GB2256885A - Method of exploring for oil - Google Patents

Abstract

The earth's subsurface is explored for formations having large numbers of fluid inclusions in the rock matrix thereof characterized by relative abundance of water soluble hydrocarbons. Further exploration for oil occurs adjacent such formations.

Description

OIL EXPLORATION The invention relates to exploring the earth's subsurface for producible hydrocarbons. In a particular aspect the invention relates to a method of exploration comprising exploring the earth's subsurface for formations characterized by large numbers of fluid inclusions of diagnostic composition.

Producible oil accumulates when hydrocarbons generated from source rock migrate through the subsurface to a trap. Benzene and toluene present in the trapped oil can diffuse outwardly from the oil accumulations through connate water in the pore space of formation rock. Samples of connate or formation water have been taken and analyzed for benzene and toluene and explorationists have attempted to use presence of these hydrocarbons in formation water as an indicator of nearby trapped hydrocarbons.

Formation water samples, however, are not available, for instance, in the case of plugged wells; and, in any case, require interrupting of drilling or other operations to obtain the sample. During acquisition of the water sample, handling may cause losses of the indicator species such as benzene or toluene. Further, techniques based on determining benzene and toluene are only effective where the formation water is in contemporaneous diffusional communication with trapped hydrocarbons. The technique also fails to detect trapped hydrocarbons depleted in the indicator hydrocarbons and traps where seals have formed isolating the water zone from the oil zone of the trap. Further, formation samples can be contaminated with drilling mud additives and sensitive to mud types, such as oil based muds.

A new technique is needed which overcomes deficiencies of the above described method.

A new technique of exploring for oil is needed which can use materials commonly archived for old wells and which does not require interruption of drilling or other operations to obtain.

A new technique is needed where handling of the sample does not cause loss of indicator species, even though the samples may have been archived for many years.

A new technique is. needed which can detect zones which have been in diffusional communication with trapped hydrocarbons in the past even though the diffusional com munication may not have continued to the present due to interfering seal formation, depletion of indicator species and the like.

A new technique is needed which does not become significant or at all contaminated by drilling mud additives or impeded by sensitivity to mud type.

A new technique is needed which is formation specific.

This invention relates to identifying formations in the earth which are characterized by large numbers of aqueous fluid inclusions in the matrix of the rock having compositions comprising water soluble hydrocarbons present in oil and using this information as an indicator of adjacent trapped oil or hydrocarbons. The term "fluid inclusion" is used in its ordinary and accepted meaning to refer to a tiny cavity in the matrix of the mineral, 1.0 micron or less to 100.0 microns or greater in diameter, containing liquid and/or gas formed by the entrapment in the mineral of fluid, usually ambient fluid, present when the fluid inclusion was formed. Fluid inclusions are distinct from pore spaces or voids in the rock, which are usually filled with formation or connate water and contain oil or gas within a trap.

In accordance with the present invention, it has been found that formations yielding rock samples having large numbers of aqueous fluid inclusions containing water soluble hydrocarbons are diagnostic of nearby hydrocarbon accumulations and can be used in exploring for such hydrocarbon accumulations. The occurrence of such aqueous fluid inclusions in the rock samples indicates that accumulated oil has been in diffusional communication with the rock from which the rock sample was taken over a period of geological time which permitted these fluid inclusions to form.

In one aspect, the invention comprises a method of exploring for oil by identifying formations in the earth's subsurface more characterized by aqueous fluid inclusions comprising water soluble hydrocarbons found in oil than adjacent formations.

In a further aspect, the invention comprises releasing fluid inclusion volatiles from large numbers of fluid inclusions in rock matrix of each of a plurality of rock samples representative of formations in the subsurface; determining occurrence of water soluble hydrocarbons in the thus released fluid inclusion volatiles of each rock sample; comparing thus determined compositions of the fluid inclusion volatiles of the rock samples with one another; and identifying rock samples having a greater abundance of water soluble hydrocarbons relative to other rock samples.

In a further aspect, the occurrence of such water soluble hydrocarbons of fluid inclusions in each rock sample is normalized relative to a parameter representative of the rock sample such as rock sample volume, rock sample weight, and the like.

In a further aspect, the invention relates to further exploring for oil adjacent to formations characterized by abundance of water soluble hydrocarbons diagnostic of hydrocarbon accumulations.

In another aspect, the invention comprises a method of exploring for oil and gas in which rock samples are obtained from different locations in the earth's subsurface; and one or more rock samples characterized by large numbers of aqueous fluid inclusions having compositions diagnostic of presence of adjacent hydrocarbon accumulations during fluid inclusions formation are identified.

In a further aspect, the invention comprises a method of exploring for oil and gas in which a plurality of rock samples is obtained representative of formations adjacent a well as a function of depth. The composition of fluid inclusion volatiles released in bulk from such rock samples is determined and the abundance of various components of composition of fluid inclusion volatiles is normalized relative to a parameter representative of the rock sample. The relative abundance in fluid inclusion volatiles is displayed as a function of depth along the borehole. From the resulting display depth zones along the borehole characterized by greater abundance of said water soluble hydrocarbons are identified.

The Invention will now be described by way of example with reference to the accompanying drawings in which: FIGURE 1 illustrates schematically a well penetrating the water zone of an oil trap.

FIGURE 2A illustrates a simplified flow chart of a method in accordance with the invention.

FIGURE 2B illustrates the method of FIGURE 2A in detail.

FIGURE 3A illustrates, in exploded view, a preferred autosampler system for automated release and composition analysis of collective fluid inclusion volatiles samples from each of a plurality of rock samples.

FIGURE 3B illustrates a cutaway view of a portion of the autosampler 10 of FIGURE 3A as assembled.

FIGURE 4A illustrates schematically a system for mass spectroscopic analysis of collective fluid inclusion volatiles samples.

FIGURE 4B illustrates, by a simplified flow diagram, control of the autosampler/analysis system of FIGURE 4A.

FIGURE 5 illustrates, by simplified flow diagram, a system for summing mass to charge ratio (MCR) responses for each of a plurality of scans of a range of MCR for a single collective volatiles sample to produce summed MCR values for the totality of scans for the single collective volatiles sample which can be displayed as an MCR spectrogram.

FIGURE 6A schematically illustrates measurement of autosampler background data and of autosampler background data plus collective fluid inclusion sample data.

FIGURE 6B schematically illustrates distinguishing inclusion from noninclusion volatiles by subtracting background data from sample data.

FIGURES 7, 8, and 9 illustrate differences in MCR compositions among rock samples characterized by oily fluid inclusions, by aqueous fluid inclusions predominantly containing inorganic compounds, and by aqueous fluid inclusions enriched in water soluble hydrocarbons respectively.

FIGURE 10 illustrates identifying formations characterized by aqueous fluid inclusions enriched in occurrence of water soluble hydrocarbons in rock samples taken from a well penetrating the water zone of an oil trap.

FIGURE 11 illustrates formations having rock samples taken from a well penetrating the water zone of an oil trap where the fluid inclusions are not enriched in water soluble hydrocarbons.

FIGURE 12 illustrates formations having rock samples not taken from a well penetrating a water zone of an oil trap where the fluid inclusions are not enriched in water soluble hydrocarbons.

Referring now to FIGURE 1, FIGURE 1 illustrates schematically a well a penetrating the water zone b of an oil trap. The trap has a gas zone f, an oil zone c underlying the gas zone, and a water zone b underlying the oil zone. The oil migrates into the trap via a migration pathway illustrated by d from source rock e. As a result of using the invention herein described, further exploration illustrated by a well having dashed line g may be trilled to produce the reservoir, or a deviated well h may be drilled.

Where a water zone such as b underlies or is otherwise in diffusional communication with an oil zone c and the oil is sufficiently rich in water soluble hydrocarbons, over a period of time fluid inclusions will trap water soluble hydrocarbons in aqueous 'fluid inclusions in the rock matrix of the water zone penetrated by a well.

The water soluble hydrocarbons characteristic of oil accumulations include lower alkanes such as methane, ethane, propane, and the like, organic acids such as acetic acid, and aromatics such as benzene, toluene, xylene, ethyl- and other alkylated benzenes, and the like, and mixtures thereof.

Certain sets of water soluble hydrocarbons from aqueous fluid inclusions obtained from a rock sample are indicative of diffusional communication between accumulated hydrocarbons and the formation from which the rock sample was taken. These sets include the set of water soluble hydrocarbons described in the previous paragraph as well as various subsets of that set. These subsets in particular include any subset including acetic acid, any subset including one of benzene and toluene, any subset characterized by benzene (toluene plus benzene) ratios significantly higher than such ratios in accumulated oil.

Generally, the presence of methane, ethane (or lower alkanes) should be supplemented by one or more of the other members of the set to be diagnostic.

If formations yielding rock samples which are characterized by large amounts of aqueous fluid inclusions containing water soluble hydrocarbons normally occurring in oil are found, it is an indicator that the water zone or water leg of a hydrocarbon trap has been found. While it is possible, subsequent to aqueous fluid inclusion formation, that the oil accumulation could have been lost (for example, due to loss of seal, etc.), generally the presence of such water soluble hydrocarbons in aqueous fluid inclusions will be indicative of nearby oil accumulations.

This is true even if the trapped hydrocarbons become depleted, for example, due to diffusional loss, in water soluble hydrocarbons over time. The aqueous fluid inclusion record will still preserve information that water soluble hydrocarbons were once present and in diffusional communication with the formation. Thus, the fluid inclusion record can be said to integrate over time.

Conversely, where no zones enriched in aqueous fluid inclusions comprising water soluble hydrocarbons are found, it is possible that no oil accumulation is or has been present, or that water solubles normally present in the oil were lost to formation waters during migration (for example, along path d of FIGURE 1) prior to trapping of the hydrocarbons.

In some cases, for example, where traps occur close to source rock, as is known to occur in certain regions of the world, the absence of zones enriched in water soluble hydrocarbons may be indicative that nearby oil accumulations are not present. This is because the oil may not have been reservoired for sufficient time.

Judgments based on fluid inclusion data should of course take into consideration all other relevant information as is well known to those skilled in the art of exploring for oil and gas. However, the positive occurrence of zones having large numbers of fluid inclusions having relatively abundant water soluble hydrocarbons has been found to be a significant indicator of accumulated nearby oil.

This exploration technique is based upon the finding that formations yielding rock samples characterized by relatively abundant quantities of water soluble hydrocarbons, especially methane, ethane, benzene, toluene, and acetic acid determined using bulk fluid inclusion mass spectrometric analyses occur in water wet regions of oil-producing reservoirs. A high relative abundance of these fluid inclusion volatile compounds in a rock sample of one formation compared, for example, to rock samples of adjacent formations, or to an abundance response similar to that found adjacent known reservoirs, is an oil proximity indicator.

Without limiting the invention, the following explanation is provided. Waters surrounding and in diffusional communication with reservoired oil will be saturated with water soluble hydrocarbons. As secondary inclusions form in the water zone through time, for example, through the process of microcrack healing, aqueous inclusions containing these water soluble compounds will be trapped. Cements forming after filling of the reservoir will also form aqueous inclusions containing these water soluble compounds. If migration is rapid, and oil is not reservoired nearby, the number of aqueous inclusions trapped that are saturated with water soluble hydrocarbons will be small. However, if oil is reservoired nearby for an appreciable length of geological time, large numbers of secondary aqueous fluid inclusions containing these water soluble hydrocarbons will form. Thus, the formations of interest are those characterized by rock samples having large numbers of fluid inclusions containing water soluble hydrocarbons, or simply a greater abundance of such fluid inclusion water soluble hydrocarbons, than rock samples of adjacent formations.

Generally, presence only of lower alkanes such as methane, ethane, and the like will not be as reliable an indicator of reservoired oil nearby in diffusional communication with the sampled rock, though it may indicate nearby reservoired gas. It is noted, however, that the lower alkanes if present in formation waters according to the prior art technique would frequently be lost in handling, whereas these species are retained and provide significant information according to the invented technique.

However, if one or more of acetic acid, benzene, alkylated benzene, and the like is also abundantly present, relative to abundance in adjacent formations, this occurrence is strongly indicative of pooled oil nearby at some point in geological time.

Since significant periods of geological time appear necessary for formation of the fluid inclusion record of interest, it is believed that this technique is more reliable than techniques based on analysis of formation waters for water soluble hydrocarbons. In any case, its other advantages make it a significant additional tool in exploring for pooled hydrocarbons.

Occurrence of these proximity indicators has been found within a mile or less from reservoired oil and it is expected that such indicators will be detectable up to five miles or more from the reservoired oil.

A. Obtaining Rock SamDles As illustrated in FIGURES 2A and 2B, step A of the invention relates to obtaining a plurality of rock samples as a function of depth or areal distribution in the earth.

The rock samples can be washed drill cuttings, cores, outcrop samples, soil samples, sidewall cores, and the like. Preferably, drill cuttings are used. Drill cuttings are widely available and allow investigation of substantially the entire length of a borehole. Further, unlike formation water samples, such cuttings are frequently archived for old wells, and the inclusion contents remain stable indefinitely over time and can be acquired in the normal course of drilling without interruption of operations.

Where more than one well is present in an area of exploration interest, areal samples are frequently also available.

About 10 cubic centimeters or less of each rock sample are suitable for analyses using the invention.

Rock samples representative of at least every 200 ft, 100 ft, 50 ft, 20 ft, 10 ft or less provide an adequate frequency of sampling for good results. Thirty-foot spacing has provided excellent results in many runs. More frequent sampling is essentially advantageous for this technique. For regional investigations, each of multiple spaced apart wells can be sampled along substantially the entire depth of a well from adjacent the surface to total depth, or along a zone of particular interest, for example, a particular formation. Preferably, 50 to 100 or more specimens spanning a depth or areal domain of interest are selected; fewer can also be used.

B. Selectina Rock Specimens Characterized bv a Selected Class of Fluid Inclusions As illustrated at step B of FIGURE 2A and steps B1, B2, B3 of FIGURE 2B, the invention in one aspect relates to selecting one or more rock specimens characterized by fluid inclusions having an enhanced content of water soluble hydrocarbons relative to other rock samples.

Fluid inclusions are trapped fluids occluded in the matrix of rocks in tiny cavities which do not contribute to the rock's pore system. Fluid inclusions are classified as hydrocarbon or oily inclusions when liquid hydrocarbons are predominant, aqueous inclusions when liquid water is predominant, and gaseous when gases are predominant. Mixed inclusions also occur. This invention uses aqueous fluid inclusion compositions for exploring for nearby oil accumulations.

For purposes of this invention, mixed fluid inclusions are considered and treated as substantially equivalent to individual fluid inclusions having sizes corresponding to aqueous or oily portions of the mixed inclusions.

According to the present invention, fluid inclusion composition data are used to identify formations in diffusional water wet communication with pooled hydrocarbons. Sedimentary reservoir rock is characterized by large numbers of fluid inclusions usually smaller than 10 microns diameter and of different generations, i.e., formed at different times and representing different environments. The frequency of occurrence of inclusions per unit volume of sedimentary rock sample varies considerably, but can be reasonably estimated to be on the order of 103 to 109 inclusions per cubic centimeter of rock.

This order of magnitude of occurrence of fluid inclusions in sedimentary rocks such as carbonates, sandstones, and shales is referred to herein as myriad fluid inclusions and volatiles released indiscriminately from myriad fluid inclusions are referred to as released in bulk or as bulk or collective fluid inclusion volatiles.

Since fluid inclusions are ruptured in bulk the resulting composition is a composite composition of all the volatiles. Where the composition of the bulk volatiles is predominantly characterized, directly or in relation to other rock samples or adjacent formations by presence or absence of various compounds such as water soluble hydrocarbons, this is representative of there being large numbers or quantities of aqueous fluid inclusions in a rock sample which are also so characterized.

B1 - Releasina Fluid Inclusion Volatiles in Bulk from Rock Samples As indicated, an aspect of the invention can include a step of releasing fluid inclusion volatiles in bulk from each of a plurality of rock samples. The release of fluid inclusion volatiles in bulk can be accomplished by any suitable technique including those known to those skilled in the art such as placing a rock sample in a metal tube, crimping the ends, impacting to release volatiles, and opening the tube and collecting the volatiles for analysis. Preferably, the release of bulk volatiles is achieved as hereinafter described.

Referring now to FIGURE 3A, FIGURE 3A illustrates in exploded view an autosampler 10 configured with controller 16 and spectrometer 12 for releasing, delivering and analyzing composition of a plurality of fluid inclusion volatiles samples. Volatiles samples released in bulk from myriad fluid inclusions in a sedimentary rock sample may also be referred to as collective volatiles or collective fluid inclusion volatiles.

System 10 includes upper housing 27 and lower housing 28 having seal 30 therebetween for forming evacuable chamber 60 (see FIGURE 3B) when housings 27 and 28 are aligned and joined. It is desirable for chamber 60 to be as small as feasible. Seal 30 can be an oxygen-free high conductivity copper gasket. Housings 27 and 28 can be adapted with knife edges for sealing by engaging gasket 30. Evacuable chamber 60 has an outlet 11 with valve 11v which delivers released volatiles to spectrometer 12 as they are being released.

Vacuum pump 14 places vacuum stage 10 under a vacuum at the start of a sequence of analyses. Thereafter, the system can be maintained under vacuum by pumps 15' associated with the mass spectroscopic system.

See FIGURE 4A.

Lower housing 28 comprises flange 28f, sidewall 28s, and base 28b. Base 28b has a groove 32 therein holding bearings 34. Circular carousel 26 is adapted with a plurality of sample chambers 36 therein and centered slot 46 for engagably receiving shaft key 44 on stepper motor shaft 42.

Asymmetry of key 44 and notch 46 assure that the carousel 26 is positioned in the autosampler so that each sample has a uniquely determined position. Carousel 26 has groove 32b for engaging bearings 34 in groove 32a in base 28b. When carousel 26 is placed in lower housing 28, grooves 32b and 32a cooperate to align the carousel 26, and bearings 34 provide for rotation of carousel 26 in response to motor 24 turning shaft 42 having key 44 engagably connected with slot 46.

Sample chambers 36 are each effective for receiving a rock sample 38 and for maintaining it during volatiles release in a confined space between the walls and base of the chamber and the impacting means.

According to an aspect of the invention, the relative abundance of water soluble hydrocarbons is normalized relative to a parameter representative of the rock sample. While this may be accomplished by data manipulation after analysis, it is preferred to use approximately equal weights or volumes of rock sample for each analysis.

A typical rock sample is less than 10 cc (cubic centimeters) in volume which provides sufficient material for several runs, if necessary. Core and outcrop samples are usually broken prior to analysis while drill cutting samples can be used directly. Individual samples for analysis generally range from about 1/100 to about 1/2 cc, typically about 1/25 to about 1/2 cc. As indicated, use of approximately equal samples by volume or by weight is preferred.

Depths or areal locations are recorded by entry into a computer such as controller 16 as the samples are loaded into predetermined sample chambers in the carousels. The depths can later be transferred to another computer such as a mainframe for analysis of resulting data if desired.

Three pneumatic rams 18, 20, and 22 are illustrated passing through upper housing 27. More or fewer rams can be used. Illustrated carousel 26 has three concentric rings of sample chambers 36, and each pneumatic ram aligns with a respective concentric ring of sample chambers. Ram 19 is illustrated with plunger 52 and ram tip 54. Ram 19 aligns with outer ring 36O; ram 20 aligns with intermediate ring 36i, and ram 22 aligns with central ring 36c. When a sample chamber 36 is aligned with a respective ram, the ram is actuated to impact a sample 38 in the chamber effective for releasing a collective volatiles sample. Preferably, each sample chamber is also provided with a sample chamber slug 40 to prevent cross contamination of samples during impacting. Slug 40 can be considered part of the impacting means.Sample 38 and slug 40 are shown enlarged in circle 39 for clarity. However, slug 40 is adapted to cover sample 38 in chamber 36 while permitting volatiles to escape through an annulus between slug 40 and the wall of chamber 36. While only one slug 40 and sample 38 are shown, there will usually be as many slugs 40 and samples 38 as chambers 36.

Referring now to FIGURE 3B, FIGURE 3B illustrates in greater detail the operation of the rams. Pneu matic ram 19 as indicated is aligned with the outer row 36O of sample chambers on carousel 26. In response to a signal via line 18c from controller 16 (see FIGURE 3A), the pneumatic ram bellows 48 expand, driving shaft 50 and plunger 52 into contact with the slug 40, impacting sample 38 in chamber 36. Impacting of the sample may occur one or more times, preferably multiple times under control of controller 16 to ensure release of substantially all fluid inclusion volatiles. The released fluid inclusion gases then are transported within chamber 60 through a space between the lower surface of upper housing 27 and the upper surface of the carousel 26 to mass spectrometer 12 for analysis.Mass spectrometer 12 can be controlled by computer controller 16 as illustrated by line 12C.

Impacting of the sample preferably occurs while the sample is closely confined by a slug 40 in a chamber 36. The impact can be any impact sufficient for releasing a collective fluid inclusions volatile sample, for example, by crushing, pulverization, and the like.

Preferably, the impact is effective for causing a deformation or concussion of the sample effective for releasing a collective volatiles sample substantially without crumbling or powdering the sample. For most drill cuttings run, an impact of about 400 pounds per square inch is effective. The result of crushing is preferably a rock sample deformed and shaped by the sample chamber and the crushing means into a compacted aggregated mass.

Impacting can take place virtually instantaneously up to about 10 seconds or even longer. Ten or twelve seconds have provided highly satisfactory results.

In such case, the plunger impacts the rock sample and maintains fluid inclusion deforming pressure thereon for 10 or 12 seconds, for example. When iterative impacting is employed, all of the iterations can be made to occur in 10 seconds or 12 seconds or less if desired. Alternatively, the sample can be impacted and pressure maintained for a period of time such as 10 or 12 seconds, released for a period of time such as 10 or 12 seconds, and again impacted and pressure maintained and released one or more additional times. Impacting generally can be for a time effective for releasing a volume of fluid inclusion gases.

Release of substantially all, or at least a preponderance of, fluid inclusion volatiles is preferred.

As illustrated, the invention includes a controller 16 for controlling sampler 10, for example, by controlling motor 24, rams 18, 20, 22, to release sequentially in bulk from each of a plurality of rock samples fluid inclusion components and for controlling mass spectrometer 12 for removing and analyzing the released fluids.

In pumpdown configuration value 13v is open and value 11v is closed; in automated sampling configuration, 11v is closed and 13v is open. Autosampler 10 can be heated to maintain the samples at about 1500C during operation. Inlet and outlet lines to mass spectrometer system 12 can also be heated to about 1500C. Alterna tively, room or ambient temperature operation can be used to facilitate equilibration of the system. When room temperature operation is used, a polymer vacuum seal can be used instead of a metal gasket for seal 30 in FIGURE 3A.

The analysis temperature can be any temperature effective for volatilizing particular molecules of interest up to a temperature less than that at which thermal decapitations causes release of fluid inclusion volatiles.

For oil and gas exploration, temperatures in the range of about 1500 to about 2000C are particularly advantageous for volatilizing of hydrocarbons.

For operation autosampler 10 is evacuated, for example, first to very high vacuum using a turbomolecular pump such as pump 14 not open to the mass spectrometers.

The entire system can then pump down in its analytical configuration, for example, for a period of time, for example, three hours before the analytical session is begun. When the system is in analytical configuration, released inclusion volatiles from autosampler 10 can be pumped directly through mass spectrometers 12 (See FIGURE 5A). That is, gas evolved during analyses can be pumped through the ionization chambers of the mass spectrometers in order to be pumped away. If desired, automated valving can be added so that pump 14 assists in pumpdown between impacting of rock samples.

For operation, the system is maintained at a vacuum of about 10 8 to about 10 6 torr. Even during release of volatiles, the pressure will not increase much above 10 torr. Generally, the pumps evacuating the system during analytical configuration maintain low pressures to insure substantially all of released volatiles are passed through mass spectrometers for analysis.

The operation of controller 16 is described in detail below in reference to FIGURE 4B.

B2 - Determinina Composition of Released Volatiles As indicated, a step of an aspect of the invention relates to determining composition of released fluid inclusion volatiles. Any suitable means for determining composition such as gas chromatography (GC), mass spectroscopy (MS), combined GC/MS and the like can be used. A preferred method of determining composition is illustrated in FIGURES 4A, 4B, 5, 6A and 6B.

Referring now to FIGURE 4A, FIGURE 4A illustrates a preferred system for mass spectrometric analysis of collective fluid inclusion samples. Referring now to FIGURE 4A in detail, there is illustrated a source 10' of collective fluid inclusion volatiles samples, such as autosampler 10 in FIGURE 3, connected via line 11' having valve ll'v to a preferred arrangement of mass spectrometers 12'. . During analytical configuration, valve ll'v is open and samples are being withdrawn as they are released by crushing. Thus, the system depicted in 4A is dynamic, i.e., open to the sampler 10' during sample release. As illustrated, the mass spectrometers are arranged in two banks of three, each bank having a pump 15' for drawing sample from line 11' through each of mass spectrometers 12' via outlet line 13'.Each mass spectrometer is configured to sample a specific set of MCR (mass to charge ratio) responses using the optimum gain for each, for example, as follows: Mass to Charge Ratio Mass Spectrometer Responses Sampled 1 2,16,17,18,28,44 2 3,4,12,13,14,15,19-27,29-43,45-60 3 61-120 4 121-180 5 181-240 6 241-300 Generally there are no peaks at MCR 5 to 11. By assigning specific MS to sample a set of MCR responses which have comparable amplitudes, time lost in switching amplifiers for the MS can be minimized. Thus, MS1 samples the most abundant MCR < 61 and MS2 samples the least abundant MCR < 61.

The 0-10v signal outline of each mass spectrometer 12' is operably connected to a bank of five signal conditioners 17, each configured for a different optimum gain, discussed in detail below.

The outputs of signal conditioners 17 are provided to analog to digital converter (ADC) 17' and then to computer controller 16'. For simplicity, only the output of one MS 12' is illustrated but the other MS 12' are also so configured.

Since the MS system of FIGURE 3A is open to sampler 10 during sampling, volatiles are being passed through the MS system over a period of time dependent on the relative molecular weight of the volatiles and the period of time when volatiles are being released from a particular sample. Accordingly, the MS system is configured and controlled for scanning a range of MCR of interest a multiplicity of times during the period of release of volatiles from each rock sample, and the results from all the multiplicity of scans are summed on an MCR by MCR basis for each rock sample.

As described herein, the MCR range of interest is from about 2-300 MCR to encompass an advantageous range for analysis. Greater or lesser ranges can also be used.

Preferably, substantially all or at least a preponderance of ranges such as 2-60, 2-120, 2-180, 2-240, 2-300 and the like are scanned a multiplicity of times as a volatiles sample is released from each rock sample. As described herein, the multiplicity of scans is 256. More or fewer scans can be used, for example, 128. Reduction in number of scans leads to loss of precision and accuracy, but can increase speed of operation.

Referring now to FIGURE 4B, FIGURE 4B illustrates control of the MS system of FIGURE 3A integrated with control of the autosampler 10 of FIGURE 3. Generally, the system scans sampler 10 or 10' a multiplicity of times during a time when no sample is being released and sums the results on an MCR by MCR basis to provide a background reading. The system then scans sampler 10' a multiplicity of times during a time when a collective volatile sample is being released from a particular rock sample and sums the results on an MCR by MCR basis to pro vide a sample reading. The system repeats the preceding two steps until a plurality of samples has been run. In a preliminary data reduction step, background readings taken before each sample is read can be removed from the sample readings. The preliminary data reduction is described in more detail below in reference to FIGURES 6A and 6B.

Referring now to FIGURE 4B in detail, FIGURE 4B illustrates a system for control of sampler 10 and the MS system of FIGURE 3A.

Controller 16 can be, for example, a personal computer programmed for controlling autosampler 10, mass spectrometers 12 and for storing composition data produced from mass spectrometer 12 on disk together with apparatus for driving the rams, motor, controlling mass spectrometers and the like. Such equipment can readily be assembled by those skilled in the art for use as described herein. Any suitable controller can be used.

For each rock sample, controller 16 generates signals causing measurements and recording of background data, causing a rock sample to be impacted, causing measurement and recording of background plus fluid inclusion volatiles, causing storing of preliminary recorded data on disk and querying whether all samples have been run. If all samples have not been run, controller 12 generates a signal controlling motor 24 for causing carousel 26 to position for crushing of the next rock sample. When all samples have been run, controller 12 can perform end of run procedures such as releasing the vacuum on the system, data transfer, and the like. The operation of controller 16 is illustrated in more detail in FIGURE 4B discussed below.

As indicated at 220 in FIGURE 4B, certain preliminary operations can be controlled by controller 16.

Thus, controller 16 can generate signals for formatting a data disk in controller 16, for calibrating mass spectrometer system 12, and for positioning carousel 26 for impacting of a predetermined first rock sample.

Step 222 is for setting the beginning of the MCR range (MCR = 2). Step 224 is for controller 16 sampling the output of the MS configured for MCR2 and step 226 is for the computer selecting a signal conditioner for optimum gain for MCR2 signal and causing the selected conditioner output to appear on the output line of ADC 17' where the computer samples it (step 228). Step 230 is for sending the MS to the next MCR to be tested. Step 232 is for storing the sampled ADC in the appropriate summer.

The computer then by Steps 234 and 236 are for the computer sampling in the same way via loop 244, the MS assigned to the next MCR until the full range of MCR 2-300 has been scanned.

The step for sending the appropriate MS to the next MCR is illustrated in FIGURE 4A by line 121 and in FIGURE 4B by step 230. It can be accomplished using controller 16 including a DAC (digital to analog converter).

Thus, a personal computer can provide a signal selecting the next MS for the next MCR to a DAC for a particular mass spectrometer. The DAC can then cause the appropriate mass spectrometer to be configured for the next MCR to be read.

By step 238, the full range of MCR of interest is scanned a multiplicity of times for each rock sample, the data for each MCR being summed on an MCR by MCR basis for the multiplicity of scans. After 256 scans, the computer tests whether there was a scan of background data or of sample data by step 240. This can be as simple as determining the set of 256 scans is the second set since impacting the previous rock sample. Upon determining that the readings were of background data, step 242 stores the background data for the sampler in the computer's memory and generates a signal to autosampler 10 causing the first sample to be impacted and returns to step 224.

Steps 224 through 240 are then repeated and when step 240 now responds indicating that sample data have been measured, step 246 stores the sample data on disk.

Step 248 inquires whether all samples have been run and if not, by step 250 and loop 252 provides a signal via line 24c (see FIGURE 3) to sampler 10 to position the next rock sample for analysis. After step 248 indicates that all samples have been run, step 254 ends the run, and the data can then be transferred if desired (see step 256) to another computer for preliminary data reduction. All of the steps described above can be readily implemented by those skilled in the art of computerized control from the description herein using commercially available equipment.

Referring now to FIGURE 5, an autoranging routine is shown for selecting an optimum signal conditioner for each MCR reading and for summing the readings on an MCR by MCR basis. A 0-10 volt signal from each mass spectrometer is sent to a bank of five signal conditioners set at different gains. The gains of the 30 signal conditioners are calibrated using a National Bureau of Standard standard. The computer uses an autoranging routine such as shown in FIGURE 5 to select the optimum signal conditioner for each MCR scan. For each MCR reading, a particular signal conditioner is selected by, for example, 0-5v gain control 200. Then each MCR response is directed to the appropriate memory for summing by steps 202, 204, 206, 208. Thus, if step 202 indicates that the signal s for the particular MCR is > 5v, the response is sampled on the 0-10v channel and summed using the 0-10v summer 203.If l < s < 5, step 204 samples and sums the response using the 0-5v summer 205. If 0.5 < s < l, step 206 samples and sums the response using the 0-lv summer 207. If 0.1 < s < 0.5, step 208 samples and sums the response using the summer 209; if < 0.1, using the summer 211. After, for example, 256 mass scans are summed for each MCR in the range of 2-300 MCR, computer 16 can sum the responses for each MCR over all scans and can generate for each collective fluid inclusion volatiles sample a mass spectrogram such as the one shown at 220 in FIGURE 5B.

Preferably the abundance of different MCR of an MCR spectrogram are presented in logarithmic scale. This is because linear scale representations make difficult recognition of occurrence of trace elements and compounds useful in characterizing classes of inclusions. The use of a logarithmic scale which enhances MCR responses of trace organic and inorganic volatiles relative to the more abundant components of fluid inclusions is therefore preferred. The MCR spectrogram 220 in FIGURE 5 represents such a display in logarithmic scale. Linear or other scales can also be used.

Mass spectrograms for autosampler background data and for a sample are also illustrated in a simplified manner at 132 and 136, respectively, in FIGURES 6A and 6B.

During operation, controller 16 reads the output of mass spectrometers 12' 256 times in about 10 seconds as a volatiles sample is being released from an individual rock sample to collect 256 complete MCR spectra from MCR2 through 300, i.e., for each volatiles sample 256 scans of MCR 2-300 are made. A summer 213, for example, in computer 16, sums the 256 responses from each MCR from all of the multiplicity of scans as they are collected. For each MCR, after selecting the optimum signal conditioner, collecting the data, adding it to the total for that MCR, and storing the result in memory, controller 16 triggers the appropriate mass spectrometer system to proceed to the next MCR. The computer then reads a signal from the mass spectrometers configured to sample that MCR, and so on until 256 MCR scans are summed.A time interval, for example, about 100 microseconds can be allowed between each MCR sampled.

For each rock sample the summed data from the first multiplicity of scans are an analysis of the background gases in the system (see 130 in FIGURE 6A).

Once the background is characterized, the computer signals and controls the appropriate air piston one or more times to ram the appropriate steel slug thus impacting the sample (time of occurrence illustrated in FIGURE 6A by arrow 134). 256 new scans of 2-300MCR are initiated each time the rock sample is impacted or while the rock sample is crushed multiple times. The sum of the second and subsequent multiplicity of 256 scans is the analysis of the fluid inclusion gases plus the background, as illustrated by reference numeral 136 in FIGURE 6A.

Referring now to FIGURE 6B, FIGURE 6B illustrates a preliminary data reduction step in which the background gas contribution 132, characterized immediately before impacting each rock sample, is subtracted from data 136 on an MCR by MCR basis for each collective fluid inclusion volatiles sample plus background. This technique is effective for discriminating inclusion from noninclusion gases so that the final volatiles record is representative of inclusion gases.

In the mass spectrometer, the molecules in each bulk sample are ionized, accelerated, separated according to MCR, and measured. Ionization is usually accompanied by partial fragmentation of the molecules which is characteristic of specific molecules and operating conditions.

While fragmentation complicates interpretation - a given molecular weight fragment can be derived from different molecules - it also permits distinguishing between isomers and gives molecular structural information. The output can be various forms of MCR versus abundance records, mass spectrograms, and the like.

The mass values of some fragments encountered in fluid inclusion analysis and source molecules are shown in the following table.

MCR Sianatures - Inorganic Fluid Inclusion Gases Table 1 Inorganic Gases MCR Signature H2 2 He 4 H2O 18 CO2 22,44 Ar 40 N2 28,14 NH3 17 CO 28 H2S 34 O2 32 SO(1-3) 48 COS 60 CS2 76 Ne 20,22 HC1 35,36,37,38 Xe 129,130,131,132,134,136 Mass Sianatures - Oraanic Fluid Inclusion Gases Table 2 Oraanic Gases Mass Sianature Methane 15 Ethane 30 Propane 44 Butane 58 Benzene 78 Toluene 91 Xylene 105 Triterpenes 191 Steranes 217 The mass spectra for the higher mass organic compounds becomes complicated with overlapping mass spectra peaks, making it difficult to identify single compounds with certainty.Classes of organic compounds, however, share common fragments: MCR Sianatures - Hiaher Mass Oraanic Compounds Table 3 Organic Gases MCR Sianature paraffins 57 naphthene s 55 aromatics 77 toluene 91 alkylated naphthenes 97 In addition to these peaks, these hydrocarbon families tend to give responses at every 14 mass numbers due to the CH2 repeat in organic polymers: MCR Signatures - Hiaher Mass Organic Compounds With Repeating CH2 Table 4 Oraanic Gases MCR Sianatures paraffins 57,71,85,99,113,127, etc.

naphthenes 55,69,83,97,111,125, etc.

B3 - Selectina Rock Specimens Characterized by Enrichment in Water Soluble Hydrocarbon Fluid Inclusions As indicated in FIGURES 2A and 2B, a step of an aspect of the invention relates to selecting rock specimens characterized by fluid inclusion composition enriched in water soluble hydrocarbons relative to adjacent formations in the earth. This can be done in any convenient way such as by inspecting fluid inclusion compositions for selected compounds and selecting a rock specimen from a rock sample characterized by the composition. Preferably the selecting step comprises displaying selected composition data as a function of depth or areal location in the earth, identifying one or more depths characterized by greater abundance of an MCR indicator of water soluble hydrocarbons, and selecting rock specimens from rock samples corresponding to such depths.

In evaluating the subsurface for formations characterized by aqueous fluid inclusions enriched in water soluble hydrocarbons, it is useful and advantageous to generate plots indicative of three classes of fluid inclusions: aqueous inorganic, aqueous enriched in water soluble hydrocarbons, and oily.

A person skilled in examining MCR spectrograms can readily distinguish compositions characteristic of aqueous inorganic fluid inclusions from compositions characteristic of aqueous fluid inclusions enriched in water soluble hydrocarbons from oil fluid inclusions.

Figures 7-9 illustrate MCR spectrograms of these compositional types.

Formations characterized by oily inclusions are characterized by relatively large amounts of paraffins and naphthenes, particularly those with repeating CH2 groups such as set forth above in Table 4. Referring now to FIGURE 7, FIGURE 7 illustrates an MCR spectrogram for a rock sample rich in and characterized by oily fluid inclusions. Generally, oily inclusions can be distinguished by the occurrence of the peak, valley, peak, valley sequence illustrated in Figure 7, with the peaks for fragments for each carbon number generally occurring at repeating CH2 fragment intervals.

Referring now to FIGURE 8, FIGURE 8 illustrates an MCR spectrogram for rock characterized by aqueous fluid inclusions containing inorganic compounds and relatively insignificant amounts of organic compounds. In this regard, it is noted that ordinate of the MCR spectrogram is a logarithmic scale. Formations having aqueous fluid inclusions characterized by inorganics show strong MCR peaks characteristic of water, of a CO+ and CO++ fragments from CO2, and CO2 and much weaker MCR peaks, even on a logarithmic scale, characteristic of water soluble hydrocarbons and of paraffins and longer chain hydrocarbons.

Such other hydrocarbons may be frequently present in only baseline, trace amounts.

Thus, FIGURE 8 has strong peaks at MCR 18 (H2O), MCR 28 (CO+), NCR 22 (CO2++), 44 (CO2), and the like, but substantially lacks longer chain hydrocarbons having repeating CH2 groups.

Formations characterized by aqueous fluid inclusions enriched in water soluble hydrocarbons display, relative to formations characterized by aqueous inorganic fluid inclusions, strong MCR peaks characteristic of water soluble hydrocarbons and relative to formations characterized by oily fluid inclusions, relatively weak peaks char acteristic of paraffins and long chain hydrocarbons with repeating CH2 units. Relative to both aqueous inorganic and to oily inclusions, such formations also show higher ratios of acetic acid/(acetic acids + paraffins), aromatics/(aromatics + paraffins), benzene/(toluene + benzene), and similar ratios.

Whereas in oil accumulations, toluene is usually more abundant than benzene (benzene/toluene) ratio of 1:2 or more, in aqueous fluid inclusions characterized by abundance of water soluble hydrocarbons, toluene is usually less abundant than benzene (benzene/toluene) ratios as high as 10:1 or even higher.

For convenience, MCR indicators of some water soluble hydrocarbons are set forth in Table 5 below: Table 5

Water Soluble Hvdrocarbon MCR Signature Acetic Acid* 60 Benzene 78 Toluene 91/92 Ethyl benzene # (weak) 105/106 Xylene Methane 15 Ethane 30 *Acetic acid may be depleted in carbonate minerals due to reaction with the mineral.

Referring again to FIGURE 9, FIGURE 9 illustrates an MCR spectrogram for a rock sample characterized by aqueous fluid inclusions containing abundant water soluble hydrocarbons. Thus it can be seen that MCR 15, 30, 34, 60, 78, 91/92 and 105/106 representing respectively methane, ethane, acetic acid, benzene, toluene, and ethyl benzene/xylene have strong peaks. Particularly noteworthy are the acetic acid, benzene, toluene, and xylene/ethyl benzene peaks. Paraffins and naphthenes having repeating CH2 groups may be present but are much less abundant (even orders of magnitude less abundant) than the water soluble organics. In the aqueous fluid inclusions containing water soluble hydrocarbons, water insoluble hydrocarbons may be present in only relatively trace, insignificant amounts.

Preferably composition data resulting from analysis of collective fluid inclusion volatiles are displayed as a function of depth along a borehole or of areal location in the earth. Since the composition data are representative of heterogeneous fluid inclusions, MCR can be selected representative of particular compounds of interest and displayed relative to depth of rock sample.

Such displays may be referred to as fluid inclusion composition log displays.

Further, it is useful to compare one or more types of molecules to one or more others, such as A to B, by determining a ratio A/(A+B). This permits a semiquantitative evaluation from well to well. A is referred to herein as normalized with respect to B. Either A or B can represent one or more MCR.

MCR Ratio indicators of aqueous fluid inclusions enriched in water soluble hydrocarbons are set forth in Table 6.

Table 6 Acetic Acid/(Paraffins + Acetic Acid) 60/60+57 Aromatics/(Paraffins + Aromatics) 77/77+57 Benzene/(Toluene + Benzene) 78/78+91 In particular, it is useful to generate logs as a function of depth along a wellbore of MCR peaks representing two or more water soluble hydrocarbons and ratios of MCR peaks such as set forth in Table 6. Such plots facilitate identifying formations characterized by the three classes of fluid inclusions identified above relative to adjacent formations as a function of depth.

FIGURES 10, 11, 12 illustrate MCR logs for methane, ethane, acetic acid, benzene, toluene, and acetic acid/(paraffins + acetic acid) for three wells.

FIGURE 10 illustrates identifying formations characterized by aqueous fluid inclusions comprising water soluble hydrocarbons in fluid inclusions of rock samples taken from a well penetrating the water zone of an oil trap. At around 15,000 ft, it can be seen that methane, ethane, acetic acid, benzene, and toluene are present and that the ratio of acetic acid to paraffins plus acetic acid shows a strong response relative to adjacent formations. Thus a strong response of these MCR is indicative of occurrence of nearby oil accumulations.

FIGURE 11 illustrates such MCR logs for another well adjacent an oil field. It can be seen that the oil proximity indicators are substantially absent compared to FIGURE 11. This indicates that such indicators are not necessarily present even in the water zone adjacent an oil well. However, as shown by FIGURE 10, where aqueous fluid inclusions are enriched in water soluble hydrocarbons, it is a strong indicator that trapped oil is nearby.

FIGURE 12 illustrates such MCR logs for another well which is not adjacent an oil trap. FIGURES 11 and 12 together indicate that nonoccurrence of aqueous fluid inclusion volatiles is not necessarily indicative that trapped oil is not nearby.

Thus, the presence of fluid inclusions enriched in water solubles is a strong indicator of nearby accumulations of oil. As usual in oil and gas exploration, this indicator should be used with other available information in evaluating a prospect.

Fluid inclusion composition data can also be usefully displayed in ternary diagrams.

Useful ternary displays include, for example, displays of MCR indicators of paraffins, acetic acid, and CO2 representative generally of formations characterized respectively by oily, aqueous enriched water soluble hydrocarbons and aqueous inorganic fluid inclusions.

Table 6 above illustrates some useful binary displays; however, many other selections for display of relative or absolute abundances of elements and compounds in fluid inclusions can be used in accordance with the invention.

It will be apparent that suitable displays of such data as a function of depth permit rapid selection of rock specimens characterized by particular classes of fluid inclusions which are of technical and economic significance.

While binary ratios, tertiary ratios and the like can be advantageously used to select rock samples dominated by aqueous fluid inclusions enriched in water soluble hydrocarbons, relative abundance of the various components found in fluid inclusion volatiles is preferably selected and displayed relative to a parameter representative of the rock samples themselves, i.e., abundance (moles of gas) per weight or volume unit of rock sample ("raw" or "absolute" response). A logarithmic or suitable linear scale is preferred for scaling relative abundance of components, preferably a scale which scales a peak indicative of greatest abundance of a series of rock samples to full scale on the display. When raw responses are to be obtained, it becomes more important to insure that the volume or weight of each rock sample is approximately equal.Good results have been obtained, however, even with somewhat disparate volumes of rock or weight samples.

Thus, by inspecting fluid inclusion compositions or by displaying selected compositions as a function of depth or areal location, rock samples from formations characterized by fluid inclusions of selected compositions can be identified, and therefore such formations are themselves identified.

According to a further aspect of the invention, after identifying a zone characterized by aqueous fluid inclusions enriched in water soluble hydrocarbons, there can be further exploration for oil and gas based on the resulting information.

The further exploration can comprise, for example, drilling additional well(s) (see g in Figure 1) or sidekick a deviated well such as h in formations adjacent the indicator zone in areas which available geological/geophysical information indicates as likely locations of the oil trap. Other additional forms of exploration known in the art such as further seismic exploration, well logging and the like may also be advantageously used. Such techniques are well known to those skilled in the art and need not be described here in detail.

The invention has been described in terms of specific embodiments and examples but is not limited thereto but by the claims appended hereto, interpreted in accordance with applicable principles of law.

Claims (15)

CLAIMS;

1. A Method of exploring for oil comprising: identifying a formation in the earth's sub surface characterized by aqueous fluid inclusions in rock matrix thereof having compositions comprising water soluble hydrocarbons found in oil in greater abundance than in an adjacent formation.

2. The Method of Claim 1 comprising: releasing fluid inclusion volatiles from large numbers of fluid inclusions in rock matrix of each of a plurality of rock samples representative of formations at depths in the subsurface; determining occurrence of water soluble hydrocarbons in the thus released fluid volatiles; comparing compositions of fluid inclusion volatiles of the rock samples with one another; and identifying rock samples having fluid inclusion volatile compositions significantly enriched in water soluble hydrocarbons compared to other rock samples of the plurality of rock samples.

3. The Method of Claim 1 wherein rock samples are taken at intervals not less than an interval selected from the group consisting of 10, 20, 30, 50, 100, and 200 feet along a wellbore.

4. The Method of Claim 1, wherein-the water soluble hydrocarbons comprise one or more hydrocarbons selected from the group consisting of methane, ethane, propane, acetic acid, benzene, toluene, ethyl benzene, and xylene.

5. The Method of Claim 1 wherein the occurrence of water soluble hydrocarbons in a rock sample is normalized relative to a parameter representative of the rock sample.

6. The Method of Claim 5 wherein the occurrence of water soluble hydrocarbons in the rock sample is normalized to a parameter representative of the rock sample selected from rock sample volume and rock sample weight.

7. The Method of Claim 2 comprising: displaying a measure of abundance of at least one water soluble hydrocarbon found in oil as a function of location represented by the rock samples; and comparing compositions of fluid inclusion volatiles of the rock samples with one another using such display.

8. The Method of Claim 7 comprising: displaying a measure of abundance of at least one of acetic acid, benzene, toluene, xylene, and ethyl benzene as a function of location repres ented by the rock samples.

9. The Method of Claim 8 comprising: displaying variations in a ratio as a func tion of location represented by the rock samples wherein the ratio is selected from the group compris ing benzene/(benzene + toluene) and acetic acid/(acetic acid + paraffins) and aromatics/(paraffins + aromatics).

10. The method of Claim 1 comprising: further exploring for oil and gas in adja cent formations within a distance of 5 miles of the thus identified formation.

11. A Method of exploring for oil and gas comprising: obtaining rock samples from different locations in the earth's subsurface; and identifying one or more rock samples char acterized by aqueous fluid inclusions having compos itions diagnostic of presence of adjacent hydrocarbon accumulations during fluid inclusion formation.

12. The Method of Claim 11 wherein the compositions diagnostic include one or more of the following sets of components: a. acetic acid b. acetic acid and at least one of methane, ethane, and propane; and c. acetic acid and at least one of benzene, toluene, xylene, and ethyl benzene.

13. The method of Claim 11 comprising: further exploring for oil and gas in adja cent formations within a distance of 5 miles of the thus identified formation.

14. A method of exploring for oil and gas comprising: obtaining a plurality of rock samples representative of formations adjacent a well as a function of depth; determining composition of fluid inclusion volatiles released in bulk from such rock samples; wherein the abundance of various components of composition of fluid inclusion volatiles is normalized relative to a parameter representative of the rock sample; displaying relative abundance in fluid inclusion volatiles of at least one water soluble hydrocarbon occurring in oil as a function of depth along the borehole; and identifying from the resulting display depth zones along the borehole characterized by greater abundance of said water soluble hydrocarbon relative to other depth zones

15. A method of exploring for oil or gas substantially as hereinbefore described with reference to and as shown in the accompanying drawings.