of the project."
Integration was core to uncovering critical new knowledge on the Earth's crustal structure in his recent research of the Eastern Mediterranean Basin (EMB). "The Eastern Mediterranean project is a classic example of integration," says Longacre."We were able to collect all the data we needed—not just gravity and magnetic data but ocean bottom seismograph (OBS) refraction data, and very long offset reflection data."
He says the integration of all the geological and geophysical data enabled them to better define the crustal structure, which was key. Although much had been published on the hotly debated and complex EMB structure, it had remained poorly understood and suffered from diverging opinions among geologists. Obvious seafloor magnetic anomalies were lacking, making it difficult to effectively map the distribution of oceanic crust.
Comparing their final model with the initial input profile which was based largely on existing published data, they noted several major differences. For example, depth to the top of the oceanic basement and the thickness of the overlying sedimentary section is greater than that first modeled, crustal thinning across the continental margin beneath the onshore and near-shore portions of the Nile Delta is higher than initially constructed, and some previously undetected changes in geometry of the MOHO (boundary separating the Earth's crust and mantle) were found.
Longacre and his associates, together with researchers from BP Egypt Exploration and the National Oceanography Centre in Southampton, UK, shared their EMB research findings in a presentation at the EGM 2007 International Workshop held last year in Capri, Italy. It was well received, says Longacre. "Structural geologists and petroleum technologists were able to come up with a new interpretation of how and when the Eastern Mediterranean Basin actually opened, and we discovered that the EMB opened in a completely different way than we previously thought."
This discovery provided new insight on the direction and age of the initial rifting, crustal structure and sediment thickness. And it led to new understanding of the basin heat flow, maturation of source rocks and hydrocarbon migration pathways.
All this information has enabled exploration decisions to be more defined. And, since the project was completed, Longacre notes there has been substantial drilling activity and exploration success with significant discoveries on the Offshore Nile Delta in Egypt.
"There were big faults that we couldn't understand," says Longacre. "But once we understood how the basin actually opened, we were able to achieve a better mapping of the tectonic elements. This has all helped to focus exploration in certain areas."
For Oil and Gas Explorers, knowing where to focus is critical. "The cost of a frontier exploration dry hole is 100 to 120 million dollars and costs are going up," says Longacre. "That's a lot of money. Spending a fraction of that to really make sure there are no surprises when you're ready to drill makes good sense."
Interpreted distribution of crustal type and key basement fabric within the EMB. 1
1 Peter Bentham, Ibrahim Hanbal, James Cotton, Mark Longacre & Rose Edwards 2007. Crustal structure and early opening of the Eastern Mediterranean Basin: key observations from offshore northern Egypt and the Levant (EAGE 2007 Abstract)
As explorers look at deeper, more remote and more costly targets to meet global energy demand, proven, low-cost techniques like gravity and magnetic are being brought in earlier in the project cycle to minimize the risk of conducting expensive seismic investigation in potentially less productive basins.
Moreover, several areas scanned decades ago can be re-examined utilizing higher resolution data and new techniques such as gravity gradiometry and satellite gravity. And software advances are enabling integration and interpretation of these data to levels unheard of a decade ago.
"There's definitely more interest and use of gravity and magnetic methods today," says Longacre. "The cost of exploration has risen dramatically and the end result is that Oil companies are making sure they look at everything." This includes a lot of previously unexplored areas of the world's geology.
"We're pushing the envelope geologically. We're going out and exploring in areas we've never looked at before. We're being brought in much earlier in projects and really high grade areas even before they shoot 3-D seismic."
It's also driven by stronger interest in the tectonic evolution of basins, adds Longacre. "We're more interested in the crustal aspects, like crustal structure and depth to MOHO, than ever before. These sorts of things can play a big part in how the basin formed, sedimentation rates, petroleum systems, maturation and paleo-continental margins. And gravity and magnetic methods are ideally suited to help answer these types of questions."
Being able to better integrate potential field data with the other kinds of geophysical and geological data has been key. "We're much more integrated than we've ever been," says Longacre. "With today's software and technology, integration from the gravity and magnetic world to the seismic world is a seamless one. Data and maps can be easily shared and used in GIS, petroleum systems software and seismic workstations."
Advances in technology have also provided the ability to turn things around much quicker, in hours instead of days. "Technology has enabled results 'on demand'. You're much more flexible, and able to turn in different directions based on what the data tells you," he says.
For his integrated consulting needs, Longacre is a dedicated user of Geosoft's Oasis montaj and GM-SYS software for processing, data integration, modeling, interpretation, and exploration analysis.
"Geosoft is still by far the best software for generating gravity and magnetic interpretations, and products that can be easily integrated into the seismic world," he says. "You can add your own tools and your own software with the powerful Geosoft GX toolkit. This means you can still differentiate yourself with software that other people don't have, and customize it to suit your needs."
And given that his clients use a variety of different technological platforms, Longacre notes that the main advantage is the seamless integration of the software. "I can process data, work on maps, and build the model in the same platform," he says. "I can generate something and send it knowing that in a few minutes, they can have it at their work station and on their screen."
Going forward, Longacre sees huge potential for gravity and magnetic methods to add more value as the Oil and Gas industry moves to more integrated exploration approaches.
"In general, the gravity and magnetic consulting community is full of talented people that are pushing the envelope," says Longacre "Gravity and magnetic methods have huge value, and as the integration continues, more and more value can be added. We're generating knowledge that others can run with and build on."