There has never been a greater need for the explorer to have an accurate picture of subsurface geology. One relatively new technique that is delivering real answers for the oil

Figure 1. Qualitative and quantitative interpretation targeted at Debolt level. (Clockwise from top left). (i) Gzz, the vertical derivative of the gravity gradient with qualitative interpretation. (ii) Debolt inversion solution (in meters) with interpretation. (iii) Debolt structure map and (iv) Debolt structure map on topography. (Image courtesy of ARKeX)
and gas industry is Gravity Gradient Imaging (GGI). Available in the oil and gas industry only in the last few years, the early results have shown what a remarkable technology this is. Covering large areas rapidly, the surveys have yielded benefits including lease decisions, seismic location determination, prospect generation and drilling locations.

GGI is conducted from a light aircraft flown at low altitudes. The instrument measures the gradient of the Earth’s gravity field, effectively logging density variations in the underlying rocks. In contrast to traditional airborne gravity surveys, airborne GGI delivers results on both shallow and deeper targets with a vast improvement in image quality due to a superior signal-to-noise ratio. This provides a much more detailed exploration dataset, increasing returns and reducing risks for exploration and production companies.

Onshore exploration is costly, with access to an area often difficult because of environmental and legislative issues. Airborne GGI is being used to survey thousands of square kilometres of terrain, significantly improving the geological understanding of the subsurface in a non-invasive, low-cost, time-efficient manner. An added advantage is the low environmental impact. The ability to overfly difficult terrain is very valuable, and ARKeX has conducted surveys in mountainous terrain up to 9,840 ft (3,000 m).

GGI has been deployed to survey difficult terrain, and it can be as little as one-tenth the cost of a seismic survey. GGI is an excellent way to pre-screen areas so that seismic activity can be concentrated where it will be most effective. It has also been used in a complementary way with existing seismic data to provide a complete picture. This is especially useful when surveying geology that hinders the penetration of seismic energy (for example, where basalt or carbonates are present) or distorts the wavefield beyond recovery (salt domes and complex structures). GGI on its own has also proven to be able to verify prospects and identify structural targets.

ARKeX’s BlueQube service uses advanced gravity gradiometry technology to incorporate data from a number of instruments (including gravity gradiometry, magnetic gradiometry, gravity, digital terrain mapping [LiDAR technology] and digital video), with proprietary processing, inversion and interpretation. This approach allows clients to quickly evaluate acreage for leases, target their exploration prospects and choose drilling locations. Access issues are overcome, and time to production is reduced significantly.

The company also is developing a proprietary gravity gradiometer that will be capable of even greater resolution and clarity. Airborne trials will commence this year, and it will be commercially available in 2008.

Case study
An example of an unforgiving geographical area where BlueQube and the GGI technology has been used is in Muskwa-Kechika, a foothills area in the British Columbia portion of the Western Canada Sedimentary Basin.

The challenge was to efficiently and cost-effectively explore the foothills area and to shed light on the technical and economic failure of the Thunder-Cypress well drilled in 1993. Most exploration had previously taken place in the low-lying foothills area, but the Thunder- Cypress well was meant to open up the rugged mountainous area to the west. Technology such as airborne GGI was not commercially available at that time, so the company wanted to assess, with hindsight, the impact that airborne GGI would have had on the exploration strategy and well location had it been available at the time.

In the summer of 2005 along with JEBCO Seismic Canada, ARKeX began acquiring airborne GGI over 1,150 sq miles (3,000 sq km) of the foothills of Muskwa-Kechika. The survey was specifically designed to target two working plays: shallow rotated Triassic fault blocks approximately 3,168 ft to 6,336 ft (1,000 m to 2,000 m) beneath the surface, and deep-seated carbonate structures greater than 12,000 ft (4,000 m) beneath the surface.

Because of the relatively benign topographic relief and therefore easy access, most exploration in the area had targeted the Triassic play. In this locality seismic data, although relatively expensive, readily identifies structures, and drilling has been along shallow elongated north-northwest/south-southeast trends that follow the thrust front.

Further west into rugged terrain, Triassic rocks outcrop at the surface, and the main focus of exploration tends to be the deeper and highly fractured carbonates of the Debolt Formation. These structures are often masked by complex surface geology that bares no resemblance to the deeper target play. Here seismic is extremely expensive and, even when acquired, cannot guarantee clear images of Debolt geology. Even so, there is significant interest in this play as, when a Debolt structure is found, the rewards far outweigh the risks.

It is clear from the data gathered from the airborne GGI survey and the resulting analysis that, if airborne GGI had been available, the dataset would have added important knowledge for the team behind Thunder-Cypress.

Figure 1 clearly shows that the well could not have been drilled in a worse location. It has been located along the plane of a bifurcating wrench fault clearly identified in the GGI dataset.

Furthermore, the GGI analysis shows that the main structure lies some 6,336 ft (1,932 km) to the southeast, suggesting that the sidetrack deviation of the Thunder-Cypress well was chasing dip data that indicated an anticline axial plane further to the south. If this is the case, then the dip data indicated a smaller anticline west of the main structure.
The Muskwa-Kechika project is still in its infancy. A structural map over the entire survey area, outlining potential targets with similar signature to the Thunder-Cypress play, has been produced.

Initial analysis indicates several large Debolt level structures around which clients have taken up land leases and seismic acquisition programs are now planned.