The third force

Seismic imaging is considered the mainstay of the explorationists’ toolkit. From the early 2D single fold surveys to today’s 3D and 4D time lapse imaging, seismic has proved invaluable in delineating subsurface structure and detailing stratigraphic conditions.

In more recent times, controlled source electromagnetic imaging (EM) has been added to the toolkit to create resistivity profiles over potential oil fields. This represents a technological breakthrough and is providing valuable information about the possible presence of hydrocarbons.

EM and seismic are now used side by side to help better understand the sub surface picture. The more information an explorationist can have about their acreage, the better their decisions and the lower the exploration risk.

What is going to be the next game changing technology to sit alongside seismic imaging? There are only so many rock properties that can be measured, but one key property is rock density. Rock density can be measured by recording the gravity signal of the earth. Traditional airborne gravity surveys do provide density information, but at a relatively low resolution. The resultant information although valuable does not contain sufficient resolution to be useful at the prospect scale. Traditionally the gravity information gathered is used only to provide regional interpretations of the geology and guide explorationists at the basin scale.

In order to be valuable at the prospect level a much higher resolution measurement is needed. Such a measurement is now possible through the availability of gravity gradient measuring systems.

Gravity gradiometry
Gravity gradient imaging (GGI) or gravity gradiometry, measures the rate of change in rock density. It differs from conventional gravity measurements in that it has a much wider bandwidth and increased signal to noise. Due to a gradiometer’s ability to reject inertial accelerations the systems are ideally suited to high dynamic environments such as an airplane or boat. The resultant data from a gradiometer means that much more of the earth model is measured with increased sensitivity. An example of this difference can be seen in figure 1. The same area has been surveyed using both a marine gradiometer system and a conventional marine gravity system. The resultant data clearly highlights the increased frequency content measured with the gradiometer.

In this marine example the density information acquired from the gradiometer was converted to velocity and used to assist the seismic interpretation. The results were an improved understanding of the prospect.

Gravity gradiometry gives an explorationist a very high resolution 3D model of the subsurface geology that can be used for frontier exploration, prospect generation/ evaluation, pre-screening areas for 3D seismic and overcoming seismic imaging difficulties.

BlueQube
Over the past 3 years ARKeX has been performing GGI surveys in North America for oil, gas and mining clients using its BlueQube technology and workflows.

BlueQube uses GGI instrumentation in conjunction with magnetic gradiometry, digital video and digital terrain mapping (LiDAR) to build up a detailed subsurface image.

BlueQube has proved invaluable in frontier zones where the cost of surveying large areas of land with seismic has been prohibitive. With the BlueQube instrumentation mounted in an airplane, terrain obstacles such as mountains, swamps and deserts can simply be flown over, whist still obtaining a detailed signal. In addition, many land-based activities are met with local resistance from environmental groups because of their invasive nature GGI is a non-invasive, passive technology that doesn’t impact the surroundings. The detailed information that is acquired from the surveys can then help steer future land-based activities and limit environmental impact.

Gravity gradiometry and BlueQube can also be of use to compliment seismic and electromagnetic data in verifying prospects in geologically challenging situations. Even the best seismic can fail to deliver in difficult terrain, complex geology or when surveying particular rock types, e.g. salt, basalt and carbonates. Gravity gradiometry’s high resolution imaging capabilities can add another dimension to the geological picture enabling the explorer to obtain the complete 3D geological model.

Case study
One particular area where airborne gravity gradiometry has been used with great success is Muskwa Kechika, in the foothills area of British Columbia in the Western Canada Sedimentary Basin.

The survey, carried out in 2005 by ARKeX for JEBCO Seismic Canada, aimed to cost effectively explore the area and provide a detailed picture of the subsurface geology. The area has some production but the exploration challenges are difficult and 3D seismic is not routinely acquired.

The survey was specifically designed to target two working plays: shallow rotated Triassic fault blocks approximately 1 to 2 km beneath the surface and deep seated carbonate structures, greater than 4km beneath the surface.

Due to the terrain and other issues, most exploration in the area had targeted the Triassic play. Here seismic data readily identifies structures and drilling has been along shallow elongated trends that follow the Rocky Mountain 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 bears no resemblance to the deeper target play. Here seismic is extremely expensive and even when acquired, cannot guarantee to image clearly the structures. 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 undoubtedly added important geological knowledge for the team behind Thunder-Cypress Well drilled in 1993, which although a discovery, was not economic.

Figure 2 clearly shows that the well was not optimally located. It has been located along the plane of a bifurcating wrench fault clearly identified in the GGI dataset. The GGI data shows that the main structure lies 2 km to the south east.

GGI has also identified several other potential targets in the area, around which a seismic acquisition program is planned.

Airborne GGI has demonstrated that it truly is the third force in geophysical exploration, with its ability to identify structural targets and provide information to evaluate prospects while reducing drilling costs. Complimenting existing techniques, GGI is becoming an essential exploration tool.

For more information on how BlueQube technology can aid your exploration efforts please contact phill.hougton@arkex.com.