Exploration always has relied on remotely acquired data to depict the subsurface. In the industry’s early days, a wide variety of measurements were acquired and analyzed. With the advent of modern 3-D seismic, many of these faded into the background. Now, with higher resolution sensors, greater computing power, and improved data integration and visualization techniques, these legacy methods appear to be making a comeback.

Over the last several years, companies have gained prominence in the exploration services arena by offering subsurface imaging technologies that go above and beyond seismic. For example, ARKeX offers advanced gravity gradiometry surveys to better image complex geological settings such as subsalt or to fill in the areas between sparse 2-D or 3-D seismic surveys, while companies like EMGS and OHM Rock Solid Images offer electromagnetic (EM) surveys to better delineate fluid saturation anomalies in the subsurface, most frequently offshore.

Earlier this year, another company – Houston-based NEOS GeoSolutions – entered the global exploration services arena with an ambitious vision. The company simultaneously interprets as many geological, geophysical, and geochemical datasets as possible, including datasets that are accessible in the public domain, available for license from third parties, resident in client or NEOS’s own data archives, or newly acquired using NEOS-owned and operated airborne systems. At present, the company focuses exclusively on onshore E&P projects.

Multiple geological, geophysical, and geochemical datasets are interpreted simultaneously. (Images courtesy of NEOS GeoSolutions)

The goal is not to displace conventional seismic imaging but to add a suite of geological and geophysical measurements that improve seismic datasets where they exist and, in geographies where they do not, to serve as placeholders until E&P companies have a better idea of where they want to commission new seismic acquisition programs.

This approach delivers a more unique, highly constrained answer about what is going on within the subsurface. A seismic image, when it exists, can be extremely useful in revealing the structures within the earth, but other G&G measurements – including gravity, magnetic, radiometric, EM, and hyperspectral – can bring even more to the interpretation as they reveal important things about rock properties, fluid saturations, and fracture systems that seismic alone might not define adequately.

Potential fields boom

The remote sensing boom is driven by several factors. One involves the operational challenges associated with new seismic acquisition. Land-use permits can be difficult and time-consuming to obtain. Environmental restrictions limit the amount of heavy equipment – like Vibroseis vehicles – that can enter an area. And seismic acquisition programs typically involve dozens if not hundreds of crew members to deploy, maintain, and operate the instrumentation, a reality that increases the HSE risk of any project. These issues drive up the cost and cycle time associated with new acquisition programs and often limit coverage to hypothesized, high-potential “postage stamp” areas.

By contrast, many of the non-seismic geophysical measurements can be obtained from airborne acquisition platforms, including satellites, fixed-wing aircraft, and helicopters. This allows large, basin-scale areas to be surveyed quickly, efficiently, and with minimal issues of access and ground-based personnel deployment. Insights from these programs can be used to help focus follow-on seismic acquisition programs on the most prospective areas within the basin or to guide leasing decisions when time is of the essence.

The science behind the method

NEOS’s methodology involves cross-correlating the geological and geochemical conditions in the subsurface with the geophysical responses that result. For instance, most hydro- carbon reservoirs are not sealed perfectly but instead are penetrated by small faults or fractures that allow light-end hydrocarbons to seep upward. Bacterial degradation of leaching hydrocarbons can cause a reduction zone to develop above the reservoir and either pyrite or sulfur precipitates to form. These geochemical reactions in the subsurface can be detected by looking for geophysical anomalies in resistivity or magnetic measurements.

Closer to the surface, the migration of trace quantities of hydrocarbons can cause carbonates to precipitate and oxidizing zones to form. Once again, these geochemical reactions can be detected by looking for a geophysical anomaly (in this instance, a high-resistivity response). At the surface, leaching hydrocarbons can result in high gamma “halos”; oil seeps; trace quantities of natural gas; or distressed vegetation due to the presence of hydrocarbons in the air, soil, or groundwater, any of which can be detected using an appropriate radiometric or hyperspectral sensor.

A gas distribution map for the Bossier formation shows zones of highest gas saturation in hot colors.

Unconventional gas exploration in the Rockies

A multimeasurement approach can add value in many play types, from the frontier to the mature arena and from conventional to unconventional reservoirs. In Colorado’s Piceance Basin, NEOS was engaged by a client whose acreage was underlain by prospective hydrocarbon and mineral deposits. The terrain in the area was rugged with highly variable topography and a mix of public and private lands having both access and use restrictions. Producing gas wells had been drilled on the acreage, but individual well productivity was highly variable. Client geoscientists theorized that higher production wells were drilled in the vicinity of naturally occurring fracture swarms that were associated with nearby faulting.

To better explain the assumed correlation between fracture intensity and well production, NEOS acquired new airborne geophysical datasets over the project area. Hyperspectral images helped to identify surface-penetrating fault lineaments and trace quantities of natural gas at the surface. A combination of gravity and legacy seismic datasets helped to establish the regional fault picture, while magnetic data helped to identify zones of intense fracturing within the reservoir interval. Geoscientists theorized that the fracture zones contained mineralization anomalies caused by higher water throughput over the course of geologic time.

Based on a multimeasurement interpretation of the available and newly acquired data, NEOS identified fracture swarms. Although the area had been drilled and was under production for a couple of years, the actual well and production data were not revealed to NEOS at the start of the project, essentially making it a blind test. Once the results were delivered, the client confirmed that their most productive wells were located in the areas with the highest mapped fracture density.

Sand package identification in the Bossier

The Bossier formation lies just above the Haynesville shale throughout a large portion of northeast Texas and northwest Louisiana. While the Bossier contains a variety of play types, the operator was interested in identifying gas-filled sand packages that were deposited along shelf-edge deltas in the Jurassic. Although some seismic data existed in the area, legacy velocity models often were inadequate to properly depth-migrate the data. By bringing in gravity measurements, NEOS was able to improve the velocity models and the resulting prestack depth migrated data.

Geochemical reactions in the subsurface can be detected by looking for geophysical anomalies in resistivity or magnetic measurements.

Other geophysical datasets also were acquired and analyzed. High-frequency EM data were used to map near-surface resistivity anomalies, delineating oxidizing zones that could have been associated with hydrocarbon leakage from below. Radiometric data were analyzed to identify potential halos associated with trace quantities of hydrocarbons in the near-surface. Hyperspectral data were used to characterize subtle topographic features on the surface, which were correlated with other measurements to tie surface insights with subsurface structures, and a proprietary spectral decomposition algorithm was applied to discriminate lithology and fluid changes within the reservoir interval.

In the end, more than a dozen geological, geophysical, and geochemical measurements were used in the simultaneous joint inversion, a geostatistical method that delivers a 3-D probability cube highlighting subsurface rock, fluid, and fracture patterns along with high-potential drillable sweet spots. One end-product resulting from the project described the probability that a commercially viable, gas-charged sand package lies beneath that portion of acreage within the Bossier formation. A well had been drilled into the Bossier in the middle of the reddish-brown oval and penetrated a large gas-filled sand unit.

The client had withheld the existence of (and data from) that well during the project, only to have its productive potential validated by the multimeasurement survey.

The task of finding and producing hydrocarbons grows increasingly complex with each passing year. Fortunately, new technologies and new techniques to extract maximum insight from proven methods continue to be added to the industry’s exploration arsenal.