US3847549A

US3847549A - Method of geochemical exploration

Info

Publication number: US3847549A
Application number: US00340357A
Authority: US
Inventors: K Schorno
Original assignee: Phillips Petroleum Co
Current assignee: Phillips Petroleum Co
Priority date: 1973-03-12
Filing date: 1973-03-12
Publication date: 1974-11-12
Anticipated expiration: 1991-11-12

Abstract

A method of geochemical exploration wherein the rock sample from a subterranean formation is treated under conditions such as to artificially accelerate the aging of the sample. Following the aging period the sample is analyzed to compare the percent extractable oil, the ratio of extractable oil to total oil, and the n-alkane distribution of the original and aged rock to determine the maturity of the rock and its capability for generating oil.

Description

United States Patent "1191 Schorno Nov. 12, 1974 METHOD OF GEOCHEMICAL 3,649,20l 3/1972 Scalan 23/230 EP EXPLORATION OTHER PUBLICATIONS [75] lnventor: Karl S. Schorno, Bartlesville, Okla. J Shaw, Anal. Chem 1, 3 929; [73] Assignee: Phillips Petroleum Company,

Bartlesville Ok|a Primary Examiner-Robert M. Reese [22] Filed: Mar. 12, 1973 [57] ABSTRACT PP NOJ 340,357 A method of geochemical exploration wherein the rock sample from a subterranean formation is treated 52 us. c1. 23/230 EP, 208/ll under conditions Such as I0 artificially accelerate the 51 im. c1. Cl0g 1/02, ClOg 1/04, 01011 33/24 aging f the sample- Followmg the agmg paved the [58] Field of Search 23/230 EP; 208/ l 1 Sample 15 analyzed to Compare the P extractable oil, the ratio of extractable oil to total oil, and the n- [56] References Cited alkane distribution of the original and aged rock to de- UNITED STATES PATENTS termine the maturity of the .rock and its capability for generating oil. 2,854,396 9/1958 Hunt et al. 23/230 EP 3,446,597 5/1969 Bray et al 23/230 EP 4 Claims, N0 Drawings This invention relates to geochemical exploration for petroleum minerals. More particularly, the invention relates to a geochemical exploration method which involves the identification of petroleum subsurface rock formations.

Hydrocarbon energy sources such as petroleum and natural gas are found in commercial quantities in subsurface rock formations such as sandstones and limestones. Since the presence of such hydrocarbonaceous materials in a subterranean formation is not ordinarily manifested by readily discernible suface indicia, various techniques have been developed to aid in exploring for oil and gas. One such technique is geochemical exploration.

Petroleum geochemistry utilizes the fundamental concepts of genesis, migration and accumulation of petroleum in the development of procedures for use in exploration and development programs.

In most geochemical exploration methods, a search is made, generally at or near the surface of the earth, to determine the presence of components of petroleum, precursors of petroleum, or derivatives thereof. There procedures are based upon the theory that these hydrocarbonaceous materials are fugitive and have migrated to or near the surface of the earth from an underlying petroleum reservoir. The presence of such hydrocarbonaceous materials which are normally constituent componentsof petroleum has been found to be indicative of the presence of a subterranean reservoir of petroleum in the general area.

However, such surface methods of geochemical exploration give little indication of the location of the underlying petroleum reservoir. Other exploration procedures such as seismic surveying can be utilized to identify the possible petroleum reservoirs at subterranean locations in the earths crust. Such methods give valuable information as to subterraean structures favorable to the accumulation of petroleum; however, these methods are ineffective in determining which, if any, of such structures actually contain petroleum deposits.

In order to adequately evaluate whether such subsurface structures contain petroleum, in recent years there has been developed geochemical exploration techniques based upon the so-called source rock concept. Under this concept, it is assumed that petroleum hydrocarbons are formed during long periods of burial in fine-grained sediments of high organic matter content, the petroleum hydrocarbons being derived from organic matter of biologic origin which was deposited with these sediments in a marine environment. The exact mechanism by which oil and/or gas are formed in said sediments, i.e., source rocks, is not known with certainty. Generally, it is presently considered that the transformation of the original organic matter contained in such sediments is effected in a first stage before deep burial of the sediments by biogenic processes which must necessarily include an oxidant such as molecular oxygen, elemental sulfur, sulfate ion, and the oxidized states of certain heavy metals, e.g., ferric hydroxide. As the sediments become more deeply buried, the transformation of the remaining unconverted organic matter is thought to be effected abiogenetically. These processes, i.e., biogenic and abiogenic, are considered to be responsible for what can be termed the primary. and secondary genesis of petroleum, respectively. After the hydrocarbon-forming mechanism has taken place, it is thought that the petroleum hydrocarbons in the source rock then migrated to more permeable reservoir rockswhere they accumulated in the concentrated deposits found today.

The application of the concepts of petroleum geochemistry to subsurface exploration problems thus involves the evaluation of shale and limestone sidewall cores and core samples to determine whether the rock is or can be a source rock. The objective of such evaluation is the ascertainment of likely sources for petroleum and estimation of their productivity. Such source rock evaluations permit outlining areas favorable to prospecting because of the proximity to rocks geochemically identified as having generated petroleum and from which oil has subsequently migrated.

By definition, a source rock is a rock in which significant quantities of oil havebeen generated and from which the petroleum has migrated. Nonsource rocks are subdivided into barren rocks which are devoid of organic matter; lean rocks which contain a low concentration of organic 'matter; and immature rocks which may containan adequate amount of organic matter of the proper quality to be source rocks but have not undergone major genesis of petroleum because of insufficient time or depth of burial. The rate of petroleum genesis is considered to be a function of the depth of burial because of the positive geothermal, i.e., temperature, gradient, and not of the higher pressure encountered at deeper depths; Generally, the volume of source rock is orinarily several orders of magnitude larger than the volume of most petroleum saturated rocks. Source rocks, therefore, should be evident in a basin early in the exploration. Their absence should be taken as a I basis for concern. It follows that it is important to ascertain the maturity of the rock in order to determine whether deeper drilling in the area, assuming the presence of suitable structure, would be justified.

In previous geochemical exploration techniques utilizing the source rock concept, rock formations have been characterized as source rocks on the basis of the amount and kind of hydrocarbons contained in the formation. Exemplary of such techniques are the procedures disclosed in the article by Bray et al., Distribution of N-Paraffins as a Clue to Recognition of Source Beds, Geochemica et Cosmochemica acta, l96l, Vol. 22, pages 2-15; and in US. Pat. Nos. 2,854,396 and 3,446,597. While such techniques are promising tools in exploration for oil, they are not without limitations.

Current applications of geochemistry to the solution of subsurface exploration problems utilizing the source rock concept have resulted in a number of procedures which have been of value in the exploration for and development of commercial oil and gas fields. The procedures presently available have been effective for determining whether or not a sedimentary formation, which may be relatively barren of hydrocarbon content, is a likely source rock for petroleum hydrocarbons, including gaseous petroleum hydrocarbons, and also for determining the state of maturation for such sediments. However, these methods fail to provide any indicia for ascertaining the likelihood that deeper formations could be source rocks for petroleum hydrocarbons.

It is an object of this invention to provide a method for the geochemical exploration for petroleum hydrocarbons.

It is another object of this invention to provide a method for determining the state of maturation of a subterranean geological formation and oil generation characteristics of such formation.

It is yet another object of this invention to provide a method for determining whether there exists in a particular area favorable conditions for the generation and accumulation of petroleum hydrocarbons, particularly at deeper depths than that at which the sample was taken.

These and other objects, aspects and advantages of the present invention will be readily apparent from a reading of the present disclosure and the appended claims.

In accordance with the present invention, there is provided a new and improved geochemical exploration method involving the identification of petroleum source rock sediments on the basis of the degree of diagenesis of the entrapped organic matter and the degree of oil generation properties of the organic fraction of the sediment.

The present invention provides a method for determining whether or not a sedimentary formation, which may be relatively barren of hydrocarbon content, is a likely source rock for petroleum hydrocarbons, and also for determining the state of maturation for such a sediment.

More particularly, the present invention provides a method of analyzing a sedimentary rock formation in order to ascertain the likelihood of the formation being a source rock for hydrocarbons. The present invention is particularly suited for determining whether exploration effort in the deeper portion of the basin should be undertaken.

To more fully appreciate the invention, the definition and meaning of certain words of art as they are used through the specification and appended claims are as follows:

Asphaltics are mixtures of resins and asphaltenes.

Asphaltenes are dark brown compounds of high molecular weight forming part of the asphaltic fraction of petroleum and natural asphalts. They are structurally similar to the resins from which they are distinguished by their solubility in liquid propane and their insolubility in n-pentane and by their powdery consistency.

Resins are reddish brown to brown compounds of high molecular weight forming part of the asphaltic fraction of petroleum and natural asphalts. Resins are distinguished from asphaltenes by their solubility in npentane and a tacky consistency.

Kerobitumens are dark brown compounds of high molecular weight occurring as a residue and byproduct of petroleum senesis and source rocks. There materials are structurally similar to resins and asphaltenes from which they are distinguished by their solubility in methylene chloride.

Organic carbon content is the sum of the residual oil and insoluble bitumen.

Residual oil includes saturates, aromatics and asphaltic fractions.

Saurates are compounds which contain only single carbon bonds, are nonpolar and are usually hydrocarbons, i.e., compounds consisting solely of the elements carbon and hydrogen.

Aromatics are compounds characterized by the presence of a benzene ring.

Paraffims or normal paraffins are alkane linear chains terminating in methyl (Cl-I groups.

Source rocks are rocks in which significant quantities of petroleum have been generated and from which the petroleum has migrated.

Nonsource rocks include barren, lean, and immature rocks.

Barren rocks are those devoid of organic matter.

Lean rocks are those containing low concentrations of organic matter.

Immature rocks are those which may contain an adequate amount of organic matter of the proper quality to be source rocks but have not undergone major genesis of petroleum because of insufficient time or depth of burial, i.e., insufficient heat.

OEP or odd-even predominance is the ratio of odd carbon number paraffin concentrations to even number paraffin concentrations of C -C normal alkane hydrocarbons.

OEP or average odd-even predominance is the ratio of odd-carbon-numbered to even-carbon-numbered C C normal alkanes in any given sample.

del value is the difference in parts per thousand in the (C /C ratio between the sample under consideration and an arbitrary standard, commonly called PDB, Craig, Geochemica et Cosmochemica acta, Vol, 12, pages 133-149 (1957), denoted in the following equation by the subscripts (samp) and (std), respectively:

8 w/ 12) P/( is/ 12) l 1] 1,000.

It has heretofore been recognized that thermal action hastens production of hydrocarbons from organic ma terial. For example, in the article by J. D. Mulik et al., Genesis of Hydrocarbons at Low Molecular Weight in Organic-Rich Aquatic Systems, Science, Vol. 141, No. 3575, (July 5, 1963), pages 806807, there is described the results of an investigation carried out with regard to recent marine sediments. In this work, it was discovered that mild heat treatment of sediment samples resulted in the generation of certain aromatic compounds which were not originally present in the samples in detectable amounts.

Thus, in accordance with the present invention, a sample obtained from a formation in the earths crust is subjected to thermal action in order to determine if organic matter originally present in the formation has, during the course of geologic history, undergone degenerative reaction eluting hydrocarbon products. As explained in detail hereinafter, by the analysis of certain materials produced during this treatment of the sample, it can be determined whether the formation from which the sample is taken is in a relatively early stage of maturation in which certain petroleum hydrocarbons have not been generated, or in a relatively late stage of maturation where there is a strong possibility that such petroleum hydrocarbons have been generated during the course of its geologic history. Further, from the analysis it can be determined whether the formation is capable of being a source rock for petroleum hydrocarbons.

In accordance with the present invention, a sample obtained from an organic matter-containing formation is disposed in a noncombustion-supporting environment and heated over an extended time interval at a constant elevated temperature to produce hydrocarbonaceous matte'r from the organic matter ,in the sample. The hydrocarbonaceous matter thus produced is analyzed to determine the percent extractable oil, the ratio of extractable oil to total oil, and the normal alkane distribution of the treated sample, and these values are compared to the corresponding values obtained from an analysis of the original untreated rock.

More particularly, in accordance with the present invention, rock samples taken from the earths crust, e.g., well cuttings, bottom hole cores, sidewall cores, etc., are first analyzed to determine the amount of extractable oil and the n-alkane distribution in accordance with conventional procedures. Thereafter, the residue from such analysis is subjected to a treatment comprising heating for an extended period in a noncombustionsupporting atmosphere at an elevated temperature and again analyzed to determine the amount of extractable oil and the n-alkane distribution of the thus-treated residue. From this there can be ascertained the percent of extractable oil, the ratio of extractable oil to total oil, and the normal alkane distribution of the sample under each set of analysis conditions. The respective values as obtained are compared to determine the state of maturation of the sample and the petroleum hydrocarbon generating capabilities of the sample. In this regard, it has been discovegd that when the n-alkane distribution provides an OEP of greater than 1.3, the formation is a young rock, geochemically speakin and, as such, is not a source rock. However, if the EP of the sample is less than 1.3, the formation can be considered a source rock for petroleum hydrocarbons. It has also been discovered that when the ratio of extractable oil to total oil of the thermally treated residue is greater than about 0.9 percent, the formation is at least a potential source rock and deeper drilling within the basin would be advantageous in those instances wherein the OEP had indicated the formation to be geochemically young.

The thermal treatment of a rock sample in accordance with the invention is an artificially induced accelerated aging of the formation sample to evaluate to what extent the rock was capable of initial or continued secondary genesis-of petroleum. In sufficient secondary genesis in organic-rich rocks is a common occurrence irrespective of geologic age. Increase in temperature, which is associated with increasing depth of burial, is more important than time in effecting the secondary genesis. In this regard, recent studies have indicated that, in some samples, the composition of the organic matter is such as to be capable of appreciable secondary genesis of petroleum irrespective of temperature and time. The significance of this differentiation in regard to exploration is that: (1) if it were known that with higher temperatures the rock would become a prolific source of petroleum, exploration effort in the deeper portion of the basin would be indicated provided that there is indication of suitable traps; whereas (2) if the quality of the organic matter must change before petroleum could be generated in quantity, then there would be no assurance that economic accumulations of petroleum would be generated by the rock anywhere in the basin.

The thermal treatment, i.e., accelerated aging, of the formation sample normally begins with the sample after the conventional source rock evaluations have been accomplished. Thus, in accordance with the present invention, the rock containing the unaltered kerobitumen, but from which the residual oil has been removed, is reconstituted with water, deoxygenated, sealed in a pressure vessel, and subjected to a constant temperature for a fixed period of time, usually about 2 weeks. The temperatures are chosen to increase the normal geochemical and geothermal reactions but are not so high as to cause the initiation of cracking reactions common in petroleum refining but not normally observed in the earth.

Following the aging process, the gas and liquid products are removed, characterized, and their amounts determined relative to the kerobitumen content of the sample. Carbon dioxide, nitrogen, hydrocarbons and sometimes hydrogen sulfide are contained in the gas phase. A hydrocarbon gas fraction consisting predominantly of methane may be indicative of very early genesis or, particularly if accompanied by hydrogen sulfide, of a terminal stage. Methane of early genesis is recognized by a carbon isotopic composition much lighter, i.e., with a more negative del value, than the kerobitumen, whereas methane representing late or terminal genesis is characterized by del values ranging from only slightly lighter to heavier than that of the kerobitumen.

The smoothed n-paraffin hydrocarbon di st ribution and the odd-even preference, i.e., OEP, the OEP value and the amount of oil generated relative to the kerobitumen content provides the index of whether and to what extent the rock was capable of initial or continued secondary genesis of petroleum. A copious yield of oil relative to the kerobitumen content is indicative of major secondary genesis. A smooth n-paraffin distribution curve with a maxi mum in the range C to C a low OEP cruve and an OEP value near unity indicate that the secondary genesis is essentially complete. When such is not found to be the case, the accelerated aging procedure may be repeated. A low yield of oil, a persistence of peaks in the range of C -C in the smoothed n-paraffin distribution, a persisting high OEP curve and an 5E value much above unity is interpreted as indicating that the rock is incapable of major secondary genesis of petroleum.

In preparing the sample for accelerated aging, the virgin sample is first analyzed to determine the organic carbon content which is the sum of the residual oil and insoluble kerobitumen, the residual oil and its composition of saturates, aromatics'and asphaltics; the carbon isotopic composition of the residual oil and iis f raction; the average-odd-even predominance, i.e., OEP, value for the n-paraffins in the carbon number range. 25 to 35; and the ratio of residual oil to total organic matter; and the smoothed n-paraffin distribution.

The extent to which oil in migratable quantities has been genen a te d is indicated by the odd-even predominance, or OEP. This parameter expresses the average bias in Concentration between n-paraffin hydrocarbons with odd-even carbon numbers in the range G -C The GE? value is unity for an oil in which there is an equal average concentration of odd and even nparaffins. The predominance of normal paraffins of odd carbon number is indicated by values above unity; whereas the predominance of even carbon numbers is indicated by values less than unity. The significance of m is that the small amount of hydrocarbons present in the source material contains normal paraffins with a strong odd carbon number predominance. Those formed abiogenetically during secondary genesis have little or no odd-even predominance and, depending upon the extent of secondary genesis, dilute those originally present, thus causing the OEP value to approach unity. Values above 1.30 for shale and above 1.35 for carbonates indicate that there may not have been enough secondary genesis of oil to provide the amount necessary for migration and accumulation in commercial amounts, thus indicating an immature rock.

The ratio of indigenous oil to total organic matter indieates the extent of retention of oil in the rock. A rock which has retained all the oil generated is by definition not a source rock. Provided there has been adequate secondary genesis of petroleum as indicated by (YE 1T a low value for the ratio indicates the generated oil has migrated out of the rock, hence accumulations in adjacent traps are likely. Under such conditions, values for the ratio of less than 0.04 are taken as indicating the oil has migrated in sufficient quantities to provide a commercial accumulation. Above this value there is decreasing probability that migration has occurred or that the amount of oil would be sufficient to provide an accumulation. Low values of the ratio, coupled with high 6? values, simply indicate that there has been little genesis of oil.

A high value of the ratio of residual oil to total organic matter, i.e., above 0.1, or an unusualscatter of carbon isotope values for the fractions of the residual oil suggest that the residual oil may not be indigenous to the rock. In such case the carbon isotope value for the kerobitumen should be determined and compared with that of the residual oil and its fractions. If the oil is indigenous to the rock, i.e., has common origin with the bitumen, or was formed under essentially identical conditions, the carbon isotopic value of the former will be slightly lower, or more negative than the latter.

Thus, the present invention provides a process or method for geochemical exploration comprising:

extracting with a solvent soluble organic matter from a sample obtained from an organic mattercontaining formation in the earths crust to obtain a rock residue containing insoluble organic matter;

disposing said rock residue containing insoluble organic matter in a noncombustion-supporting environment in the presence of water;

heating said rock residue at a constant elevated temperature for an extended period of time sufficient to cause at least a portion of said insoluble organic matter to undergo a chemical conversion;

measuring after said heating the amount generated by said heating of soluble organic matter; and

measuring the quantity of extractable organic matter from said rock residue relative to the insoluble organic matter of said original rock sample to determine the oil generation capability of said formation.

In a preferred embodiment, the present invention provides a process or method for geochemical exploration comprising: I

extracting with a first solvent soluble organic matter from an original rock sample obtained from an organic matter-containing formation in the earths crust to obtain a fraction comprising soluble organic matter and a rock residue comprising insoluble organic matter;

evaporating said fraction comprising soluble organic matter to obtain a residue comprising soluble organic matter;

separating said residue comprising soluble organic matter into a first fraction comprising saturates, a second fraction comprising aromatics and a third fractiojn comprising asphaltics;

further separating said fraction comprising saturates into a fourth fraction comprising normal alkanes;

determining the normal alkane distribution;

reconstituting with water said rock residue containing insoluble organic matter;

disposing said water-reconstituted rock residue in a noncombustion-supporting environment;

heating said reconstituted rock residue at a constant elevated temperature for an extended period of time sufficient to cause at least a portion of said insoluble organic matter to undergo a chemical conversion;

, after said heating, extracting with a second solvent said heated reconstituted rock residue to obtain a fifth fraction comprising soluble organic matter;

evaporating said fifth fraction comprising soluble organic matter to obtain a residue comprising soluble organic matter;

separating said residue comprising soluble organic matter into sixth fraction comprising saturates, a seventh fraction comprising aromatics and an eighth fraction comprising asphaltics;

further separating said sixth fraction comprising saturates into a ninth fraction comprising normal alkanes;

determining the normal alkane distribution; and

comparing the percent extractable oil, the ratio of extractable oil to total oil and the normal alkane distribution of the original sample and the reconstituted rock residue.

In addition to the preferred methylene chloride in the solvent extraction steps, any solvent system known in the art in which the kerobitumen fraction is substantially insoluble can be used. Included among such solvents are the aromatic solvents such as benzene, toluene and the xylenes and mixtures of polar-nonpolar solvents such as mixtures of the mentioned aromatic solvents and lower monohydric alcohols; ethylene glycol, propylene glycol; ketones such as acetone and the like. However, it is critical that the same solvent or solvent mixture be employed in each of the solvent extraction steps.

Substantially any method known in the art can be employed in separating the normal alkanes from the saturate fraction. As noted, urea adduction is a preferred method.

More particularly, the virgin rock sample, containing the organic matter dispersed therein in its natural state, is crushed, e.g., to p. size particles. A representative portion of the sample is analyzed by conventional means, such as by burning, to determine the amount of total organic matter which the original rock contains. At least a portion of the remaining sample, which is in particulate form, is weighed and placed in a suitable extractor, such as a Soxhlet extractor, and extracted with methylene chloride for a suitable period, such as 48 hours. The methylene chloride extract is evaporated to obtain an oily residue, which is weighed.

The oily residue is chromatographically separated into its constituent saturate, aromatic and asphaltic fractions.

The saturate fraction is urea adducted to obtain the normal alkane and the normal alkane distribution is determined by gas chromatography.

The rock residue from the methylene chloride extraction is disposed in a noncombustion-supporting environment. The desired noncombustion-supporting environment for the sample may be obtained by placing it in a suitable airtight chamber and thereafter diluting the air in the chamber by repeated injection and evacuation of nitrogen. The chamber then is evacuated to a vacuum, e.g., of about 2X psi, and a small amount of water is added in order to provide a water vapor atmosphere in the chamber. The chamber is provided with any suitable means for heating the sample and also for measuring the temperature of the sample and controlling it at the desired value. Suitable means also are provided for collecting the gaseous. matter produced during heating and for analyzing the gaseous matter for its components. For example, the produced gases may be collected over water and analyzed by mass spectrometry or gas chromatography. Since such systems and procedures for collection and analysis of gases are well known to those skilled in the art, they will not be described further.

The reconstituted rock residue is heated at a constant temperature in the range of about 200 to about 300 C. for an extended period of at least 7 days, preferably at least 10 days, and, more preferably, from 12 days to 3 weeks.

Analysis of the gaseous matter produced during heating may be made with regard to one or more designated index substances. Carbon dioxide and nitrogen are both associated with relatively low time-temperature equivalents, and therefore analysis is carried out with regard to at least one of these substances. Since nitrogen is the most consistent and reliable indicator of low time-temperature"equivalents, it is preferred to carry out analysis of the gaseous matter with regard to this substance. In most cases, of course, it will be desirable to analyze the gaseous matter produced during heating for both nitrogen and carbon dioxide.

As noted previously, methane is associated with relatively high time-temperature equivalents. This condition also is indicated by the relative paucity of nitrogen and, to a lesser extent, carbon dioxide in the gaseous matter produced during heating. It is preferred to carry out analysis of such gaseous matter with regard to methane, as well as nitrogen and carbon dioxide. This will supplement the data obtained from analysis with regard to nitrogen and carbon dioxide and also will indicate to some extent the past potential of the organic matter for the generation of methane and other gaseous hydrocarbons. The higher gaseous hydrocarbons, specifically ethane, propane, butane, and pentane, are associated with time-temperature equivalents intermediate those predominantly associated with carbon dioxide and methane. Analysis also may be carried out for these heavier hydrocarbon gases although such analysis alone normally will not provide abasis for a conclusive determination of the state of maturation of organic matter in the formation under investigation.

At the conclusion of the accelerated aging thermal treatment, the rock residue is extracted with methylene chloride as before; the oily residue obtained by evaporation of the methylene chloride is again separated into its constituent fraction; the saturate fraction is urea adducted to obtain the normal paraffins and the OE? determined as before.

After the heating step, the metane thus produced can be analyzed to determine the concentration of the carbon isotope C therein. The C- concentration for the methane may be measured and recorded on any suitable basis. It is preferred, however, to expres the C concentration in terms of parts per million of C on the basis of total carbon content. Measurement of the C concentration may be accomplished by any satisfactory procedure. For example, the methane may be converted to carbon dioxide and water by combustion and the resulting carbon dioxide then analyzed for its C concentration through the use of a mass spectrometer. For a more complete description of a suitable analysis procedure to be followed, reference is made to H. Craig, The Geochemistry of the Stable Carbon Isotopes, Geochemica et Cosmochemica acta, Pergamom Press Ltd., London, 1953, Vol. 3, pages 53-92.

A similar procedure may be followed in measuring the C concentration in the original organic matter contained in the formation. Thus, the second sample from the formation under investigation may be crushed and then burned in order to convert the carbon in the organic matter to carbon dioxide. The carbon dioxide thus produced then can be subjected to analysis for its C concentration by the use of a mass spectrometer.

The ratio of the C concentration in the methane to the C concentration of the formation organic matter is an indicator of whether the formation is source rock of hydrocarbon gases. By way of example, a formation exhibiting a methane C concentration of 10,790 parts per million and an organic matter C concentration of 10,800 parts per million would be a more likely source rock of gas than a formation exhibiting a methane C concentration of 10,700 parts per million and an organic matter C concentration of 10,800 parts per million.

In accordance with the present invention, the G3? and ratio of extractable oil to total oil of both the un aged and aged rock are correlated to determine on the one hand, the present and future state of maturity of the rock and, on the other hand, thgfl generation capability of the rock. In this regard, OEP values greater than 1.3 indicate a geochemically young rock and one which is not, as such, a source rock. An m value of less than 1.3 indicated that the rock can be considered,.

at least geochemically, a source rock for petroleum hydrocarbons. A ratio of extractable oil to total oil of the aged residue greater than about 0.9 indicates that the rock is capable of substantial secondary genesis of petroleum and is at least a potential source rock. Such ratios of extractable oil to total oil of the aged residue thus are indicia that deeper drilling within the basin would be advantageous, assuming the exist en ce of suitable traps, in those instances wherein the OEP, particularly of the unaged sample, had indicated the formation to be geochemically young.

The following examples are illustrative of the invention.

A series of tests were carried out on samples obtained from organic matter-containing subterranean sediments. In each case, the sample was comminuted and a representative portion of the comminuted sample was analyzed for organic carbon content by burning. The remainder of the comminuted sample was placed in a Soxhlet extractor and extracted with methylene chlo- 11 ride for 48 hours. The methylene chloride extract was flash evaporated to leave an oily residue which was weighed. The oily residue was chromatographically I crust. The sampling stations may be chosen randomly separated in a column containing silica gel into a saturate fraction,'an aromatic fraction and an asphaltic fraction. The saturate fraction was urea adducted to obtain the n-alkanes. The n-alkane distribution was determi n e d by gas chromatography to obtain the OEP andOEP values of the unaltered rock.

The rock residue from the methylene chloride ex traction was placed in a glass ampoule. The ampoule was evacuated to 100 microns, water was added to the ampoule and the ampoule was sealed. The sealed ampoule was heated at 250C. for 12 days.

After cooling, the rock from the ampoule was weighed, placed in a Soxhlet extractor and'extracted with methylene chloride for 48 hours. The methylene chloride extract was flash evaporated. The oily residue which remainder was weighed and chromatographically separated in a column containing silica gel into a saturate fraction, an aromatic fraction and an asphaltic fraction. The saturate fraction was urea adducted to obtain the n-alkanes. The n-alkane distribution was de-' termgred by gas chromatography to obtain the OEP and OEP values of the rock residue heated at 250. Q for 12 days.

The present extractable oil, the ratio of extractable oil to total oil, and the n-alkane distribution, particularly the OFF value, of the unaltered rock and heated rock residues for each sample is reported in the following table:

or on the basis of a predetermined scheme and the samples may be obtained at the surface of the earth or by drilling to subsurface locations within the earth s crust.

The samples thus obtained are analyzed by one or more of the above-described heating procedures in order to ascertain the character of the gaseous matter produced during heating. As noted previously, analysis of the gaseous matter must be made at least with respect to carbon dioxide or nitrogen. Preferably, analysis will be made with regard to nitrogen, carbon dioxide, and methane and also the methane and formation organic matter C concentrations. The measurements taken for the respective samples then are correlated with each other and the locations in the earths crust at which the samples are taken in order to ascertain possible source rocks of petroleum hydrocarbons in the area. This may be accomplished by plotting the significant gas generation and C characteristics for the samples at their respective location on a geographical map of the area surveyed.

The instant invention also may be utilized in carrying out supplemental exploration in areas within which other prospecting operations already have been carried out. For example, a seismic survey of a particular locality may indicate the presence of subsurface sedimentary structures favorable to the accumulation of petrosuch hydrocarbons.

leum hydrocarbons. In this instance, the present invention can provide a valuable tool in determining which of these subsurface structures may actually contain Unaltered Rock Samfigle Methylene Chloride-Extracted Altered Rock Residue ercent atro xtracta e Percent Ratio Extractable Extractable Oil/Total Oil Extractable Oil/Total Oil Run No. Oil X 100 OEP Oil X 100 OEP The foregoing results demonstrate that formations as exemplified byRuns 1, 3, '4, 5, 6, and 8 are indicated by the OT values to be potential source rocks. The oil 1 generation characteristics of these rocks as indicated by the ratio of extractable oil to total oil, particularly of the methylene chloride-extracted rock residue, indicate that in those areas of the basin where the format i9 n s am pled is found at a deeper depth, drilling to such deeper depths could be of advantage. The results further indicate that the formations as exemplified by Runs 2, 9, and 10 are nonsource rocks, regardless at what depth encountered. The formation as exemplified by Run 7 is borderline, as a potential source rock, but,

in view of the present squeeze on oil reserves, neverthel ess could justify deeper drilling.

The invention may be utilized in various exploration procedures. For example, theinvention may be utilized case, it usually will be preferred to obtain samples from In this use of the invention, at least one and preferably a pair of samples may be obtained from each of the source rock type formations in the area. Source rock type formations usually may be considered to be sediments such as shales of not more than six percent by weight organic matter content and carbonates of not more than one percent by weight organic matter content, and which have permeabilities of not more than one millidarcy. The samples then are analyzed for one or more of the index substances, as described above, in order to identify possible source rock formations. C measurements also may be carried out in accordance with the instant invention in order to ascertain for each formation the ratio of the methane C concentration to the formation organic matter .C concentration. These ratios then are correlated with each other and their respective rock formations in order to ascertain the formation having the highest such ratio, and a well is drilled into a reservoir rock formation which is in fluid communication with the designated source rock formation and which has a permeability greater than- 'the designated source rock formation. Preferably, the

I well is drilled into a'reservoir rock formation, atleast which lies next to the source rock formation and thus provides a ready acceptor for petroleum hydrocarbons formed in the source rock and migrating therefrom. However, in some cases it may be desirableto drill a well into a promising rock formation which is not in a contiguous relationship with the source rock formation, but which is in fluid communication therewith by other means such as through subterranean faults or joints.

Having described a specific embodiment of the instant invention, it will be understood that further modifications thereof may be suggested to those skilled in.

the art, and it is intended to cover all such modifica-,

I claim: 1. In a method of geochemical prospecting, the steps comprising:

tions as fall within the scope of the appended claims.

extracting soluble organic matter from a sample ob-,

tained froman organic matter-containing formation in -the earths crust to obtain a rock residue containing insoluble organic matter;

disposing said rock residue containing insoluble organic matter in; a noncombustion-supporting environment in the presence of water;

heating said rock residue at a constant elevated temperature for an extended period;

measuring the quantity of extractable organic matter from said rock residue relative to the insoluble organic matter of said original rock to determine the oil generation capability of said formation.

2. A method of geochemical exploration comprising:

: formation in the earths crust to obtain a first frac:

tion comprising soluble organic matter and a rock residue comprising insoluble organic matter; evaporating said first fraction comprising soluble organic matter to obtain a residue comprising soluble organic matter;

separating said residue comprising soluble organic matter into a fraction comprising saturates;

separating from said fraction comprising saturates a fraction comprising normal alkanes;

determining the normal alkane distribution;

reconstituting with water said rock residue contain-,

ing insoluble organic matter;

heating said reconstituted rock residue at a constant elevated temperature for an extended period of time;

solvent extracting said heated reconstituted rock residue to obtain a second fraction comprising soluble organic matter;

evaporating said second fraction comprising soluble organic matter to obtain a residue comprising soluble organic matter;

determining the normal alkane distribution; and

comparing the percent extractable oil, the ratio of extractable oil to total oil and the normal alkane distribution of the orginala sample and the reconstituted rock residue.

3. A method according to claim 2 wherein said solvent employed in the solvent extraction of said original rock sample and said reconstituted rock residue is methylene chloride.

4. A method according to claim 3 wherein said reconstituted rock residue is heated at a temperature of 250 C. for 12 days.

Claims

1. A METHOD OF GEOCHEMICAL PROSPECTING THE STEPS COMPRISING: EXTRACTING SOLUBLE ORGANIC MATTER FROM A SAMPLE OBTAINED FROM AN ORGANIC MATTER-CONTAINING FORMATION IN THE EARTH''S CRUST TO OBTAIN A ROCK RESIDUE CONTAINING INSOLUBLE ORGANIC MATTER; DISPOSING SAID ROCK RESIDUE CONTAINING INSOLUBLE ORGANIC MATTER IN A NONCOMBUSTION-SUPPORTING ENVIRONMENT IN THE PRESENCE OF WATER; HEATING SAID ROCK RESIDUE AT A CONSTANT ELEVATED TEMPERATURE FOR AN EXTENDED PERIOD; MEASURING AFTER SAID HEATING THE AMOUNT GENERATED BY SAID HEATING OF SOLUBLE ORGANIC MATTER; AND MEASURING THE QUANTITY OF EXTRACTABLE ORGANIC MATTER FROM SAID ROCK RESIDUE RELATIVE TO THE INSOLUBLE ORGANIC MATTER OF SAID ORIGINAL ROCK TO DETERMINE THE OIL GENERATION CAPABILITY OF SAID FORMATION.

2. A method of geochemical exploration comprising: solvent extracting soluble organic matter from a sample obtained from an organic matter-containing formation in the earth''s crust to obtain a first fraction comprising soluble organic matter and a rock residue comprising insoluble organic matter; evaporating said first fraction comprising soluble organic matter to obtain a residue comprising soluble organic matter; separating said residue comprising soluble organic matter into a fraction comprising saturates; separating from said fractiOn comprising saturates a fraction comprising normal alkanes; determining the normal alkane distribution; reconstituting with water said rock residue containing insoluble organic matter; disposing said water-reconstituted rock residue in a noncombustion-supporting environment; heating said reconstituted rock residue at a constant elevated temperature for an extended period of time; solvent extracting said heated reconstituted rock residue to obtain a second fraction comprising soluble organic matter; evaporating said second fraction comprising soluble organic matter to obtain a residue comprising soluble organic matter; separating said residue comprising soluble organic matter into a fraction comprising saturates; separating from said fraction comprising saturates a fraction comprising normal alkanes; determining the normal alkane distribution; and comparing the percent extractable oil, the ratio of extractable oil to total oil and the normal alkane distribution of the orginala sample and the reconstituted rock residue.

4. A method according to claim 3 wherein said reconstituted rock residue is heated at a temperature of 250* C. for 12 days.