Airborne geophysics for hydrocarbon exploration

By Stewart Walter

With more than 13,000 employees located in 250 offices around the world, Fugro has the ability to provide exploration solutions on land, offshore and from the air. It was therefore no surprise when a major oil company came to Fugro to help with its exploration in Libya.

Having secured an exploration permit in a relatively isolated area with difficult access and limited data coverage, the company awarded Fugro Airborne Surveys (FAS) the contract to acquire airborne magnetic and gravity data. Airborne surveying is an environmentally friendly technique that rapidly provides regular data coverage at relatively low cost. Measuring small changes in the earth's gravitational and magnetic fields caused by changes in rock type, these techniques allow geoscientists to interpret the subsurface geology, extrapolating from surface outcrop and interpolating between any sparse seismic lines or well data.

Working with staff in Fugro's Tripoli office and building on previous in-country operating experience, FAS were able to complete the permitting processes and mobilise a specially equipped survey aircraft within four weeks of contract award. With interpretation of the gravity and magnetic data also provided by Fugro, the client observed that, "the resulting integrated interpretation clearly contributed to resolving our exploration questions, in particular providing useful guides for seismic interpretation". The company further commented that the project was "performed with the utmost professionalism and with respect to contractual specifications and our required exploration timing".

Following the success of this survey, the same company selected Fugro for an airborne gravity gradiometer survey in East Africa. In this case, higher resolution data was required so the Falcon gravity gradiometer system operated by Fugro was chosen. This is the only gravity gradiometer system specifically developed for airborne operation and offers a resolution 20 times better than airborne gravity. This additional resolution enabled the company to meet its geological objectives of detailing the structural framework and delineating fault patterns within the area.

To get the full benefit from the Falcon technology requires much closer line spacing and the increased amount of data results in increased project cost. It is therefore important that an oil company fully understands its objectives in order to choose the most appropriate technology. This is achieved by pre-survey modelling of the responses of the anticipated geological structures for assessment against the available technologies, something routinely undertaken by Fugro.

For example, Fugro are currently preparing for an extensive airborne survey and interpretation project for a major Libyan oil company over the Sirte Basin. There is a lot of exploration data available, but this is focused mainly on individual oil fields. The prime objective of this project is to gain a better regional perspective and predict where to concentrate future exploration efforts. The data will also be used to identify the presence and distribution of pyroclastics within the reservoir section, because these important tuffaceous and volcanic ash beds cannot be identified on seismic data.

Fugro undertook feasibility modelling to confirm that these pyroclastics could be detected by aeromagnetic data, and also advised the client that rather than acquiring new airborne gravity data, the objectives could be achieved using existing land gravity data. This saved the company more than US$3 million and will reduce the time required for data acquisition, so allowing the exploration team to have the interpretation results more rapidly.

Fugro is very pleased that these projects have demonstrated that its thorough technical evaluation, detailed planning and careful execution are the keys to success in the ongoing search for hydrocarbons.

Service: Fugro Gravity and Magnetic Services

Industry: Oil and Gas Exploration

Challenge: Lack of data for new hydrocarbon exploration

Solution: Pre-survey modelling to identify the best technology; detailed survey planning, efficient acquisition and then integrated interpretation of the data by Fugro specialists

Benefits: Development of geological models for reduction of exploration risk over known areas and generation of new targets for future exploration

About

Stewart Walter is a geophysicist and a member of Fugro's Business Development Team covering the Europe, Africa and Middle East Region