Exploration Technology
Unconventional exploration technologies: Take another look
Some have been around for many years, others are new. They can all prosper in a boom market, but are they worth it?
Perry A. Fischer, Editor
There’s something about human nature that wants it both ways. We like it when some simple technology, something supposedly overlooked, succeeds wildly. It’s a bit cultural too: The less educated especially like it. It’s like poking a stick in the eye of megabucks PhD research and development.
Conversely, we are suspicious of anything that’s too cheap, too easy. Surely, we think, the "big boys" with all their money and know-how, didn’t overlook this simple idea. They probably looked into it, and deemed it unworthy.
All too often, the inventor or practitioner of the technology is unwilling to allow the technique to be critiqued, examined or make any attempt to prove its utility. "Why should I? I’ll find all the oil and make all the money!" they would say. (But that doesn’t stop them from asking me to publish them!) In such cases, it is fitting that their technology should remain largely unused.
Finally, there’s the disruptive effect that such technology, especially when it's cheap, could potentially have on the status quo. Sometimes, the disruptive effect is real, such as when railroads and automobiles replaced wagons and horses. Sometimes, it’s just logic with a touch of paranoia, such as when people believe that technology breakthroughs are being bought and squelched to prevent them from encroaching upon billions of dollars of current investment. Such conspiracy theories are almost always wrong.
What follows are technologies that the author neither endorses nor ridicules, but out of the large number of unconventional, even maverick, technologies, these, in the author’s opinion, have the potential to reduce exploration risk.
SURFACE EXPRESSION
In many ways, the expression of an oil or gas reservoir can trigger anomalous readings across many technologies. Structurally tilted strata that form deep traps can become shallow or outcrop, possibly resulting in anomalous readings that relate to the formation, such as mineralogy, radioactivity or electrical conductivity, and only coincidentally relate to hydrocarbon pore fluids.
Surface expression of seepage along transmissive faults, bedding planes or directly upward (microseepage) is often related to a deeper reservoir. This expression, in turn, can be revealed in alteration of microbial communities and the presence of soil gases, such as methane, ethane, butane, etc.
Sometimes, you can see the surface expression with your eyes, either as an early or late seasonal color change caused by stress in vegetation, plant species distribution (Fig. 1), crown density or vigor (dwarfs or giants). More subtle changes due to seepage are shown in spectral reflectivity, sometimes called hyperspectral analysis. Even early versions of Landsat, with a relatively small number of channels, showed field outlines.
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Fig. 1. A tree island of maples sticks out among other species near Lost River field, West Virginia. Had you drilled on this, you would have come up dry. Successful well was slightly offset from this anomaly.1
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Landsat continues to be used in exploration work today (Fig. 2), as do several other newer satellite systems, although more sophisticated airborne platforms yield much better spectral information on seeps, vegetation, mineralogy, and so on. An extensive library of spectral signatures is held by government agencies such as NASA (ASTER) and USGS. In all cases, ground truthing is needed for fine calibration.
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Fig. 2. Digital spectral satellite map (left), interpretation around the Millers Chapel Area in Overton County, Tennessee, as it appeared in 1977. The wells shown were drilled after 1977. Right, the same area in 2003 after over 25 years of production. Note the dramatic color change, and that very few successful wells were drilled outside the "good" areas predicted in the 1977 map. Courtesy Mammoth Geophysical.
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Furthermore, seepage effects can result in mineralogical changes from oxidation/ reduction reactions that might be revealed in changes in some attribute of the overburden, including electrical properties, such as capacitance and conductivity, magnetic properties and radiological properties. These can take the form of anomalous concentrations, deficits or halos. For example, consider the following case of radiometric anomalies.
An interesting study was done over Helez and Kochav oil fields in Israel. These fields have halo-type radioactivity anomalies associated with depletion of eU, eTh and K-40 in sediments overlying these fields, Fig. 3. Since their aqueous chemistries are quite different, it is difficult to account for such a uniform depletion – with concomitant flanking highs – by a process of vertical aqueous transfer of daughter nuclides; especially from a reservoir in which redox changes have led, primarily, to uranium accumulation. Nor can continuous gaseous-transfer processes, requiring Rn daughters of Th and U, be responsible for the surface elemental distribution (K has no gaseous precursor).2
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Fig. 3. Radiometric analyses (eU, eTh and K-40) of sediment in section overlying Helez (a) and Kochav (b) oil fields, taken at 16-ft depths. Results shown are for total sediment analysis, as well as considering the individual activity of nuclides associated with clay, silt and sand components in total sediment.2
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"However, these elements do tend to behave similarly when entering the crystal structure of rock-forming minerals, and when adsorbed onto clay surfaces, mainly due to similarity in the ionic radius of these large cations. It thus appears that an upward flux of radionuclides leaking from the reservoir does not explain the observed features. Rather, it may be the hydrocarbon flux itself that can be corrosive, which leads to mineral alteration in the overlying sedimentary rock and the release of associated cations. These would tend to migrate laterally, away from the altered area, and become immobile at the periphery by adsorption on clays." 2
Like some other geophysical phenomena, these "pathfinder" anomalies can be specific to a basin or region. For example, radiometric surveys carried out over several oil fields in Oklahoma and Texas show a low in K-40, but a symmetrical, relative high in uranium (and daughters). The hypothesis is that the surface-reducing environment above hydrocarbon accumulations might cause K-40 to be preferentially leached, while U is preferentially precipitated as UO2.
Some practitioners make the claim that their technologies are better at giving "don’t drill" indicators than "do drill." Given that the majority, say, 80%, of exploration wells are dry (depending on how you define an exploration well), it would take a large number of wells to prove this. Deet Schumacher, an expert in geomicrobial soil surveys, and a principal in the company, GMT, has compiled a list (Table 1) using a large number of wells to make the case that pathfinder surface methods, such as microbial, geochem, radiometrics and magnetics, can definitely reduce exploration risk.3
| TABLE 1. Compilation of "do drill" and "don’t drill" anomalies.3 |
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Perhaps the best way to test the efficacy of these techniques is through blind, hindsight testing. While double-blind testing may not be possible (where both the tester and the tested do not know the location of oil fields and wells), it should be possible to design a test where the subject does not know the location of fields, field outlines, or successful/ dry wells. If reservoir pressure is a consideration, only newly discovered fields could be used. Organizations, such as the Rocky Mountain Oilfield Testing Center, are invaluable for this sort of thing.
These are 2D technologies; they cannot determine reservoir depth. And, like most exploration technologies, they usually cannot determine the difference between commercial and non-commercial accumulations. Most important, they should only be used in conjunction with other techniques.
ELECTROMAGNETIC MEASUREMENTS
There’s a well-established theory that the passive EM field that exists around the Earth is created when particles from the solar wind interact with the Earth’s magnetic field. This EM energy is generally less than 1 Hz. Higher frequency EM energy (>1 Hz) is believed to come from lightening strikes, which may originate anywhere around the world, though usually at low latitudes, and travel around the Earth, bouncing between the ionosphere and Earth’s surface.
Another source of passive EM energy might come from what is hypothesized to be a potential loop, created by dielectric differences in subsurface formations, perhaps induced or strengthened by space- or lightening-based EM. Pirson4 described a "redox cell" or "fuel cell" that resulted from the catalytic cracking of oil at depth, which created a negative charge, interacting with a positively charged surface.
Regardless of its source, this EM energy can be detected anywhere around the world. These EM fields are weak, but measurable. They are both passive and dynamic, and can vary over minutes, hours, days, and even over the 11-year sunspot cycle. This dynamic behavior creates a wrinkle, in that a baseline, against which anomalies can be measured, must be set and re-set often.
Airborne electromagnetic systems were first developed in Canada and Scandinavia to find electrically conductive sulfide ore deposits.5 The depth of mining targets and mining methods both favored the use of active EM, where an electromagnetic source is used. While this method is very useful in the mining industry, it is limited in depth to a few hundred feet or so, not the thousands of feet needed for petroleum exploration.
Passive systems rely on the naturally occurring EM energy that is continually penetrating into, and emanating from, the Earth’s surface, as described above. This energy can be mapped in either time or frequency domains, both of which can relate to depth. Unlike most active systems, passive EM can probe deeply. Russia pioneered the use of passive EM in the 1950s and 1960s, using it to find several oil and gas fields in Western Siberia and, most notably, structural uplift in Paleozoic basement that resulted in Urengoy field – one of the largest gas fields in the world.
Many of the problems in passive EM are similar to the problems of Controlled Source EM, now fully commercialized for marine use. Both give non-unique results, so both become inversion problems that need complimentary data and modeling to reduce uncertainity in the result. With increasing depth comes decreasing resolution; and the observed effect must derive from a larger anomalous source. New ideas, new patents and venture capital are behind a couple of new entries in this area that try to overcome some of these problems.
EM expert Steven Constable is a professor at the University of California at San Diego’s Institute of Geophysics and Planetary Physics. He says it is very difficult to image oil fields based on accumulated charge from passive EM, especially from the air, since the source of the charge is difficult to pin down. "It’s a tricky problem," he says. "Certainly it will be pretty innovative if they have solved all the issues associated with it." 6 But Constable also says that, although passive EM is not currently fashionable, he firmly believes that natural EM combined with Controlled Source programs will soon be carried out by the oil industry, at least in the marine environment.7
eField’s technology. This company uses an airplane with a sensitive EM receiver to detect telluric energy in the 480-Hz range and lower, but especially very low-frequency energy below 3 Hz, all the way down to DC (nearly zero). Sampling frequencies well below 1 Hz (1 cycle per sec.) would normally take seconds to minutes for just one cycle, let alone the many cycles needed for accuracy; an airplane can cover a lot of ground in that time. Somehow, this time is shortened. Exactly how, this writer does not know.
The company has managed to get substantial financing, according to one of its principles, Ed Johnson. The company has gathered data over areas of Texas and is hoping to see a well drilled soon. Johnson also says that they are "writing a proposal for one of the majors" that are interested in the technology. The company is apparently using a technique explained in a patent that appears on the company’s website. The 2004 patent was filed by well-regarded geophysicist, Anthony R. Barringer.
According to the patent, the idea is that magnetotelluric currents induce a polarization effect at the boundary between substances of different dielectric constants, such as water and hydrocarbon, hydrocarbon and sandstone, etc. When viewed in the frequency domain, this effect can be seen in the data. An example from the inventor’s 2004 patent is shown in a line flown over Colorado’s Rulison field, Fig. 4, where a clear correlation with the field can be seen.
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Fig. 4. Rulison gas field, Piceance basin, Colorado, (left) producing wells and flight line. Very low-frequency EM data (right) seems to be associated with field. From patent US6765383B1, July 20, 2004.
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The company also flies a magnetometer and intends to add more equipment, such as a hyperspectral camera. The challenge, like most EM methods, is that the data may not give a unique solution, thus it must be constrained (inverted). The company uses a suite of software to help in this process. Mr. Johnson says that he is trying to get the technology independently tested by the folks at the Rocky Mountain Oilfield Testing Center.
Pinemont’s technology. This can be used in on the ground or in the air, but typically in the air. This technology was presented at the 2006 AAPG Annual Meeting. In an abstract authored by LeSchack and John R. Jackson, the inventor of the device, some examples were given.
"Over 30 discovery wells have been documented as being drilled on positive transient-pulse anomalies; to our knowledge, no dry holes have been drilled on any of our positive anomalies. In blind tests using this prospecting method, Jackson had correctly identified Devonian Leduc pinnacle reef reservoirs previously located by LeSchack using independent geophysical methods." Such reefs are difficult to locate using conventional seismic techniques since they are so small, relatively speaking, to the types of formations typically detected by seismic.
The method does not identify the formation depth(s) that are the source of the recorded anomalies.
Here’s how it works. From the 2005 patent (US6,937,190): "Within Earth’s primary magnetic field exist randomly occurring impulses of energy. These impulses, which occur within the audio frequency range, exist in the random vertical components of the Earth’s primary magnetic field." "...Secondary magnetic fields associated with the microseepage plumes over hydrocarbon reservoirs (Pirson redox cells), interact with the random impulses in the primary field and generate secondary impulses....The number of impulses is related to the strength of the secondary magnetic fields."
The method comprises traversing above the Earth’s surface with an antenna, detecting electromagnetic radiation naturally emanating from the Earth’s surface. An electrical signal is generated from the detected electromagnetic radiation. "...filtering the electrical signal frequencies below 65 Hz, preferably below 100 Hz and more preferably below 800 Hz, and above 12,000 Hz, preferably above 8,000 Hz, to provide hydrocarbon information and converting the filtered signal to a prospecting voltage signal, comparing the prospecting voltage signal to reference voltage (e.g., ~2.5V), and generate an output signal which provides information regarding the presence or absence of the deposit of interest. The output signal is preferably in the form of a voltage or as counts in analog or digital format." The output is, of course, GPS referenced.
The patent says that it is necessary to establish a base output signal corresponding to the output signal of an area devoid of the deposit of interest, and analyze the difference between these output signals to determine the absence or presence of the deposit of interest.
At least for now, this technology is exclusively leased to a group of investors.
A FINAL WORD
As one of the practitioners in this report noted, "I keep reminding people not to use this by itself. If, by itself, this technology would allow me to find 1,000-barrel-a-day wells, I’d be out there drilling those wells, instead of working for you."
These technologies should not be used alone. More work, more data and good geologic reasoning are required. Unfortunately, sometimes, even the integration of various data types does not coalesce to a unique solution. In such cases, whether and, if so, where, to drill reminds me of DeGoyler’s wisdom:
"Prospecting is like gin rummy. Luck enough will win, but not skill alone. Best of all are luck and skill in proper proportion, but don’t ask what the proportion should be. In case of doubt, weigh mine with luck."
– Everette DeGolyer 
LITERATURE CITED
1 Harbert, W. and V. T. Jones, J. Izzo1, T. H. Anderson, "Analysis of light hydrocarbons in soil-gases, Lost River region West Virginia: Relation to stratigraphy and geological structures." AAPG 2006 Annual Meeting, April 9 – 12, 2006, Houston.
2 Yanaki, N. E. and D. Ashery, J. Kronfeld, "Careful analysis reveals root cause of gamma-ray anomalies," World Oil, Vol. 221 No. 10, Oct. 2000.
3 Schumacher, D., "Managing exploration risks – Lessons learned from surface geochemical surveys and post-survey drilling results," Presented at AAPG Annual Meeting, Houston, 2002.
4 Pirson, S. J., "Significant advances in magnetotelluric exploration," pp. 169-195, in: Unconventional Methods in Exploration for Petroleum and Natural Gas II, Symposium: Institute for the Study of Earth and Man, Southern University Press, Ed. B. M. Gottlieb, 1981.
5 Morrison H. F. and A. Becker, G. M. Hoversten, "Physics of airborne EM systems," Exploration Geophysics, Vol. 29, pp. 97 – 102, Dept. Mineral Engineering, University of California, Berkeley, California, USA, 1998.
6 Bullis, K., "Spotting oil from the sky," MIT Technology Review, July 26, 2006.
7 McBarnet, A., "Constable’s magnetic attraction to the marine EM business," first break, Vol. 23, January 2005.
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