Gaining an understanding of sub-basalt geology has up to now been an extremely difficult task for explorers; the tried-and-true method of relying on seismic surveys to map the subsurface has been ineffective in areas of basalt flows. Explorers believe that basalt prevented the penetration of seismic energy, making the resulting seismic survey data ineffective for imaging sub-basalt structure.

However, recent work by Offshore Hydrocarbon Mapping (OHM) has revealed that it is now possible to use controlled-source electromagnetic (CSEM) imaging in conjunction with other survey techniques to understand what lies below the basalt. With increasing numbers of exploration and production (E&P) companies searching for hydrocarbons in frontier regions where the focus is often on sub-basalt sedimentary formations, a solution to the sub-basalt imaging problem is welcomed with open arms by the industry.

Imaging challenges
Basalt presents explorers with a number of sub-surface imaging challenges: Basalt is highly

Figure 1. Electromagnetic and magnetotelluric data combined help explorers better define sub-basalt geology. (Images courtesy of Offshore Hydrocarbon Mapping)
reflective to seismic energy, acting as a barrier to seismic signals. Heterogeneity within the basalt itself results in scattering of seismic energy, further degrading the image recovered. The result of seismic surveys in a basaltic environment is poor seismic image quality, offering explorers little information about the sedimantary layers beneath the basalt or the depth to basement.

This is particularly frustrating when the target is potential hydrocarbon-bearing formations that are sub-basalt. Until recently explorers had few alternatives to seismic methods to address this problem.

A new methodology
Recent work has now demonstrated that CSEM can be used to complement traditional geophysical survey methods and gain an understanding of sub-basalt environments.
CSEM has been used for more than 20 years. Developed originally for the understanding of marine hydrothermal systems around volcanoes, it has been applied in the offshore oil industry for the last 7 years. The CSEM method uses an array of seabed receivers and an electromagnetic dipole source towed in the water above to provide images of the resistive structure of the subsurface. CSEM provides explorers with a method of identifying hydrocarbon accumulations because these are generally more resistive than the surrounding rocks.

OHM has been delivering CSEM surveys and interpretation since 2002, and more recently the company has identified a drill-ready prospect in the Falkland Island region for Rockhopper Exploration.

Whereas seismic can demonstrate where geologic structures may exist, it cannot tell explorers where the hydrocarbons may lie. The resistive contrast between hydrocarbon-bearing structures and the surrounding strata makes CSEM a powerful de-risking tool. And in the case of regions of basalt, we are proving that CSEM can provide subsurface understanding with an integrated approach.

West of Shetlands study
In 2006 OHM successfully carried out a sub-basalt survey on behalf of a client in the West of Shetlands (WoS) region in the United Kingdom. Prior to the survey, companies active in the region expected to find reserves similar to those found in the North Sea. However, initial drilling results were disappointing. Explorers found that they could not accurately map below the basalt flows using seismic methods. Since many significant targets for explorers in the region lie below the basalt layer, obtaining structural images beneath the basalt is critical yet has been difficult to obtain up to now.

The OHM WoS survey involved the combination of CSEM data with natural-source marine
Figure 2. A CSEM source is lowered into the water during a survey.
magnetotelluric (MT) data in an effort to constrain the basalt layer and structures beneath. CSEM data is particularly sensitive to resistive features in the subsurface and therefore can be used to constrain the properties and thickness of the basalt layer. MT data relies on natural fields generated in the earth’s atmosphere and ionosphere and is more sensitive to conductive sedimentary structure. The depth below seafloor to which MT data are sensitive is typically larger than for CSEM data, allowing the depth-to-top basement to be resolved.

However, MT data can be relatively insensitive to the types of resistive structures compared to the resolution provided by CSEM surveying. The two methods therefore provide complementary information on the subsurface and, when combined, provide better constraints on seafloor structure and properties than can be gained from either technique alone.

Careful survey design, data acquisition and data interpretation are crucial to these surveys. Resistive bodies in the background — basalt sills, shallow gas, carbonate layers or hydrates, for example — further complicate CSEM data interpretation.

Understanding a geological structure that is concealed beneath basalt or salt requires an integrated approach so that the strengths of complementary geophysical techniques can be exploited. Marine CSEM sounding can make a significant contribution in the context of sub-basalt imaging and that, by combining MT and CSEM data, better constraints on the geometry and properties of sub-basalt sediments can be gained.

Using the company’s experience of sub-basalt WoS gained from its 2006 survey, the company is planning a wider basalt delineation program in 2007 in the Shetlands area, specifically to delineate the base of the massive basalt flows that run from the Faroes shelf southwest into the UK WoS acreage. The survey will map base of basalt and target the depth of the basement using a combination of CSEM and MT survey and interpretation techniques.

After many years of explorers encountering problems with sub-basalt imaging using seismic techniques, the E&P industry now has a promising method to understand what lies below the basalt. This is especially important for frontier exploration, where much of the world’s sub-basalt interest lies.

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