Multiclient seismic surveys have represented an established tool in frontier exploration and in supporting licensing rounds for many years. Recently, multiclient full-tensor gravity gradiometry (FTG) surveys have begun to share the same status.

FTG measures the spatial rate of change of the Earth’s gravitational acceleration in response to small variations in subsurface density. In frontier exploration areas such high-resolution, broad-bandwidth, and strong signal-to-noise ratio data allow operators to quickly and accurately map zones of structural complexity at a variety of scales and depths.

Operators can use FTG data to understand the regional geological setting of a prospect and to quantify the 2-D and 3-D architecture of the geological setting in which it resides. FTG data are fast becoming a key component of risk evaluation prior to making commitments on exploration licenses.

One particular application where FTG data have proved to be valuable is in areas where seismic faces illumination problems. For example, in salt basins the relatively low density of salt in comparison with the surrounding strata and the complex morphology of salt bodies are ideal targets for detection and modeling using FTG data. The result is accurate mapping of the density interface between salt and the surrounding strata in 3-D. With a greater understanding of the structure and density/velocity relationships derived from modeling FTG data, seismic data can be reprocessed and interpreted with greater confidence.

FTG data complement seismic data, not only as a means of enhancing and understanding existing seismic data but in helping to shape, design, and optimize future seismic surveys. FTG data provide increased spatial awareness in frontier exploration regions and are particularly useful for interpolating between sparse 2-D seismic lines or between areas of 3-D seismic. This helps interpreters make better decisions about fault linkages and reduces uncertainty by aiding the construction of more accurate structural models.

The FTG instrumentation can be deployed on either airborne or marine platforms depending on how far the target area is offshore, with the resulting data indistinguishable from each other.

Data are acquired quickly and efficiently with a single instrument, resulting in a rapid exploration of vast frontier regions. The acquisition technology is passive, has minimal adverse environmental impact, and limits the risk to personnel and equipment in the field. In addition, FTG data can be processed and delivered to clients within weeks of survey completion, making them a highly cost-effective alternative and/or complement to seismic surveys.

Multiclient FTG surveys are a win-win for all sides. For the operator, they mean high-quality data sooner and at a lower cost than exclusive proprietary ownership. This offers the operator an enhanced earth-model, leading to improved exploration success rates and better investment decisions around licensing.

For governments, increased understanding of exploration targets means more efficient and profitable development of their oil and gas resources.

Barents Sea case study

The Barents Sea region has seen strong interest from oil companies during the last few years, with 20 new licenses being awarded in the Barents region during the 22nd licensing round. To date 40 companies have submitted nominations according to their interests in the 23rd Licensing Round, and already more than 30 companies have purchased the NPD 2-D seismic data from the southeast Barents Sea area.

The Norwegian energy ministry announced its proposal of areas to be included in the 23rd Licensing Round in February 2014. This will be followed by the formal announcement of the included blocks before the summer.

ARKeX began acquiring a multiclient FTG survey in the southeast Barents Sea in late 2013, optimal timing to coincide with the 23rd Licensing Round. Data already available from acquisition in 2013 cover parts of the Nordkapp and Tiddlybanken basins and the Fedynsky High. Acquisition was postponed in 2013 due to deteriorating weather conditions; however, the survey was to resume in mid-March 2014. Due to the exploration interest in the northern part of the southeast Barents Sea region, ARKeX has confirmed an extension of the original survey plan northward, with all fully processed deliverables available from August 2014.

The Barents Shelf is a structurally complex mosaic of platform and basinal areas. Structural trends formed during the Caledonian and Uralian Orogenies dominate the basement and have exerted a strong influence on the structure and development of the overlying basins and highs.

To this end, mapping areas of structural complexity and understanding the area’s regional architecture is crucial to reducing exploration risk. Furthermore, the Nordkapp and Tiddlybanken basins both contain large quantities of Carboniferous and Permian salt, which form large, subvertical diapirs, the majority of which extend upwards to the seabed. The flanks of the salt domes and inversion structures in these basins have significant potential as hydrocarbon traps.

To investigate such geologically complex areas it is necessary to create the best possible geological subsurface models. ARKeX uses XFIELD, a powerful interpretation tool, to build 2-D/2.5-D density, velocity, and susceptibility models of the subsurface. XFIELD enables the user to combine geological and geophysical data and interpretations in one 3-D visualization space.

XFIELD also allows users to test the gravity and magnetic signal response of their geological model and compare it to observed data. The interpreter can use the constraints offered by integrating gravity, gradiometry, magnetic, seismic, and well data to build more geologically sound models.

The ARKeX multiclient southeast Barents FTG survey will provide a high-resolution dataset for exploration geoscientists. The FTG survey has been designed to complement existing 2-D seismic and planned 3-D seismic surveys. When combined with 2-D and 3-D seismic data, FTG data provide a powerful tool for structural interpolation on a regional to prospect scale and a quantitative basis for interrogating seismic interpretations.

Preliminary results already demonstrate the value of FTG surveys as a key exploration, interpretation, and planning tool in frontier hydrocarbon-bearing regions.

Reducing exploration risk

Whether applied at a regional or prospect scale, in frontier or mature hydrocarbon-bearing regions, FTG is a powerful derisking tool.

Multiclient surveys such as the ARKeX FTG survey in the southeast Barents Sea will be an important tool when evaluating prospects in frontier areas. FTG multiclient surveys also are increasingly becoming an important element in future licensing rounds.

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