Services > Applications > Oil and Gas Exploration

Introduction

Airborne geophysics has been used for petroleum exploration for many years, chiefly aeromagnetic surveys. Recent years have seen the application of airborne electromagnetic data to the detection of hydrocarbon alteration plumes and shallow gas reservoirs, and the development of effective airborne gravity.

Structural Mapping

Airborne magnetic surveying has been used extensively in oil exploration, for mapping bedrock structure and depth to basement. Capable of detecting subtle faulting and folding deep within the earth, aeromagnetic surveys are fast, low-cost methods of covering large areas. Salt dome mapping is another application where the susceptibility contrast between the sediments and the salt is measurable and salt diapirs, domes and ridges can be mapped. Airborne gravity surveys are a more recent addition to the exploration toolbox, and can map structures represented by density variations. The regional coverage of aeromagnetic and airborne gravity surveys can be used to characterise entire basins, linking together more sparsely distributed seismic data.

Shallow Gas

In the Western Canadian Sedimentary Basin, natural gas is being produced from very shallow, incised valley systems with no surface topographic expression.  Some wells have been completed at less than 100 m. The gas has migrated from deeper reservoirs and is contained in porous fluvial sand and gravel sediments sealed within clay layers. The sand and gravel reservoirs appear as low resistivity targets to an electromagnetic survey. The clay caps necessary to trap the gas appear as conductive zones. The three dimensional renditions of conductivity, which are possible with multi-frequency or multi-channel time-domain EM surveys, provide an indication of the size, shape, and thickness of the incised valley systems.

Case History Shallow Gas

Hydrocarbon Alteration Mapping

Hydrocarbons are continuously moving toward the surface, creating alteration features that can be mapped by geophysical methods. Typical alteration plumes have a resistive carbonate-silica cap close to surface. Below this is a much more electrically conductive clay alteration plume. Diagenetic magnetite may also be formed in the alteration zone. Airborne electromagnetic and magnetic surveys have been used to detect these alteration zones.

Case Histories Colorado and Railroad Valley

Oil Sands

Geological complexity and relatively shallow depths of investigation necessitate the use of innovative, proven and cost effective methods for exploring and developing surface mineable and in situ heavy oil sand leases in the Athabasca Oil Sands near Fort McMurray, Alberta.

Airborne electromagnetics (AEM) has proven its worth for mining exploration for 40 years, is inexpensive compared to ground geophysics on a line-km basis and provides information that is easily integrated with borehole, well logs, seismic, resistivity and ground EM. The airborne resistivity measurements are similar to electrical and inductance logging data in that they provide useful information about geoelectrical contrasts between the geological horizons of interest where sufficient contrast exists. The high density, 3D airborne data set comprises continuous measurements at intervals along line of as little as 3 m, from depths of 0 m to about 300m. Although vertical and spatial resolution are lower than borehole and ground geophysical methods, when integrated with ground based methods, AEM can yield:

• heavy oil reservoir quality;
• thickness and extent of clay or shale seal (such as the Clearwater shale) for SAGD feasibility;
• delineation of rich surface mineable oil sands;
• mapping of surface sand, gravel and aquifers for environmental investigations;
• overburden characteristics and drilling hazards;
• shallow gas reservoir delineation; and
• mapping soils and overburden for facilities site investigation and engineering purposes.

Forward modeling using the available well logs can help determine if the exploration or development objectives can be achieved using AEM.

The economic advantage of AEM is that, for the cost of a few shallow boreholes per township, it provides a multi-purpose, 3D data set, comprising geoelectrical information between sparse and more expensive ground based investigations. When acquired early in a project, AEM can be used to make future drilling and surface geophysical programs more effective by increasing the probability of successfully intersecting pay zone or engineering related targets and also by early identification of areas with unfavourable horizons that don’t require further investigation.

See how Deer Creek Energy Limited used Fugro’s RESOLVE system for their surface mineable Joslyn oil sands project.

An Integrated Approach to Oil Sands Exploration and Delineation : June, 2004