Reed Tompkins
Energy Exploration Inc.
Houston
Since the early 1970s, extensive re-examination of what has been called "unconventional exploration technology" has led to some major advances in this field.
These tools come in the form of geochemistry, radiometrics, helium surveys, micromagnetics, metal halos, induced polarization, and a host of lesser known geophysical toys.
The personal computer and advanced analytical techniques have allowed scientifically minded entrepreneurs to advance into realms of exploration research normally reserved for major companies. With the diversification of research, higher oil prices, and diminishing drilling results, a flood of advanced exploration concepts has hit the industry.
Unfortunately, each area of research has tended to create cause and effect theories that catered to that particular science, giving rise to many seemingly unrelated theories regarding the properties of oil reservoirs and their effect on surrounding systems.
In an attempt to create a unified theory, the author endeavored to study every form of "black box" technology that has had some degree of success. The study has resulted in a unified theory relative to what might properly be called direct location technologies (DLT).
DIRECT LOCATION TECHNOLOGIES
Direct location technologies include all methods that relate to processes or phenomena directly or indirectly produced by hydrocarbons.
Indicators such as seismic (except bright spot technology), subsurface mapping standard magnetics and gravity are considered symptomatic in nature. Symptomatic technologies give structural or lithological indications as to the possible locations of hydrocarbons but tell nothing as to their direct occurrence.
On the other hand, DLTs indicate hydrocarbon presence but tell nothing relative to physical entrapment. DLTs include some of the old standards such as geochemistry and Landsat and also the newer forms of micro-magnetics, earth radiation analysis (ERA), and magnetic electrical tellurics.
All direct location phenomena appear to be directly related to a single underlying chemical process. In order to better understand that primary generating agent, the following theory is first presented prior to the discussion of individual technologies in order to show that one underlying agent can create numerous effects.
This DLT process theory is built upon data taken from numerous technologies, all of which demonstrated a fair degree of success and repeatability. The work of other researchers, field data published and unpublished, and laboratory experiments contributed to this concept.
OLT PROCESSES THEORY
All details are not currently understood, but it is felt that the following concept will hold true for future testing and is outlined as follows:
I. In the oil and/or gas reservoir, natural subsurface hydrostatic pressure forces the saturation of overlying sealing agents (shales and/or limestone) with hydrocarbons from the reservoir (Fig. 1). No known shale is absolutely impermeable, and with the addition of micro-fracturing, all shale seals are considered permeable.
Numerous studies indicated all oil and gas fields to be in a steady state of leakage and depletion allowing for continual saturation of seals (Kontorovich, 1984).
As long ago 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 (Jones, 1984).
II. 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:
C4H10 + catalyst - 2 C2H5-secondary reactions allow for:
2 C2H5- + H2 - 2 C2H6 + 2 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 (Pirson, 1981). This cracking process moves through several stages of varying reactions and rates until the operation stops and the shale returns to its normal oxidized state.
If fresh oil is re-injected, replacing that previously cracked, the reaction begins anew. In the deep subsurface, free oxygen is in short supply, and the generated ionization potential is not strong enough to break down water molecules thereby liberating O2. Therefore the total seal area is saturated with a non-reducible charge.
Field confirmation of these reactions comes from ion measurements gained through advanced 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 zones (Fig. 1).
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 (English, personal communique).
III. The near surface (0-2,000 ft) environment is strongly oxidized due to the steady influx of free O2 and carbolic acid.
With the near surface being oxidized (forming an anode), a column of electrical flow is established from the electron generating oil reservoir (the cathode) directly to the electron poor surface (Fig. 1). 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" (Pirson, 1981).
Pirson thoroughly documented these giant earth batteries through the use of electric logs and surface redox potential (Eh) measurements. Field studies by Parker not only provide additional evidence that all oil fields generate redox cells but also that cell activity ceases and dies when the reservoir is depleted (Parker, personal communique).
Depletion of the oil 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.
IV. Electrical flow from the redox cell creates a near surface low or -Eh reducing environment. This type of reducing situation is not the norm for sedimentary rocks but is found in association with oil fields, some fracture zones, and certain saline water interfaces (Fisher, 1986; Campbell, 1977).
Consequently, the redox cell stands out as a brown blemish on the white plain of oxidation. It is this contrast of oxidized versus reduced environments, and their associated interaction, that generates various types of measurable geochemical phenomena.
V. Although numerous types of chemical reactions occur near surface, there are basically four types of phenomena existing relative to oil fields: (A) direct leakage, (B) surface ion movement, (C) reduction mineralization, and (D) direct electromagnetic force (emf) measurements.
A) Direct leakage
Leakage detection has been used for more than 100 years to locate oil fields. Since most of the tar traps, paraffin deposits, and oil seeps have been drilled, modern direct leakage technology has moved into the area of advanced geochemistry in its various forms.
Microseepage hydrocarbon anomalies generally surround most oil and gas fields.
As stated before, microfracturing of the seals allows for a steady leakage of hydrocarbons to the surface. As they pass through the seal, cracking occurs leaving the broken hydrocarbon chains with a residual negative charge in a negative environment. This causes the upwardly mobile molecules to alter their course and move away from the redox cell.
This upward and outward motion creates an inverted cone configuration that gives the halo type patterns surrounding most fields (Fig. 1). The intensity of the cell voltage, hydrocarbon type, field density, fracture-joint configuration, and other variables will determine leakage patterns.
B) Surface ion movement
Ion movement is probably the biggest single activity of the near surface relative to redox cells. Positive ions such as UO2, ThO2, Ca, SO2, Fe, FeO, I, K, Ni, Cu, and Zn will move to various redox cell locations relative to their ionic nature, the strength of the cell charges, and the geomorphic makeup of the near surface rocks.
Positive ions located outside the redox area migrate like spokes of a wheel inwardly until they reach a lower Eh potential, which causes precipitation and/or crystallization.
Low Eh levels capable of causing chemical deposition are normally found on the outer edges of redox cells, thereby creating zones of mineral concentration surrounding the oil field. This process constructs numerous types of DLT phenomenon such as radiation, metal, CaCO3 (Delta C), iodine, and various other soil anomalies that appear as halos.
Gallagher reports the elements I, Br, Ni, Ca, P, V, Zn, Fe, Mn, Mg, Co, Cr, Sr, and a host of others to be concentrated (Fig. 9) in the halo areas (Gallagher, 1984).
Radioactive elements such as uranium and thorium are also concentrated in halos. Variations in size and symmetry may be influenced by local ground water migrations.
Since the oxidized soil directly overlying the redox cell is most affected, a strong downward leaching action occurs. In some cases the action is of significant strength to create surface tonal variations and vegetation altering mineralization.
The easiest form of leach mineral zone identification is radiation survey. Many of the minerals involved are radioactive and can be used as "tracing" agents.
Laboratory soil analysis can also be used to trace ion, nickel, cooper, zinc, and other metals, but this technique is slow and expensive relative to radiometrics, which gives the same basic results.
C) Reduction mineralization
One of the most interesting aspects of redox cell activity is the formation of reduced (low Eh) minerals. Most mineral alteration occurs directly over the oil field and thereby provides detectable anomalies that conform generally to the field confinements.
In response to cell activity, increased cementing occurs over the field, and consequently as erosion continues a pseudo-structure (Fig. 1) develops over the field in response to the hardened soils (Matthews, 1985). These non-structurally related surface features have been key indicators in past years for oil exploration and are used today with Landsat imagery to work selective trends.
A second and most promising aspect of secondary mineralization is the near surface formation of reduced altered zones containing magnetic iron and sulfide compounds (Sternberg et al., 1984; Foote, 1984).
Normally, only iron minerals such as hematite and pyrite are formed and remain in the oxidized surface; however, in a redox environment recrystallization occurs, producing magnetite, magnetic hematite, and the magnetic sulfide greigite (Foote, 1986). These compounds form only inside the cell, which proceeds vertically from the reservoir.
Detection can be made by either shallow core samples producing magnetic susceptibility logs or by airborne magnetic surveys, the latter (micro-magnetics) being the most cost effective (Foote, 1986).
These reduced altered zones can also be detected with some induced polarization methods (Sternberg et al., 1984) and the magnetic electrical tellurics of Pirson.
D) Direct EMF Measurements
Live-time measurement of redox cell activity must be viewed as the most fundamental of all DLTS, for it directly measures the life of the field. If it were not so extremely difficult, direct cell measurements would make most other tools obsolete.
At present only one tool, ERA, exists that can directly identify redox cell live-time activity, and no technical papers have been published to date (Parker).
DLT TOOLS
Whether the oil field and its redox cell are detected by direct hydrocarbon detection, radiation or magnetic measurements, soil analysis, or erosional and tonal patterns, the same electrical chimney generates or controls the surface abnormalities.
The list of commonly accepted anomalies includes, but is not limited to:
- Radiation, metal, calcium carbonate, iodine, and various other soil analysis phenomena and associated halos.
- Geochemical, gas, and delta C halo patterns.
- Landsat, tonal, vegetation, and erosional anomalies.
- Induced polarization phenomena.
- Telluric and magnetic electrical telluric patterns.
- ERA anomalies.
- Micromagnetic anomalies.
- Radon and helium anomalies.
Of the generally accepted tools, a handful work remarkable well on a regular basis and should be considered before drilling any prospect. The selection of technologies will depend on locality, pocketbook, and lead time.
Of all the technologies studied, six were found relatively reliable. However of these, two were found to give more precise drill site information, requiring division into two categories, general and precision drill site locators.
GENERAL LOCATORS
General locators provide phenomena that are normally found in the surface but can be related to oil fields when present, and-or normally only associated with hydrocarbons but which must be pattern interpreted to locate the general field area. This group includes:
A. Geochemistry, the oldest form of geophysics developed. It was oil seepage that encouraged Drake to drill the first oil well in the U.S., and again, it was hydrocarbon leakage that pointed the way to salt dome drilling along the Gulf Coast.
During the 1930s, Russian geologists racked up impressive 80% wildcat success ratios using early gas sniffers and "bugology," the detection of hydrocarbon eating microbes that proliferate in hydrocarbonous soils.
This "bugology," though a crude form of geochemistry, is being studied by at least one major company.
Modern geochemical surveys, with computer and instrumentation advancements, have moved far beyond these early attempts and can now detect hydrocarbon leakage in parts per billion. A typical anomaly will develop a broken ring or halo surrounding the oil-gas field (Figs. 1, 2).
However this is not always the case because numerous fields develop anomalies of single and multiple out-gassings updip from the reservoir and possible single gas "bubbles" directly over the field. Consequently, out-gassing patterns cannot be used solely for drill site locations due to their irregular nature.
What a geochemical survey tells, though, is that hydrocarbons are present and possibly producible; this is an excellent tool for frontier areas.
Most geochemical surveys are done by either taking soil gas samples or soil samples and measuring through chromatographs the background hydrocarbon content. However sampling can be done through radar detection of propane gas with results similar to ground surveys (Sandy, 1988) and additionally, delta C surveys can also give data patterns equal to or better than standard geochemical testing.
Delta C (_C) analysis consists of measuring carbon dioxide produced through the thermal dissociation of carbonates taken from soil samples (Duchscherer, 1982).
Calcium carbonate cements are formed in response to the passage and oxidation of hydrocarbons over time. These surveys are considered paleoanomalies covering geologic time and therefore are not directly affected by recent leakage changes (Figs. 1, 4).
B. Magnetic electrical tellurics is the surface detection of redox cell current flow.
Using surface magnetometers, Pirson converts magnetic residual anomalies into electrical gradients corresponding to the cell discharges.
Herzfeld reports a success of 86% on wildcat production predictions (Herzfeld, 1984). In general, the magnetic electrical telluric anomalies are greatly enlarged relative to production but are normally centered on the field.
In addition, Pirson's work attempts to determine the depth and configuration of the redox cell source (oil seal) through the interputation of inferred electrical flow patterns. This appears to be one of the best attempts yet at predicting reservoir depth.
However, recent research by the author indicates these voltages to be extremely small (.1 nano Tesla) and beyond the resolution of the magnetometers used.
Rather than being actual current induced magnetic variances, these anomalies may be caused by near surface magnetic altered zones. Their proximity to the surface distorts the earth's normal background magnetics, thereby allowing detection by vector analysis (Fig. 1). But despite the cause, magnetic electrical tellurics is an excellent tool when used in combination with others.
C. Landsat and high altitude photography are advanced forms of surface geology; geomorphology, one of the most fundamental tools of the trade.
However photogeology does have a few advantages over its century old predecessor. Surface photography can rapidly cover large areas revealing tonal, vegetation, drainage, and outcropping patterns; all are indicators for actual or pseudo-structural anomalies.
Drilling successes range from 28-77% depending on the area, with some locales yielding worthless results (Saunders, 1984). These high flying pictorials tend to work best in areas of moderate erosion and can be used successfully on both structural and stratigraphic traps.
Cost is the lowest per mile of any tool with high altitude photography being limited to the U.S. Landsat provides world coverage.
D. Radiometrics can be described as the "Reader's Digest" version of geochemical analysis. Done properly, it can give an on-site general overview of soil leaching, mineralization halos, and helium out-gassing that closely matches lab analysis of soil and gas samples (Figs. 1, 5).
Radiometric surveys measure relative soil containment of the various radioactive components such as uranium, thallium, radon, bismuth, and thorium, all of which form soluble compounds and migrate relative to electrical conditions.
Therefore an area that has been leached downward due to redox cell activity will not only be deficient of Fe++ but also of UO2++.
Areas such as mineral halos will not only have an overabundance of iodine and calcium carbonate but will also concentrate thallium and thorium due to their ionization energy and solubility.
Areas surrounding oil fields have long been known for concentrations of uranium deposits (Fisher, 1986). The former Sun Oil reported correlations between production and radioactive lows of 50% (Weart et al., 1981).
PRECISION LOCATORS
Precision drill site locators are the newest forms of DLTs and have to be considered the next step in direct hydrocarbon location.
They have a distinct advantage over general locators being that they are direct redox cell identifiers and typically display anomalies that fit the field configuration. Some tools such as radiometrics and magnetic electrical tellurics form anomalies over the fields but are too confused to give drill sites.
The best of the precision locators are:
A. Micromagnetics (magnetic horizontal gradient anomaly) is a relatively new tool detecting near surface secondary digenetic magnetite/mag-hematite mineralization related to deeper oil and gas reservoirs (Figs. 1, 3, 6, 8, 11).
As stated previously, altered zones of reduced mineralization have been found directly over oil fields taking on the general outline of the fields. These zones can most easily be detected from airborne magnetic surveys.
To confirm that (1) near-surface altered zones existed and that they (2) corresponded with field locations and (3) also matched airborne generated anomalies, more than 1,600 shallow magnetic susceptibility logs were produced over and around oil fields, The results confirmed that magnetic altered zones exist over almost all oil fields and, for the most part, did not exist in areas known to be dry.
Correlation between measured altered zone anomalies and airborne magnetics resulted in a 90% plus match, indicating the magnetic surveys to be extremely accurate. Field testing to date has shown 64% accuracy on production picks (Foote, personal communique).
B. Earth radiation analysis (ERA) is a direct redox cell locator using radiation detectors to measure Radon out-gassing rates (Figs. 7, 12).
Radon is one of several gases produced and exhaled by the soil. Although radon is just one of many gases migrating into the atmosphere, it is one of the few that is in fair abundance and also radioactive, thereby making it easy to identify and measure.
Specific radon out-gassing rates and patterns are produced in a live-time sequence relative to redox cell activity. Production estimates average 50%, with dry hole picks being in the high 90 percentile range.
Being a live-time cell indicator allows the ERA system to work over depleted production revealing deeper untapped pays. Cell activity diminishes as production pressures drop, with old fields showing little activity (Parker).
EXPLORATION ECONOMICS
Direct location tools are not panaceas that will solve all the oilman's problems over night. They are technical advancements that can give a tremendous edge in the risky exploration business.
Many assume that by moving to the use of DLTs that the proven tools of classic exploration are thrown out, but nothing could be further from the truth.
In fact, DLTs allow for the optimum use of existent technology. Since DLTs give extremely accurate answers relative to no drill situations, they can be used as a rapid, cheap submittal checking device that will eliminate dry prospects prior to large expenditures.
The single greatest advantage of DLT exploration is the ability to increase wildcat success ratios from an industry average of 1 in 8 to 1 in 3.
The average company with its 13% success rate spends $16-22/bbl of oil in finding and development costs.
When the success ratio is moved to 35% in a standard industry environment, cost drops to $5-7/bbl. However, this is an extremely rare situation and normally only occurs in new trend development.
When DLTs are added to an exploration program, finding and development costs should be reduced even if success ratios don't change. This is due to the reduced acreage and seismic expenses and the ability to better determine the actual size of a prospect, thereby rejecting small reservoir plays.
All of this adds together to immediately drop the cost to approximately $8-10/bbl. And again with a DLT program, the expected success ratio is 35%, not the industry standard 13%.
Consequently, the normal finding and development cost should be $3-5/bbl.
A Midland company using the DLT approach in 1 year raised its rank wildcat discovery percentage to 29% from 8% and lowered its total costs to $3.59/bbl.
CONCLUSION
With proper use, DLTs offer major technical advances.
One who chooses to pursue this advanced path must not cut corners. The tools are relatively cheap, not free.
One of the biggest mistakes made by independents is the misconception that they can skimp on DLT in the same manner that they skimped on seismic.
Single line geochemical profiles, although cheap, will not substitute for a detailed grid survey.
In addition, one must be careful as to whom they use for DLT work. No industry standards exist as with electric logs and seismic, and many former house painters have been known to do radiometric surveys, thereby giving everyone in the trade a bad name.
However, if one will expend the time and money to do the job properly, great rewards can be had from DLT.For a list of references, contact the author at 4703 Cypressdale, Spring, Tex. 77388. (713) 350-0960.
Copyright 1990 Oil & Gas Journal. All Rights Reserved.
