REDOX IDENTIFICATION TECHNOLOGY -- A DIRECT LOCATION TECHNIQUE

Reed Tompkins
Eschaton Exploration
Spring, Tex.

Recently a new technology has been developed whereby the direct detection of oil reservoirs generated by reduction-oxidation (redox) cells can be made. This technique is described as redox identification technology (RIT).

The oil related reduction-oxidation cell was first popularized by Sylvain J. Pirson, a Schlumberger engineer who developed the redox cell concepts from SP base line shifts relative to production zones.

It was Pirson who conceived the model of oil fields as giant negative charge generating earth batteries. These batteries were thought to move limited amounts of current to the surface, thereby creating columns or cells of reduced chemical stability, something normally not seen in sedimentary rocks.1

These reduced environment cells, according to Pirson's theories, could then be measured through the detection of the governing electrical currents (Fig. 1). The development of RIT technology is a furtherance and continuation of Pirson's earlier work and theory.

CELL THEORY

The theory of redox cell formation is quite simple.

In the oil/gas reservoir, natural subsurface hydrostatic pressure forces the saturation of overlying sealing agents (shales and/or limestone) with hydrocarbons from the reservoir (Fig. 2). There is no known shale that is absolutely impermeable, and with the addition of microfracturing all shale seals are considered porous.

Numerous studies have indicated all oil and gas fields to be in a steady state of leakage and depletion allowing for continual saturation of seals.2 As far back as 1933, geochemical surveys proved that major oil deposits allowed direct leakage of hydrocarbons to the surface. With the advent of advanced analytical techniques it appears that 70% or more of all known reserves have a definable hydrocarbon anomaly.3 4

Natural zeolites (clay minerals in reservoir seal) act as catalytic cracking agents, breaking long hydrocarbon chains into smaller molecules. This cracking action results in the negative ionization of the saturated zones along the general chemical reaction of:

C2H10 + catalyst 2 C 2H5-

Secondary reactions allow for:

4 C2H5- +2 H2O

4 C2H6 + O2 + 4 e-

Repeated experiments by the author and Pirson have shown that when shale cuttings are immersed in crude oil a negative charge (reduction) is generated.1 5 This cracking process moves through several stages of varying reactions and rates until the operation stops and the shale returns to its normal state. If fresh oil is reinjected, replacing that previously cracked, the reaction begins anew.

In the seal cracking zone, it is believed that the Eh is so negative as to create meta-stable water, allowing for the breakdown of H2O to provide H2 molecules. The H2 combines with the ionized hydrocarbon chain, completing the chain and discharging its negative charge to the surrounding rocks.

Therefore the total seal area is saturated with a non-diminishable charge. The released O2, upon migrating to the more positive Eh near-surface, will combine with hydrocarbons to form H2O and CO2, the latter forming Fe-Ca-CO3 cements and carbolic acid (H2CO3).

Field confirmation of these processes comes from ion measurements gained through advanced ion mud-logging techniques. These "ion logs" confirm that most if not all oil and gas reservoirs are enveloped in a zone of negative charge.

This reducing environment has been found extending from 25-500 ft above the oil reservoir and 5-10 ft below the pay base creating an ion "bag" or "envelope" surrounding the pay zone (Fig. 2). Neither the structural nature of the trap nor its sealant lithology seem to play a major role in this enveloping cathode development.

Data were collected from numerous wells located across the U.S. and Canada.6

The near-surface (0-10 ft) environment is strongly oxidized due to the steady influx of free O2 and carbolic acid (rain water). Additionally the near-surface air provides a strong positive charge which on average is stronger than that developed in the ground.

This charge is developed as an offsetting countercharge to that generated in the upper atmosphere by cosmic radiation. The positive air charge, combined with the other positive charging agents, creates a near-surface anode and establishes a column of electrical flow from the electron generating oil reservoir (the cathode) directly to the electron poor surface (Fig. 2).

This flow, following the path of least resistance, forms a reduction environment column or chimney centered directly over the field. The chimney is referred to as a redox cell.1

Pirson documented these giant earth batteries through the use of electric logs and surface redox potential (Eh) measurements.

During the 1930s, the Russians also noted low Eh soil conditions over oil fields and developed technology that was used as an exploration tool.1 4

Additional support of the redox cell comes from the occurrence of authigenic magnetite/maghemite deposition directly over oil reservoirs.7 This magnetite occurrence is normally formed directly above and in the same configuration as the reservoir at depth. Maghemite is considered an alteration of magnetite, and both are indistinguishable except under sophisticated lab analysis.8

Authigenic magnetite does not form in patterns that respond to gas leakage patterns and therefore does not appear to be connected to oxidation of surface leaks. Conoco Inc. recognized altered zones of low Eh over most oil fields tested with their form of induced polarization called Indepth.9

Depletion of the oil reservoir allows for reversal of hydrostatic pressures creating reversed flow downward from the seal into the reservoir. This action effectively kills the cracking reactions, and the cell activity greatly diminishes. Hence the RIT technology cannot be used on depleted or partly depleted reservoirs, although virgin pressure reservoirs in association with depleted zones can be identified.

RIT CURRENT EVIDENCE

With the redox cell being formed from current flow between two oppositely charged plates, flow patterns appear to be fairly uniform along the entire cross-sectional area of the cell.

Convergence does not seem to exist, but current patterns appear to be in nearly vertical lines within the cell area. This is contrary to Pirson's concepts, which indicated the flow to be conical and comparable to magnetic force lines.

With this diverging flow pattern, Pirson used surface measurements (magneto-electric tellurics) to calculate an originating point for the magnetic/electrical field focus. If the Pirson current flows had in fact been configured in such fashion, then it would be possible to predict relative depths to reservoirs. This was the extended conclusion of Pirson's work: the prediction of oil depth along with the vertical position.

Additional evidence for vertical flow lines is from the work by Robert Foote concerning location of near-surface authigenic magnetite.7 Foote's primary work has been centered on the identification of near-surface magnetite from airborne magnetic data and their relationship to entrapped hydrocarbons.

In an attempt to verify the cause of the magnetic anomalies and to prove a direct natural occurrence relating to hydrocarbons, Foote has measured the borehole cuttings of several thousand wells comparing magnetic events to production results.

With approximately 1,500 field wells tested, 80-85% found magnetic mineralization in the upper 4,000 ft. If the field edge wells are excluded from the count, then the percentage rises to 90-95% correlation between production and magnetic alteration.

Of the approximately 1,500 dry holes measured, only 5-16% exhibit any form of alteration. When the field edge dry holes are discounted, the magnetite rate drops to less than 5% for dry areas.10

Magnetic mineralization percentage diversity is due to variances between different areas. Foote's technology is called sedimentary residual magnetics (SRM).

This magnetic alteration does not normally occur except where the Eh values have dropped to below -.1 volts, and this is not a normal Eh value in most sedimentary rocks (Fig. 3).11 12 Consequently sedimentary magnetite only occurs authigenicly due to a strong reducing environment, i.e. a redox cell.

With magnetite placement being vertical over the field location, it must be concluded that cell formation and its generating current are vertical in nature.

Studies of RIT measured electrical currents show their magnitude to be 100 times below the sensitivity of Pirson's equipment, giving potent evidence that he was looking at near-surface magnetic alterations and not current flow.

The recognition of magnetite deposits was just being discovered at the time of Pirson's death and therefore it is not likely that he could have related the two facts. The detection of Foote's SRM anomalies is somewhat possible with Pirson's equipment, and it is proposed that Pirson was in fact seeing magnetite alterations instead of electrical flow patterns.

RIT surveys, where run in conjunction with SRM or ME tellurics, show close comparison with each other, providing additional evidence that the SRM, ME tellurics, and RIT surveys are interrelated, all being created by the cell currents.

Similarly, both Pirson's ME tellurics and Foote's SRM surveys have almost the same success rate of 80-90% accuracy on wildcat production picks.1 10 13

RIT SURVEYS

The RIT system consists of several surface sensors, electronics, and recording packages. Sensor arrays do not require contact with the ground, thereby eliminating the problems associated with most surface probes.

RIT is not a form of induced polarization, Geoprobe, Petro-sonde, or any other heretofore described technology.

RIT data are normally collected from a truck mounted unit. However, the equipment is small enough to be packed by all terrain vehicles or a two man crew. Data collection allows for seven to 15 data points/day, depending on land accessibility.

The sensor placement is best organized on a standard grid pattern, but unlike many geochemical surveys that must form patterns, RIT surveys are site specific. By being site-specific, the anomalies are directly located over production areas and are first measured from the center of the prospective area (Fig. 4). Data are then collected away from the main area of interest.

If an RIT anomaly is located, then a detailed grid is performed. If no anomaly is seen, then a minimum of 10-15 data points are patterned over the area to verify that the entire prospective area is nonproductive.

This process allows for the prospect to be defined as positive or negative without the greater expense of a full survey. If the initial work indicates a positive aberration then a detailed pattern grid is preferred.

Spacing of data points depends on detail required. RIT surveys have been performed with grids stationed from 250-2,500 ft, looking for prospects from 20-5,000 acres.

One of the unique qualities of RIT technology is its ability to distinguish oil from gas reservoirs. Due to cracking differences between methane and heavier hydrocarbons, variations in the current flow can be identified, distinctly separating oil from gas.

Although there is no absolute GOR point established relative to this phenomenon, high-valued GORs (10,000 or greater) will generate an explicit gas current, and low-range mixes (3,000 or less) will exhibit a definitive oil anomaly.

Mixtures between these extremes will produce variations of both currents. Consequently, the system has been useful in mapping shallow gas-caused bright spots, allowing for separation from other pay zones. Redox cells, and hence RIT anomalies, exist with all fields irrespective of depth. RIT surveys have defined traps from 700-13,000 ft.

The technology has its greatest strengths in being able to define stratigraphic pinchouts, channels, production fault contacts, and virgin plays in old fields.

CONCLUSION

An old concept has been revisited with new techniques and new ideas, providing an exciting innovative technology in the hunt for oil.

With the ability to directly measure the redox cell currents, the actual heartbeat of the oil field can be seen. It is this oil-related current that controls most associated surface geochemical reactions such as radiometrics, SRM anomalies, geochemical halos, induced polarization surveys, delta carbonate, and various other surface events.

Consequently, since the redox identification technology is the actual mapping of the base causality agent, then this is the fundamental mapping tool for hydrocarbon induced geochemical events.

Data are analyzed in the field through on-site computer modulation, and less data recovery is required than most geochemical surveys to make decisions. RIT technology has the unique ability to see through depleted reservoirs to detect and map virgin reservoirs. Cost is low when compared to dry holes and most other forms of geochemical tooling.

RIT technology is totally passive in nature, doing no damage to crops, flowers, or landowners' egos. The system is truck mounted but in extreme situations can be hand carried to the site.

In summary, the redox identification technology is a new form of oil exploration tool with the ability to see the primary redox cell generating agent-oil generated electrical currents. The technology is recapitulated by the following points.

RIT is a primary, not secondary oil/gas mapping tool.
RIT has the unique ability to distinguish oily from gaseous hydrocarbons.
The tool is site-specific, providing anomalies directly over reservoirs.
Being site specific, data grids, although preferred, are not necessary allowing for limited data points on prospects that appear to map dry.
RIT surveys are not limited by reservoir depth. Shallow fields may have a differing signature than deep fields, however there is no depth projection method known.
Real time computer analysis allows for on-site interpretation of data, permitting maximized data location and fast decisions as to next steps.
RIT surveys are passive and nondestructive in nature providing no harm to crops, cattle, and fragile landowner emotions.
Since all virgin reservoirs generate redox cell activity, RIT technology can see through depleted reservoirs in old fields and identify missed pays.
Survey cost is low when compared with most other technologies and dry holes.

REFERENCES

Pirson, S.J., Advances in magneto-electric exploration, Unconventional Methods in Exploration, SMU Symposium II, 1981.
Kontorovich, A.E., Geochemical methods for the quantitative evaluation for the petroleum potential of sedimentary basins, AAPG Memoir 35, 1984.
Jones, V.T., Overview of Geochemical Exploration Technology, workshop of Rocky Mountain Assn. of Geologists, Denver, Jan. 30, 1984
Kartsev, A.A. et al., Geochemical methods of prospecting and exploration, Chapter 12, English translation by Witherspoon, P.A., and Romey, W.D., University of California Press, Berkeley, 1959.
Tompkins, W.R., Direct location technologies, OGJ, Sept. 21, 1990.
English, R., English Geological Consulting, personal communication, 1985.
Foote, R. S., Use of magnetic field aids oil search, OGJ, May 4, 1992.
Mason, B., and Berry, L.G., Elements of Mineralogy, 1968, pp. 280-282.
Sternberg, B.K., and Oehler, D.Z., Electrical methods for hydrocarbon exploration, Unconventional Methods in Exploration for Petroleum and Natural Gas, Southern Methodist University, 1984.
Foote, R.S., personal communication, 1993.
Garrels, R.M., and Christ, C.L., Solutions, Minerals, and Equilibria, Jones & Bartlett Publishers, 1990.
Krauskoph, K.B., Introduction to Geochemistry, McGraw-Hill Publishing, 1967, pp. 237-280.
Herzfeld, R.M., Magneto-electric Exploration, OGJ, Feb. 13, 1984.