A Spectraseis crew sets up camp in the desert of Libya. (Photo courtesy of Spectraseis)

Frontier exploration in both new and well-established provinces has always been a vital activity for the oil and gas industry, but has experienced a renaissance in focus and investment since 2003.

Just as the spending on exploration has increased, so has the role of new technologies in illuminating opportunities and generating commercial successes from previously intractable geologies.

For example, the contribution of new subsalt imaging techniques to large finds and economic field development in the Gulf of Mexico are well reported. Yet these discoveries, and the 90% success rates attributed to the sophisticated use of advanced geophysical techniques by some of the super-majors in deep-water Angola, for example, are not representative of the industry as a whole.

The more typical experience is reflected in the much lower drilling success rate and a reserve replacement ratio that has been less than 1% for the past five years.

No one is claiming that the future will be easier. With the industry’s prime exploration portfolio increasingly made up of complex and remote reservoirs, often characterized by poor seismic responses, there is growing industry demand to improve the value of conventional seismic data and adopt techniques that complement or even precede it.

Deeper and more complex geological settings and challenging or environmentally sensitive operating conditions at the surface highlight the limits of conventional seismic techniques, which have been struggling to keep up with operators’ needs.

A new data source

Fortunately, however, technology does not stand still. Recent innovations in extracting attributes from low-frequency data — itself a new frontier for the industry — are showing strong applicability to high-grade frontier areas and de-risk exploration investments at costs that won’t confine these new technologies to the oil and gas majors for long.

Low-frequency passive seismic (LFPS) draws on spectral analysis of the passive seismic wavefield of the earth around 1 to 6 Hz.

Resonant amplification and, possibly, resonant scattering mechanisms occurring uniquely in multiphase (oil-gas-water) fluid systems modify the background seismic wavefield. They then generate characteristic spectral attributes in high-sensitivity seismic recordings made at the surface over hydrocarbon reservoirs, thereby providing the petroleum explorationist with valuable new data about high-potential hydrocarbon-bearing zones.

Resonant amplification is a micro-scale effect wherein certain frequencies of the natural seismic energy spectrum of the earth are trapped in the multiphase fluid system of the pore space of hydrocarbon reservoirs to be subsequently emitted from the reservoir with detectable energy levels.

The data from developed fields suggests that some of the low-frequency signal energy associated with producing hydrocarbon reservoirs results from the dynamics of the production environment.

However, even in a production environment, it has been possible to identify low-frequency hydrocarbon-related anomalies, which are distinct and often spectrally isolated from the anthropogenic broadband noise. Furthermore, in the absence of production activity, such as in frontier exploration settings, the identification of hydrocarbon reservoirs has subsequently been confirmed by drilling.

Time-reverse imaging techniques developed by Spectraseis and its research partners have successfully localized the source of the anomalies at depth, opening the way to direct imaging of hydrocarbon fluids in 3-D.

Input to the processing includes data from synchronous arrays acquired using highly sensitive three-component sensors. The reverse-time data process is applied to the synchronized array measurements along with a velocity model to display a plurality of dynamic particle parameters associated with subsurface locations. In contrast with conventional time-reverse imaging methods, no single event or first arrival phase identification is required as input to the modeling.

The anomalous low-frequency effects of hydrocarbon reservoirs on seismic data have been known for many years, but only recently have the research and commercial development efforts, centered on a rapidly growing volume of survey data, converged to deliver powerful and reliable applications.

Gas, oil, and heavy oil reservoirs have been targeted with the technique with successful outcomes. Spectraseis, recently joined in this field by WesternGeco and several independent concerns, has collected dozens of passive seismic datasets over the last five years in diverse basins in the Middle East, North Africa, Latin America, and, most recently, Texas. Anomalies in the low-frequency 1 to 6 Hz spectra of data collected at the surface have consistently correlated with hydrocarbon accumulations.

Implications

So what are the implications of LFPS for frontier exploration?

LFPS surveys offer the explorationist a rich and completely new dataset at a fraction of the time and cost of conventional seismic surveying.

With small teams and light equipment, coarse, staggered survey grids can be used to rapidly scan and highlight potential leads within a large concession. Very large areas — thousands of square miles — can be evaluated for hydrocarbon potential in a few months, including acquisition, processing, and analysis.

The set of coarse grids may be followed by staggered, synchronized swaths to further evaluate prospective areas either before or after seismic. LFPS can be used to plan a targeted seismic program, thereby reducing seismic costs and shortening cycle time to drilling. And, where seismic is already available, integration of LFPS data can be used to high-grade structural interpretations with hydrocarbon indications.

In many frontier exploration locations, the challenge is not one of scale but of difficult site conditions. Poorly consolidated surface layers, for example, can dull active seismic response. but do not detract noticeably from LFPS recordings.

Similarly, LFPS signals are not strongly affected by lithology variations or attenuated at depth and thus carry new information unavailable from other technologies. By integrating LFPS results with other subsurface data, operators can better develop plays hampered by poor seismic imaging and target stratigraphic traps, which are not mappable on 2-D/3-D seismic.

Light equipment and limited manpower requirements can be particularly valuable in more remote areas where costs and safety risks escalate rapidly. The minimal environmental footprint of an LFPS survey enables operations to move into areas off limits to conventional exploration techniques, thereby paving the way for new resource plays.

Two recent studies in the Middle East highlight the technology’s promise as a frontier exploration tool. A study conducted by Spectraseis over deep gas prospects showed attribute patterns generally consistent with the prospect structure. The announcements on the well results are pending.

Separately, survey results reported by SRAK, a Shell joint venture with Saudi Aramco, showed a striking correlation with the reservoir outline of known gas accumulations.

In North America, Spectraseis recently successfully delineated a prospect area in a location where commissioning a conventional seismic survey was impractical due to cultural resistance engendered by unsatisfactory earlier experiences with seismic operations.

By applying the LFPS methodology over a known producing field and extending the survey to include the same play type into adjacent fault blocks, the operator successfully drilled new wells in areas where LFPS results indicated good hydrocarbon potential. This was both in the producing field area as well as previously untested areas in the adjacent fault blocks.

LFPS technology, with its potential to operate in areas off limits to traditional exploration techniques and its ability to generate valuable new data at a fraction of the time and cost of conventional seismic surveying, has a vital role to play in frontier exploration.

The end result will be a growth in new resource plays. At a time when more than 70% of the world’s oil and gas production comes from fields that are more than 30 years old, according to the World Energy Organization, the timing could not be better.