Gravity and magnetics can seem a bit like the “flavor of the month” – sometimes the measurements are considered critical for successful exploration and other times ignored or viewed suspiciously by the uneducated.

Luckily for companies that acquire these data, potential fields measurements are back in vogue, helped tremendously by technical improvements such as high-resolution aeromagnetics and gravity gradiometry. Many oil and gas companies will acquire a gravity or magnetic survey prior to shooting any seismic to get a general sense of the “lay of the land” with a low-cost geophysical acquisition tool.

“We’ve been in a teaching mode for most of my career,” said Jeff Rowe, vice president and senior geophysicist for Fugro Gravity & Magnetic Services. “We help companies understand the potential value of gravity and magnetics. I think there’s more of an openness to utilizing all geophysical methods and building a better workstation toolkit that can allow for a superior interpretation.”

He added that younger geoscientists often have been exposed to classes in non-seismic techniques in their studies, giving them a better feel for the value of these data.

Given the reconnaissance aspect of potential fields data, they lend themselves very well to a multiclient model. Rowe said Fugro has done close to 6,000 non-exclusive surveys, all of which are accessible at the company’s website. “Our game plan with non-exclusive data is to spend time talking to clients to find out where they’re looking at doing projects, where there’s open acreage, and where they have needs, and to provide a cost-effective option rather than shooting a proprietary survey” he said. “This way they can see their concession in a broader geological context.”

An aeromagnetic survey will be acquired with a twin engine Cessna 404 with a stinger-mounted magnetometer. (Image courtesy of Fugro Gravity & Magnetic Services)

Uruguay

Recently ANCAP, Uruguay’s state-run oil company, invited Fugro to do a complete aeromagnetic survey of the Pelotas, Punta del Este, and Oriental del Plata offshore basins. ANCAP hopes the data will allow a better understanding of the tectonic evolution of the basins and identify the structural elements to define play concepts. ANCAP already has multiclient agreements with CGGVeritas for approximately 7,380 miles (12,000 km) of 2-D seismic data; has signed a contract with ION/ GXT for the acquisition, processing, and interpretation of the UruguaySPAN for 1,800 to 2,130 miles (3,000 to 3,500 km) to link the Uruguay basins with the neighboring offshore basins in Brazil and Argentina; and has awarded an additional 2-D seismic survey to Reflect Geophysical.

According to an ANCAP press release, this large amount of data will provide additional knowledge of the offshore basins, which will help oil companies that are evaluating exploration opportunities in Uruguay. The data will be made available to interested companies in anticipation of a licensing round to be held in 2012.

Fugro plans to acquire 31,250 miles (50,000 km) of aeromagnetic data over all of offshore Uruguay. This will consist of a 4,500-ft by 18,000-ft (1,500-m by 6,000-m) grid in the shallow-water blocks (less than 3,300 ft or 1,000 m) and a 9,000-ft by 36,000-ft (3,000-m to 12,000-m) grid in the deeper waters.

Rowe explained that the Pelotas Basin is an extension of a basin in Brazil where Fugro acquired data several years ago. ANCAP also is studying the similarity of the Punta del Este Basin to the Orange Basin offshore Namibia and South Africa.

According to ANCAP, the stratigraphy of Uruguay’s offshore basins is characterized by large depositional sequences that overlay Paleozoic sedimentary rocks and Precambrian crystalline basement rocks. Sequences include aluvio-fluvial and lacustrine deposits interbedded with volcanic rocks as well as a transgressive transitional sequence followed by regressive deposits.

Diverse play types have been identified, including anticlinal structures related to basement highs, compaction synclines, truncation of synrift deposits by breakup unconformity, pinchouts, channels, and basin floor fans.

“The play types in Uruguay frequently are related to basement highs, horst blocks, graben structures, and tilted and rotated fault blocks,” Rowe said. “In many cases, the sediments are draped on these features, creating the stratigraphic traps.

“All of the areas in Uruguay are fairly frontier,” he added. “There’s not a lot of drilling history there, so it’s an ideal place to start an exploration project with a high-res magnetic database.”

A strike seismic section in the Punta del Este Basin shows a rift of the northwest-southeast structural trend (left), A dip section in that basin indicates the same rift (right). (Image courtesy of ANCAP)

The survey will be acquired less than 450 ft (150 m) above sea level. This allows the company to map not only the crystalline basement but also magnetic susceptibility changes from within the sedimentary section. Data also are acquired at a faster sample rate, which provides resolution an order of magnitude greater than in the past. The survey, which is scheduled to begin in summer 2011, should take four to five weeks to complete, weather permitting. Results from the survey should answer several questions regarding:

The influence of the magnetic basement on the overlying sediments;

The relationship between sedimentary anomalies in the section with the magnetics;

The presence or absence of volcanics;

The orientation of faults observed on 2-D lines;

The location of depo-centers;

The location of mini-basins; and

Determination of migration pathways from the magnetic basement architecture. The data also should be able to be used as a trend/ mapping tool to extrapolate features viewed in the widely spaced seismic lines. Without existing magnetic data to demonstrate, it can seem like a tough sell to get operators interested in investing in this dataset. Rowe said the client company needs to look at the bigger geological picture.

“In a frontier play, we talk about a game plan and an approach,” he said. “By acquiring a uniform grid of data like we’re proposing, companies will be able to map the crystal basement surface and topography and identify basins, mini-basins, and depo-centers that could indicate areas that would have higher prospectivity.” An airborne magnetic survey like this provides a good framework upon which to build an exploration project.

“We try to encourage companies to take an approach where the first step in an exploration project is a reconnaissance-type of airborne survey,” Rowe said. “The second phase would be a more detailed survey utilizing airborne gravity. And then they might acquire a gravity database in conjunction with 2-D seismic and develop prospect leads that are then followed up by 3-D seismic and drilling.” He added that the airborne surveys allow areas to be high-graded and down-graded, enabling exploration investment to be more “strategically positioned.”

Already the company has a list of more than 20 companies that have expressed some level of interest. “Before we started to negotiate with the government, we met with a number of significant players to see if Uruguay was on their radar and, if so, how they would respond to our proposal to undertake this sort of project,” Rowe said. ANCAP negotiations took almost a year because the country had to pass a governmental decree signed by the president to allow the survey to go forward. Since then, the dealings have been easier. “Their offer to industry seems to be very straightforward ,” he said. “Companies that are looking for new places to explore should consider Uruguay. They will be pleased to discover ANCAP and the opportunities that await there.”