US3835710A

US3835710A - Geo-chemical exploration method

Info

Publication number: US3835710A
Application number: US00139347A
Authority: US
Inventors: L Pogorski
Original assignee: Individual
Current assignee: Individual
Priority date: 1971-05-03
Filing date: 1971-05-03
Publication date: 1974-09-17
Anticipated expiration: 1991-09-17

Abstract

This is a method for prospecting for hydrocarbon, bitumen and radioactive mineral deposits by collecting relatively small subsurface soil gas samples and analysing same to determine areas of anomalously high helium content therein. The samples are collected by using a long slender shaft capable of being driven into the ground which has a capillary bore running substantially its entire length. The inlet to the lower end of the capillary bore is controlled by a cap which may be opened or closed by turning the shaft when the device is in the ground and the upper end of the bore is sealed by a septum comprising a flexible sleeve fitting over the end of the shaft and a flexible plug closing the sleeve. The sample is collected in a container having a sharpened hollow tube as an inlet port. The sharpened tube may be inserted through the flexible plug to communicate with the capillary bore of the shaft for withdrawing a gas sample from the subsurface soil. The container is the type which may be deformed into a collapsed configuration and will return to its expanded configuration to draw in a gas sample.

Description

United States Patent 1191 Pogorski 1451 Sept. 17, 1974 GEO-CHEMICAL EXPLORATION METHOD [76] Inventor: Louis August Pogorski, 660 Eglinton Ave. West, Toronto, Ontario, Canada [22] Filed: May 3, 1971 [21] Appl. No.: 139,347

[52] US. Cl. 73/4215 R, 23/230 EP, 73/432 R [51] Int. Cl. G01n 1/22 [58] Field of Search 73/421.5 R, 422 GC, 432 R; 23/230 EP [5 6] References Cited UNITED STATES PATENTS 1,845,709 2/1932 Haddon 175/20 2,141,261 12/1938 Clark 73/4215 R 2,210,546 10/1940 Hassler 23/240 EP 2,330,829 10/1943 Lundberg 23/230 EP 3,490,288 l/l970 Patnode 73/4215 Pogorski 73/4256 Primary Examiner-S. Clement Swisher Attorney, Agent, or Firm-Weldon F. Green [5 7] ABSTRACT This is a method for prospecting for hydrocarbon, bitumen and radioactive mineral deposits by collecting relatively small subsurface soil gas samples and analysing same to determine areas of anomalously high helium content therein. The samples are collected by using a long slender shaft capable of being driven into the ground which has a capillary bore running substantially its entire length. The inlet to the lower end of the capillary bore is controlled by a cap which may be opened or closed by turning the shaft when the device is in the ground and the upper end of the bore is sealed by a septum comprising a flexible sleeve fitting over the end of the shaft and a flexible plug closing the sleeve. The sample is collected in a container having a sharpened hollow tube as an inlet port. The sharpened tube may be inserted through the flexible plug to communicate with the capillary bore of the shaft for withdrawing a gas sample from the subsurface soil. The container is the type which may be deformed into a collapsed configuration and will return to its expanded configuration to draw in a gas sample. 5 Claims, 7 Drawing Figures PAIENIEBSEP 1 11914

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1 or 2 INVE T0 LO IS AUG EAIEM nsm 7191-4 SHEET 2 (IF 2 23 FIG.7

- INVENTOR. LOU AUGUST P QRSKI. BY v GEO-CHEMICAL EXPLORATION METHOD This invention relates to a method of geochemical prospecting and exploration for valuable subsurface deposits. More particularly, this invention relates to a method for detecting subsurface accumulations which may contain hydrocarbon deposits such as natural gas, petroleum or bitumen and for detecting locations of radioactive mineral ore deposits such as those of uranium and thorium by analysing small gas samples collected from the soil near the earths surface and analysing the same for the presence of volatile indicators, especially helium, of such deposits. More particularly this invention relates to a technique for collecting gas samples which provides a sample which is essentially fractionation free, dilution free and contamination free.

Many techniques of geochemical prospecting have been known for some time. These methods of exploration depend on the fact that geochemical and geophysical forces acting on subsurface accumulations, whether they be hydrocarbons, mineral ores, or otherwise, cause many of the elements and compounds which constitute the deposits to migrate to the surrounding rocks or soil strata. During the migration decomposition or reactions involving these constituents may take place, secondary components may be formed of the original primary constituents, the primary or secondary constituents may be partially or completely adsorbed in the soil or rock or absorbed in ground fluids.

Whatever the course of events, the progress of migration introduces or superimposes the presence of components related to the subsurface deposits on the normal composition of the surrounding strata, which normal composition may be referred to as the 'background. Typically the concentration of the superimposed migrated constituents will be at a maximum near the source accumulation and will decrease to a minimum approaching that of the background composition at some distance from the source. The development of the concentration patterns is affected by many variables such as temperature, concentration, partial pressure of the given component at the source, porosity of the surrounding strata, presence of micro and macro fissures, interaction of the migrating constituents with the surrounding material, and other factors. The migration of constituents from the source deposit, and therefore the pattern of concentration of these migrating constituents, will extend from the source deposit to the surroundings in all directions and may reach the vicinity of the earths surface. Migrating components which are characteristic of their source accumulation are known as indicators and manifestations of the penetration of these indicators into the surrounding strata are referred to as seeps." The concentration level of a given constituent or component different from that considered normal for the background composition of the surroundings in a given geographical area is referred to as a geochemical anomaly. Detection and tracing of the geochemical anomaly to its source is the subject of geochemical prospecting and exploration whereby valuable subsurface deposits are discovered.

As many of the more easily detected valuable deposits are discovered the need for sophisticated equipment and techniques for detecting faint indications of valuable deposits becomes apparent.

Known geochemical exploration methods, not described herein, have involved, inter alia, collection of soil, subsurface water or gas samples followed by analysis of such samples for trace components regarded as indicators of underground deposits of a specific type. These previously known techniques of geochemical exploration have been subject to inaccuracy, errors and misinterpretations for a variety of reasons. For instance hydrocarbon anomalies may be due to hydrocarbons formed by decomposition of organic matter as well as sources of natural gases or petroleum. While attempts have been made to isolate specific components considered characteristic of hydrocarbon deposits the fact remains that many organic or organometallic compounds and a number of inorganic compounds can be formed through chemical or biochemical reactions between subsurface components other than the deposits of interest thereby resulting in false anomalies.

On the other hand valuable indications of the possible presence of natural gas, petroleum or radioactive mineral deposits may be obtained by the detection of subsurface anomalies of gas concentration, particularly of helium.

It is known that trace quantities of helium (approximately 5 parts per million) are fairly uniformly distributed throughout the troposphere and throughout the earths crust. Higher concentrations of helium are however often associated with hydrocarbon accumulations such as natural gas and petroleum as well as with radioactive mineral deposits such as those of uranium and thorium.

Although the origin of the normal or background composition of helium in the earths crust may not be completely known it is known that helium is produced as a result of the process of radioactive decomposition which takes place constantly throughout the earths crust. No doubt this is one source of the helium which is found uniformly distributed throughout the troposhere and throughout the earths crust.

Helium is often found in abundance in hydrocarbon and bitumen deposits. The presence of anomalously high concentrations of helium associated with natural gas and petroleum deposits may be explained by the probability that those physical features which entrap such fluid substances as natural gas and petroleum also tend to entrap quantities of naturally occurring gases such as helium. These traps may be formed in many ways; commonly they are caused by the occurrence of a dense impermeable zone which forms a barrier preventing the fluids from flowing away through more permeable layers.

In the case of radioactive mineral deposits a higher than average concentration of helium may be explained by the fact that the helium is produced by radioactive decomposition and is therefore produced in greater quantities in the area of radioactive mineral deposits.

l-leliums low molecular weight, high volatility, inert properties, relatively low solubility and low adsorption and absorption coefficients make it a constituent likely to migrate from a source accumulation to surrounding strata and perhaps to the surface, in an unaltered state. Furthermore, unlike many other constituents associated with hydrocarbons helium is not released by chemical or biochemical decomposition or organic matter. It therefore follows that subsurface concentrations of helium higher than the average terrestrial abundance of approximately five parts per million, or higher than the local background concentration are good indicators of subsurface accumulations which might contain deposits of natural gas, petroleum or radioactive minerals. A distinction as to which of the above deposit types may be indicated by the presence of a helium concentration anomaly may be made by considering other geological or geochemical evidence.

It should also be understood that although helium is referred to as being a good indicator for the purposes mentioned other gases might serve the same purpose. Both neon and argon are also produced by radioactive decomposition butare present in such large quantities in the air that identification of anomalies is difficult. Radon is also produced by radioactive decomposition and may be detected by radioactive means but since it is found in such very small quantities it requires that larger samples be taken and this requirement introduces difficulties discussed later.

Due to the high volatility of helium and its low solubility, soil and water sampling techniques do not afford a reliable means of tracing helium anomalies.

In many cases a portion of any gases migrating from underground deposits would reach the earths surface and enter the troposphere. Under ideal conditions, if the seep was strong enough, there was no wind, no atmospheric contamination, air samples taken in the vicinity of the seep would indicate an anomolous concentration of helium or other volatile trace components. However these conditions rarely exist. Subsurface gases on the other hand are restricted in their movement by the surrounding strata and a state of thermodynamic equilibrium between all phases present is approached. Gases and other components underground that exert a recognizable vapour pressure are detectable by means of underground gas sampling. The magnitude of the gas seep will be much more recognizable than it would be from surface air sampling.

I have found that the method of collecting subsurface gas samples is of critical importance to the obtaining of accurate information and detection of anomalous gas concentrations. Typical gas sample collection techniques used prior to this invention involved drawing relatively large samples of subsurface gases through a system made up of a suction pump and a tube inserted into the ground. These previously used methods tended to disturb the equilibrium of the conditions existing in the subsurface to such a degree that collection of a truly representative sample of gas was not possible. The effect of the suction pump tends to induce movements of gases within the subsurface pores which cause differential flow or fractionation. At the same time dilution of the sample often occurs as a result of leaking of atmospheric air around the shaft or tube which is inserted into the ground. Thus inaccuracies are introduced into the sample data and interpretation is made more difficult.

I have found that to obtain a representative subsurface gas sample requires that the sample collected be relatively small; in the order of less than 100 cubic centimetres, normal pressure and temperature (NTP) and preferably about 2-10 cubic centimetres. It is also desirable that the rate of withdrawal of the gas sample should be relatively slow. I have also found that the low concentrations of helium found in such sampling requires that the accuracy of the determination of the helium content should be within one part per million.

To facilitate the taking of a sample in accordance with the desired conditions I have found that it is convenient to use a probe having a stiff, slender shaft, preferably of a strong impermeable corrosion free material such as stainless steel, capable of being driven into the ground for a distance of about 3-l0 feet and having an internal capillary bore, running essentially the entire length of the shaft, which may be opened to communicate with the subsurface soil at its lower end so thata gas sample may be collected from the soil by drawing the gas through the internal capillary bore to some suitable collecting container means connected to the upper end of the shaft.

In the drawings which illustrate one embodiment of the devices used in this sampling method,

FIG. 1 is a perspective view of the probe in its assembled state;

FIG. 2 is a perspective view of the the elements thereof;

FIG. 3 and FIG. 4 show the lower end of the probe in the closed and the opened condition respectively;

FIG. 5 is a perspective view of a sample collecting container;

FIG. 6 is a section of the sample collecting device in inflated state;

FIG. 7 is a vertical section of the sample collecting device in the collapsed state.

The probe comprises a shaft 1 which is sufficiently slender and rigid to be driven several feet into the ground. The shaft has attached to it, near its upper end, the

handle elements

2 and 3 which are adapted to be securely fastened to the shaft by means of a sleeve 4 which is drilled perpendicular to its axes as shown at 5. A similar hole is drilled in the handle element 3 as shown at 6.

The upper end of the shaft is inserted through the

holes

5 and 6, with the sleeve 4 being positioned over the slender end of the handle element 3, and the handle element 2 is attached to the end of the handle element 3 by means of the female and male threads respectively. When the threaded connection between the

handle elements

2 and 3 is tightened sufficiently the handle assembly will grip the shaft firmly.

The lower end of the shaft, seen in FIG. 2, has a reduced cross-section containing an inlet port 7 connecting with the capillary bore 8, seen in FIG. 2 and FIGS. 3 and 4. The lower end of the probe comprises a cap 9 which has a flattened lower extremity shaped in the nature of a wedge or chisel. The cap is attachable to the lower end of the shaft by means of threads so that when the cap is tightened against the shoulder of the lower end of the shaft it effectively closes the inlet port, and therefore the central bore, from the surroundings.

The upper end of the shaft has a tapered portion 10 and the central bore of the shaft emerges at the upper end thereof.

A sleeve 11, preferably a soft flexible material, which is impermeable to gases, such as rubber or plastic is designed to fit snugly over the tapered portion 10. The upper end of the flexible sleeve is sealed by a plug 12 also of a soft flexible material such as silicon, rubber or teflon. This plugged sleeve is known as a septum and effectively caps the upper end of the central bore of the shaft isolating it from the surrounding atmosphere but permits access thereto by any sharp, hollow needle-like probe showing all instrument which may be used to puncture the septum plug to draw off any gas sample collected through the bore.

A preferred method of taking a sample collected through the probe is to use a bistable fluid sample'container of the type which is the subject of another invention described in co-pending application Ser. No. 25,736 now Pat. No. 3,662,928 in the United States of America.

An embodiment of such a bistable fluid sampler is shown in FIGS. 5, 6 and 7. In the embodiment shown a container 21 for the collection and storage of fluids is formed of two thin, dome-shaped

shells

22 and 23, preferably of stainless steel, joined with an air-tight seal at their peripheral edges 24' to define an internal cavity 25 therebetween. The shells have an inherent springlike flexibility such that under the application of an external pressure one'shell may be displaced without rupture to a collapsed configuration conforming to the shape of the other shell so that, in this disposition shown in FIG. 7, there is small dead volume in the order of less than 1 cubic centimetre and preferably in the order of 0.1 cubic centimetres. The device is operated by the application of pressure through the thumb and forefinger at the periphery of the container whereby the concave shell will snap into its convex configuration shown in FIG. 6 so as to reproduce a cavity of known volume in the order of 2-l0 cubic centimetres.

Communication to the internal volume of the device is provided by a slender hollow tube 27, preferably sharpened at its external end to facilitate insertion into soft material barriers such as the septum of the probe previously mentioned. The sample collected in the container may be preserved by fitting a soft plug over the end of the tube 27 or by crimping the tube closed or by fitting a tefion sleeve over the end of the tube which may be clamped or crimped closed.

It will be readily seen that this fluid collecting container can be used in conjunction with the probe previously described by inserting the sharpened end of the hollow tube 27 through the soft plug 12 of the septum so as to form a substantially leak-proof communication between the bore of the probe and the fluid collecting container.

In operation the probe may be inserted in the ground manually by exerting the appropriate force on the handles. Ideally the probe will be inserted with the cap 8 in the closed position. When the probe is inserted to the appropriate depth into the ground the flattened end of the cap will tend to hold the cap against rotation so that the cap may be loosened simply by turning the shaft of the probe by means of the handle. By loosening the cap, as in FIG. 4, the lower end of the bore will be opened, through the inlet port, to communication with the subsurface environment from which the gas sample is to be collected. The upper end of the bore will remain sealed by the septum. A sample of the subsurface gas may then be obtained by inserting into the septum plug the sharpened end of the hollow tube 27 of the fluid collecting container, said container having been collapsed to the minimum dead volume. Upon exerting the appropriate force the sample container will revert to its maximum volume and draw into it a known volume of fluid through the bore of the probe. In this manner a sample of the gas surrounding the lower'end of the probe will be collected throuth the probe bore.

As previously mentioned it is desirable inorder to obtain an accurate sample of the gas composition in the subsurface'soil that the sample withdrawn be relatively small. For this reason it is desirable that the dead volume of the probe bore and the dead volume of the sample collecting container be as small as possible so as to introduce a minimum amount of dilutionor contamination. To this end the diameter of the bore of the probe should be in the order 0.010 of an inch in diameter. Stated in other terms it is desirable that the dead volume of the probe bore and the vessel be less than 25 percent of the volume collected. It is obvious that comtamination and dilution from a larger dead volume may be compensated for by indrawing and then exhausting the container to the atmosphere a number of times to purge the original contents of the dead volume. This technique however tends to increase the amount of gas drawn from the soil and raises the difficulties previously mentioned. Since it is desirable to keep the sample withdrawn from the soil to less than cubic centimetres and preferably in the order of 2-10 cubic centimetres the probe bore and container should be designed to constitute a dead volume of less than 2 cubic centimetres.

' The collection of gas samples may be affected in some cases by ground waters in which case it would be desirable to filter out the water. This may be done by fitting the lower end of the shaft with a filter or membrane type phase separater which would allow gas to enter the probe bore but would provide a barrier to water. Such a membrane of filter is not intended to. be described herein but could take the form of a sleeve which would be placed over the lower end of the shaft so as to cover the openings of the inlet ports. In such a case the upper end of the cap could be provided with an enlarged internal diameter large enough to clear the membrane sleeve in the closed position.

It should be understood that samples can be taken at depths greater than several feet by drilling a hole in the soil to within 2 to 3 feet of the desired sample depth. Appropriate means may then be used to drive the probe into the ground at the bottom of the hole to the additional depth of 2 or 3 feet to reach the desired location for sampling.

The sample once having been collected in the manner described may be analysed for the content of the element of interest, such as helium, by any of the wellknown methods such as a chromatograph or a mass spectrometer.

The data obtained as described may be plotted according to location to develop a chart showing the location of anomalous concentrations of the indicator, such as helium, as an area of possible hydrocarbon or radioactive mineral discovery.

The following table lists an example of anomalous concentrations of helium detected by me over known natural gas deposits containing helium.

-Continued Soil Gas Collected Sample at Depth Location He (Surface All) While the foregoing described preferred techniques and apparatus for employing my invention it will be appreciated that a variety of modifications may be used without departing from the scope of this invention.

What I claim is:

1. Apparatus for use in withdrawing subsurface soil gas samples comprising a shaft, capable of being driven into the soil, of constant circular cross-sectional dimension such that said shaft will fill the cavity made by driving it into the soil; a small capillary bore, in the order of 0.01 inches in diameter, running the length of said shaft; said bore having an inlet port adjacent the lower end and an outlet at the upper end; a cap mounted on the lower end of said shaft presenting a sharpened lower end adapted to facilitate insertion of the said shaft into the soil; said cap presenting a wedge-shaped point, of dimensions no greater than the cross-sectional dimension of said shaft, adapted to resist rotation when in the soil; said cap further being adapted to open or close said inlet port of said capillary bore when said shaft is rotated in one direction of the other direction respectively, relative to said cap.

2. Apparatus is claimed in claim 1 in which said outlet of said capillary bore is sealed by a puncturable septum.

3. Apparatus is claimed in claim 2 in which said septum comprises a substantially impermeable flexible sleeve adapted at one end for airtight fit over the head of said shaft in communication with said bore and closed at the other end of said sleeve with a flexible, substantially impermeable, puncturable plug.

4. A method of prospecting for locations of hydrocarbon, bitumen or radioactive mineral deposits comprising the steps of collecting a small sample, in the order of 2 to 10 cubic centimeters, of subsurface soil gas at various geographical locations by means of a probe and a collection container, analysing said sample to determine the percentage of indicator gas therein, correlating the analyses with the geographical location of the sample to determine areas of anomalously high indicator gas concentration in the subsurface soil; said probe comprising a shaft of constant circular cross-sectional dimension, capable of being driven into the ground, having a small capillary bore in the order of 0.01 inches in diameter running substantially the entire length thereof, a septum sealing the upper end of said bore, and an inlet port at the lower end of said bore; means for opening and closing said inlet port comprising a cap threadably attached to the lower end of said shaft, having a flattened wedge-shaped point whereby said cap is restrained from rotation when inserted in the ground; said cap being adapted to open said inlet port when the shaft is rotated in one direction and to close said inlet port when the shaft is rotated in the other direction relative to said cap; said collection container comprising a substantially airtight chamber capable of being displaced from a collapsed configuration of very small dead volume to an expanded configuration of a volume equivalent to the sample and having an inlet means communicating with the internal cavity of said collection container comprising a slender hollow tube capable of puncturing said septum.

5. A method is claimed in claim 4 in which said septum comprises a flexible sleeve, adapted at one end for airtight fit over the upper end of said shaft in communication with said bore and closed at the other end by a substantially impermeable, puncturable plug whereby said hollow tube may be inserted through said plug for communication with said bore.