Airborne survey techniques for oil and gas exploration

Paul Boughton

Timothy R Bodger looks at an oil seep detection system that can be installed on an aircraft.

After the Second World War, as the oil industry took stock of its position, logic suggested that as three-quarters of the earth's surface was water, then a lot of oil must be underwater. Offshore oil exploration and production was needed.

The first offshore exploration areas were obvious. Expansion of onshore fields into the sea's shallow margins allowed the industry to develop its skills. Techniques were developed, and emboldened by some truly amazing tools and skills offshore explorers now venture into very deep, cold and remote frontier zones that were unimaginable 25 years ago.

Oil seeps are often the oil exploration industry's first clues to sub-surface and sub-sea oil accumulations. It has been said that 95 per cent of all discoveries were made because of seepage in one form or another. [Page Break]

Detecting oil seeps from satellite data is not hugely successful. Cold water up-risings and other conditions look pretty much the same as oil from space, and if the oil seep can be seen from space, it is more likely to be a major breach, meaning it has been seeping for thousands of years, or it is a recent accident or tanker flush-out. What was needed was a seep detection system that could be installed on an aircraft and flown in a regional pattern to map seeps to determine where they might be coming from and going to.

In the early 1990s, and in collaboration with members of the oil exploration industry, Oilsearch plc, a UK based technology design group, researched and developed the early SEEPFINDER technology, and recently took on the task of developing a new gravity sensing technology, called GRAVEX.

In 2011, McPhar International, entered into a business relationship with Oilsearch and are now exclusive territorial license holders of the SEEPFINDER and GRAVEX technologies. [Page Break]

SEEPFINDER is a computer controlled airborne optical spectrometer capable of detecting fluorescence from minute concentrations of hydrocarbons on the surface of water. The ultra-violet components of sunlight cause the polycyclic aromatic compounds in crude oil to fluoresce, and the composite signature of these emissions in the visible spectrum is detected and mapped by SEEPFINDER.

Mounted in a light aircraft, together with a high-resolution magnetometer and often a gravity measurement system, survey results are presented as contour and analysis maps, showing light (L), medium (M) and heavy (H) oil types and a composite map of all seepage signatures. [Page Break]

While SEEPFINDER data is not directly calibrated in API units, an approximate assignment would give light >35°, mid 25° to 35°, and heavy below 25°. The amplitudes of the analysis maps are adjusted to give approximately the same colour balance for each map, whereas the composite map shows the true relationship between the different regions and/or types of seepage.

Seep and aeromagnetic data often correlate, as shown in the above example. Note the exact correspondences between the emergence points and the low susceptibility points in the magnetic data. They are excellent, and give additional confidence to the seep data.

Over 350,000 kilometres of survey data has been acquired by SEEPFINDER systems worldwide, for a client base of over 30 of the world's major oil companies.

SEEPFINDER is not the ultimate exploration tool; however, it is a valuable tool in the tool-box. The results of a SEEPFINDER survey will provide further insight and assessment of the geology and thus of the petroleum potential of an area. With direct access to the slick locations and their characteristics, exploration companies can analyse and plan more efficiently additional exploration activity in the region.

GRAVEX is a system of individual gravity sensors mounted in an aircraft in a configuration that allows for a greatly enhanced processing of gravity data. Unlike other gravity gradiometer systems, GRAVEX measures both gravity and gravity gradients. Soon to be available on a contract basis, GRAVEX is a unique combination of experience, new and existing technologies, all partnered with a new method for using them, with the whole concept protected by patent.

GRAVEX is a multi-sensor approach to the measurement of gravity gradiometry, applied over a much larger baseline - typically approaching the dimensions of the aircraft hosting the system. The survey aircraft is fitted with wing pods, aerodynamic wingtips and a tail 'stinger' housing a high-sensitivity magnetometer. [Page Break]

Each wing pod and the stinger house a single, temperature and axis-stabilised, miniature gravity sensor, together with one at the aircraft centre of gravity, acting as a reference. Typical dimension of the gradient triangle formed is 10metres per side.

The triangle of sensor data is compensated for aircraft movement by a laser system and then presented as a gradient vector and a slope angle. Sensor sample rate is 1KHz. Even when smoothed times 10, the delivered data rate is 100 points per second. At 140knots (260 km/hour), this is better than 1 metre along line resolution.

SEEPFINDER and GRAVEX together with high-resolution magnetometry, offer a fast, flexible solution for oil and gas exploration. The unique combination of these airborne technologies, allows detection of offshore seepage and structures with the ability to follow structural trends through the transition zone and onshore.

The lightweight and compact equipment permits the airborne platform to carry maximum fuel allowing up to 1,500Km of data to be collected per day. Fast data processing times provide a fully integrated dataset giving clear results in a format that can be easily used to tailor additional exploration activities.

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Timothy R Bodger is President, McPhar International, Uxbridge, ON, Canada.

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