Predicting hydrocarbon potential of an earth formation underlying a body of water by analysis of seeps containing low concentrations of methane

The present invention provides for on-site capture of methane at sea, for isotopic examination. Liquid and interfering gases are separated from the methane; the methane is oxidized to form carbon dioxide and water; and the carbon dioxide and water are isotopically analyzed for carbon and deuterium distribution to determine methane origin, as an aid to evaluation of hydrocarbon potential of an earth formation.

SCOPE OF THE INVENTION 
This invention relates to a method and apparatus--in general--for providing 
for isotopic chemical analysis of methane seeping from a hydrocarbon pool 
or other source of organic matter associated with an earth formation 
underlying a body of water, and--in particular--for providing for on-site 
capture of such gas whereby indications of their biogenic and/or 
thermogenic origin of the pool can be accurately forecasted. 
In one aspect, the present invention provides for the acquisition of highly 
accurate data related to the isotopic chemistry of extremely small 
concentrations of such gas, say 1 to 10 microliters per liter of the sea 
water, being constantly collected at depth. 
In another aspect, the dissolved methane can be collected in sufficient 
amounts utilizing vacuum separation and selective capture techniques in 
the presence of an inert air carrier. The sequence of steps includes: 
Carbonaceous fluids are first separated from the water collected at depth; 
then the methane present is quantitatively converted to gaseous carbon 
dioxide and heavy water, if present. Basis of later analysis is the 
isotopic composition of the .sup.13 C (or .sup.14 C) and deuterium 
associated with the collected sample. Further, since the normalized 
variation of .sup.12 C to .sup.13 C (i.e., the delta .sup.13 C 
measurement) requires less amounts of methane to be collected, such 
analytical method is preferred. The delta .sup.13 C measurement is defined 
in Petroleum Formation and Occurrence, B. P. Tissot, D. H. Welte, 
Springer-Verlag, N.Y., (1978) at p. 88 as: 
##EQU1## 
BACKGROUND OF THE INVENTION 
While marine exploration systems are presently available for continuously 
sampling water seeps so as to analyze for presence of carbonaceous fluids 
such as methane, none have the capability of providing a compositional 
parameter that is uniquely diagnostic of seep origin, and hence allowing 
the user/operator to distinguish the biogenically derived sample from a 
sample associated with a hydrocarbon source. 
Reasons: Other interpretative tests were thought to be sufficient from a 
cost/result standpoint. Also, the lengthy and complexed nature of the 
steps involved in collecting, isolating and tagging sufficient amounts of 
the samples for such analysis were thought to be beyond the capability of 
present on-site collection and analytical systems. 
MODIFICATIONS IN ACCORDANCE WITH THE PRESENT INVENTION 
When it was noted in the above-identified parent application that 
hydrocarbon potential of earth formations (underlying bodies of water) 
could be evaluated based, inter alia, on .sup.13 C (or .sup.14 C and/or 
delta .sup.13 C) content of collected methane samples, interest in 
shipboard collection and analysis techniques intensified. 
It has now been discovered that if the heavier isotope of hydrogen, 
deuterium, is also a constituent of the captured methane, then yet a 
further interpretive clue to biogenic origin of the methane is provided. 
Since the oxidation products of the collected methane (carried out at 
station 39 of FIG. 4 of the above parent application) are gaseous carbon 
dioxide and water, and since both such products are trapped in the 
subsequent trapping station in the above-identified embodiment (i.e., at 
station 40 of FIG. 4), isotopic presence of deuterium can be easily 
determined by mass spectroscopic examination around the time that the 
previously described carbon isotopic testing was performed. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, a quick, convenient and highly 
accurate technique for the acquisition of sufficient amounts of two 
constituents of methane dissolved in sea water is provided. Result: 
Indications of their isotopic character--and biogenically- and/or 
thermogenically-derived origin of the associated pool--can be easily and 
surprisingly accurately determined. 
In more detail, sea water is first collected via an electro-hydraulic 
cable, at depth by a drone trailing from a sea-going vessel, the water 
being pumped at a substantially constant flow rate in a range from about 3 
to 7 liters/minute. Up cable destination of the water: A vacuum chamber 
aboard the vessel where the water is broken into droplets under a slight 
vacuum (27-28 inches of mercury) and the gaseous constituents, liberated. 
These constituents are carried via an air stream to a continuous 
hydrocarbon flame monitor where, if the flame monitor response is 
positive, more complexed analytical equipment is brought into play; e.g., 
a multi-port valve can be energized as to allow the dissolved gases to be 
analyzed chromatographically. Or still another of the valve ports can be 
activated to allow the same constituents to flow into and through an 
isotropic trapping network where collection in microliter amounts occurs. 
Within the isotopic network, use is made of the flowing air stream (flow 
rate being preferably about 30 milliliters per minute in a range of 20-120 
milliliters per minute). Gases of interest pass, in seriate, from 
station-to-station: Methane is isolated (by removing all interfering 
species), and finally converted to gaseous carbon dioxide and heavy water, 
if present (in a catalytically-aided complete oxidation reaction), and 
both are cryogenically trapped in a U-shaped trapping chamber. Next, the 
ends of the trapping chamber are heated and collapsed, sealing them from 
the atmosphere. 
After being transported to a mass spectrometer, the chamber is re-opened so 
that isotopic analysis can occur. Using the latter results (along with 
geographic address data) allows for accurate biogenically- and 
thermogenically-associated predictions to be made.

DETAILED DESCRIPTION OF THE PRESENT INVENTION 
FIG. 1 illustrates the present invention. 
As shown, a drone 10 is positioned at a depth Z.sub.0 below a vessel 11 
floating at the surface 12 of the body of water 13. Within the drone 10, 
is a pump (not shown). The purpose of the pump: To draw water samples 
interior of the drone 10 and pump them upcable (via electrohydraulic cable 
14) to a diagnostic system 15 aboard the vessel 11. 
In addition to pumping equipment, the drone 10 is fitted with various 
oceanagraphic devices, including a depth sensor; a current monitor is also 
provided that includes a bottom-oriented sonar device in combination with 
an electromagnetic sensor for measuring the speed and direction of the 
ocean currents relative to the bottom. Signals from these devices pass via 
conductors in the side walls of the cable 14 so as to provide annotation 
(both visual and on tape), associated with geographic position (of the 
drone and/or the vessel), depth of the drone, etc., as shown. In that way, 
accurate geographic addresses for the samples as a function of drone 
and/or vessel location, is assured. 
Diagnostic system 15 also provides for a series of geochemical tests, and 
includes vaccum separation system 16 and a gas phase analysis network 17. 
Key to diagnostic results using network 17: determination that hydrocarbon 
gases are present and what types (using hydrocarbon analyzer system 18) in 
combination with a gas chromatagraph 19, and then isotopic examination via 
isotopic capture network 20. 
FIG. 2 illustrates operation of diagnostic system 15 in still more detail. 
As shown, vacuum separation system 16 includes an air-tight chamber 21 for 
separating the water into liquid and gas phases. Entry and egress from the 
chamber 21 via a series of inlets and outlets 21A-21C. Inlet 21A receives 
the water samples via cable 14 and associated manifold 22. A nozzle 24 
breaks water into droplets. Note that even though the flow rate of the 
water can be as high as 7 liters per minute, usual flow rate is usually 
about 3.7 liters per minute. The separated liquid phase is discharged from 
the chamber 21 via outlet 21B and pump 25. The gas phase exits via outlet 
21C through vacuum pump 26 and exhaust manifold 27 in the presence of a 
wet air carrier. 
Also of importance in the operation of diagnostic system 15: storage and 
display of all annotation data via the electrical network/display 34 to 
allow for determination of geographic addresses of all samples taken by 
the drone, as previously explained. 
Manifold 27 includes an arm 28 which terminates in a continuously operating 
hydrogen flame ionization analyzer 18. Remaining arm 29 of the manifold 27 
terminates at multi-port valve 30. 
One port 31 of the valve 30 is open to the atmosphere. Another port 32 is 
disconnectably connected to gas chromatagraph 19 which, when operating, 
providing gas chromatograms. Yet another port 33 of the valve 30 is 
disconnectably connected to isotopic capture system 20 of the present 
invention. 
Since the gas chromatograph 19 as used in association with operations of 
the present invention is usual, that is, the gas chromatograph 19 provides 
chromatograms of hydrocarbon components in the water, emphasis of 
description is placed on isotopic analysis system 20 of the present 
invention. 
FIG. 3 illustrates how easily isotopic capture system 20 can be transported 
aboard a vessel 11 in either an assembled or unassembled state and be 
effectively operated in any type of environment. And since all functions 
associated with the isotopic capture system 20 occur aboard a sea-going 
vessel often in a hostile environment, constructional aspects related to 
portability, reliability and ruggedness are of some importance. 
As shown, the capture system 20 includes a series of trapping and stripping 
stations 35-40 mounted to upright front panel 50 of carry-on capture box 
51. The box 51 includes side, top and back panels 52, which form an 
enclosure interior thereof, wherein equipment associated with operations 
can be stowed either temporarily as during transport (or permanently as 
required). Carry handle 53 facilitates hand-transport of the box 51 to and 
from the vessel. Note also that the front panel 50 intersects bottom panel 
58 near its center. Hence, not only can the operator use bottom panel 58 
as a floor for equipment associated with stations 35-40, but also he can 
place a separate cover 54 (shown in phantom line) in attachment with the 
panels 52, 58 as when transport of the box is required. In that way, the 
equipment comprising the stations 35-40 can be protected against breakage 
during transport. Note that the cover 54 has extending side and top panels 
55 of reverse orientation with respect to the shape provided the side, top 
and bottom panels. Result: Disconnectably connecting hinges 56 can be 
aligned with mounts 57 to releasably attach the cover 54 with respect to 
the panels 50, 52 and 58. 
FIG. 4 illustrates operation of stages 35-40 of system 20 in more detail. 
Assume the operator has allowed dissolved gases to enter the sytem 20 via 
valve 60 to station 35 to begin operations. 
The key to isotopic operation of system 20 lies in quantitative oxidation 
of the dissolved "signature" gas of interest, i.e., methane, as within 
oxidation station 39 and subsequent collection at station 40, of selected 
oxidants thereof. These operations occur after sinusoidal travel of all 
the collected, dissolved gases via the intermediate stations 35-38 as set 
forth below. The usual flow rate of the gas sample within system 20 is 
about 30 microliters per minute. The amount of collection at station 40 is 
dependent on the methane concentration in the sample sea water, the flow 
rate of the air carrier system, and the separation efficiency of the 
vacuum separation system aboard the vessel. If the normal methane 
concentration is 1 microliter per liter of water, and the extraction rate 
of the drone is 7 liters per minute at depth, then 10 minutes will be 
needed to collect about 50 microliters of the gas of interest at station 
40, assuming extraction efficiency at the vacuum system of 75%. In the 
vicinity of modest gas seeps, the concentration of methane can easily 
approach 10 microliters per liter of water (STP) particularly in deep 
water. A background sample typically containing 0.2 microliters of methane 
will take 50 minutes to collect. 
Briefly with reference to FIG. 4, the wet air carrier and the dissolved 
gases from the vacuum separation center enters station 35 at inlet 61. At 
the station 35, the gases perculate downwardly through the series of 
absorbent materials 62 supported in upright tube 63. Materials 62 remove 
both water vapor and molecular carbon dioxide. Next, the carbon monoxide 
which also occurs in variable abundance in water, is removed at station 36 
by oxidation to carbon dioxide; the latter is subsequently removed from 
the carrier system after passing via valve 64 to station 37. The carrier 
gas stream containing both air gases and low molecular weight alkanes is 
then directed to stage 38 after passage through valve 65. 
At the station 38, the lower, mid- and higher-range molecular weight 
hydrocarbons are removed, that is, all hydrocarbons above C.sub.1. The 
remaining methane then enters station 39 where it is oxidized to gaseous 
carbon dioxide and water. After passage through valve 66 the latter is 
subsequently retained at station 40. The details of operation of stages 
35-40 will now be presented in more detail below. 
STATION 35 
Purpose: To trap water vapor and molecular carbon dioxide in the gas phase 
of the separated sample. The station 35 is constructed of the tube 62 
attached to the front panel 50 of capture box 51 upright position, see 
FIG. 3, the tube 63 usually being constructed of standard wall pyrex 
tubing. A bed of absorbent materials 62 is held in place by small wads of 
glass wool 67 placed at the ends of the tube 63. The absorbent materials 
62 are conventional and are available in the industry for removing water 
vapor (viz calcium chloride, CaCl.sub.2) and for absorbing molecular 
carbon dioxide (namely, sodium hydroxide, NA(OH)). Mixture ratio 1:1. 
STATION 36 
Purpose: To remove carbon monoxide which occurs in variable amounts in sea 
water using a flow-through furnace system 70. 
As shown in detail FIG. 5, furnace 70 consists of a helix 71 wound about a 
quartz tube 72, the tube being previously wrapped with a single layer of 
asbestos tape 73. The helix 71 is then covered with additional asbestos 
tape 74 as well as with a glass wool matting 75 forming a sidewall into 
which a thermocouple (not shown) can be inserted. The ends of the helix 71 
and the thermocouple are electrically connected to thermal controller 76 
of FIG. 4. The controller 76 supplies regulated power to the furnace as a 
function of temperature. At the remaining annular space between the 
sidewalls of the wool matting 75 and the glass tube of the system 70 are 
positioned cupric oxide wire 77 along with platinized alumina pellets 78. 
The pellets 78 are placed at the downstream end of the tube 70, and held 
by quartz wool, not shown. The furnace operates about 125.degree. C. 
whereby the carbon monoxide is oxidized to carbon dioxide. 
STATION 37 
Purpose: To remove carbon dioxide previously generated at station 36. 
Station 37 is constructed of a glass tube 80 filled with an absorbent 
material 81 such as sodium hydroxide, Na(OH), held in place with glass 
wads 82, and is similar in construction to station 35 previously 
described. 
STATION 38 
Purpose: To remove lower-, mid- and high-range molecular weight 
hydrocarbons. Station 38 is constructed of a metallic tube 83 filled with 
inert chromatographic glass beads 84 in a size range of 60-80 mesh held in 
place by glass wool wads (not shown). When removal of low- mid- and 
high-range hydrocarbons is desired (removal of all hydrocarbons above 
C.sub.1) a bath 85 consisting of liquid argon (-180.degree. C.) or 
isopentane-liquid nitrogen slush (-160.degree. C.) is placed 
circumferentially about the bed of beads 84. 
If desired, station 38 can be by-passed via valve 65. Hence, clean-up of 
the tube 83 can be facilitated, i.e., a clean gas can be passed via valve 
65 through the tube 83 while the bed of beads 84 is heated to a 
temperature of about 200.degree.-300.degree. C. for several minutes. Note 
that at temperatures above 400.degree. C. however, the beads 84 will 
soften. 
STATION 39 
Purpose: To completely oxidize the methane of interest to gaseous carbon 
dioxide and water. The station 39 includes a furnace 86. It is similar in 
design and construction to the furnace 70 of station 36 shown in detail in 
FIG. 5, except that furnace 86 operates at temperatures in excess of 
600.degree. C. in a catalytically aided reaction. The temperature 
preferred is about 650.degree. C. Result: Methane is quantitatively 
converted to carbon dioxide at a lower operating temperature than would be 
normal, due in part to the effect of the cupric oxide helix and platinized 
alumina beads used as catalysts within the furnace 86. In this regard, it 
should be noted the combustion efficiency of the furnace 86 at different 
ranges and temperatures has been tested. A standard hydrocarbon mixture 
consisting of say 66 parts per million methane, 10 parts per million 
C.sub.2 H.sub.6, 10 parts per million C.sub.3 H.sub.6, and 10 parts per 
million C.sub.4 H.sub.10 in a helium carrier, was passed through furnace 
86 at 30 milliliters per minute. The test was repeated at 20 milliliters 
per minute. The vent of the furnace 86 was connected to a gas 
chromatograph equipped with a flame ionization detector. At the maximum 
sensitivity of the detector (approximately 0.5 parts per million CH.sub.4 
equivalent) with the above mixture flowing through the furnace 86 at the 
above rates, no methane was detected at the detector. The condition 
continued as long as combustion furnace 86 was above 600.degree. C., say 
preferably 650.degree. C. Larger amounts of methane were syringe-injected 
(say up to 400 microliters of methane) into the furnace with similar 
results. Thus, it is concluded that furnace 86 will quantitatively oxidize 
all methane concentrations that are likely to be obtained in the field. 
STATION 40 
Purpose: To trap microliter quantities of carbon dioxide and heavy water, 
if any, oxidized at station 39. 
Station 40 is constructed of a glass tube 87 bent into a U-shape. Its arms 
connect to transfer tubes 88 (and its inlet and outlet, respectively) 
which facilitate gas passage through the tube 87. The tube 87 is also 
filled with inert chromatographic grade glass beads 89 of average size, 
say 60-80 mesh forming a trapping bed. Beads 89 are held in place by wads 
of glass wool (not shown). Collection of the gaseous carbon dioxide and 
water is effected by immersing the tube 87 and beads 89 in a bath 90 of 
either liquid argon at -180.degree. C. or an isopentane-liquid nitrogen 
slush at -160.degree. C. Note that at the outlet of the station 40, the 
transfer tube 88 connects via valve 91 to either (i) flow meter 92 and 
vent 93 or (ii) to a vacuum pump (not shown). During collection, item (i), 
above, is connected to the tube 87. After the collection is complete, the 
valve 91 is operated to connect the interior of the tube 87 to the vacuum 
pump and the latter is turned on. The transfer tubes 88 are then sealed by 
heating them with a small oxy-propane torch. At a mass spectrometer site, 
the contents of the tube 87 (carbon dioxide and water vapor) and 
impurities, if they exist, (air gases, primarily) are introduced into a 
vacuum line where the carbon dioxide can be purified prior to isotopic 
analysis, if desired. Prior to reusing the tube 87 and beads 89, both are 
heated to 200.degree.-300.degree. C. in the stream of clean air to remove 
organic contaminents. Note that cryogenic trapping by bath 90 can present 
some problems if the bath temperatures are not maintained within a range 
of -160.degree. to -180.degree. C. For example, it has been found that at 
higher temperatures (say -125.degree. C.) using an isopentane-liquid 
nitrogen slush, the carbon dioxide can break through the tube 87 to vent 
93 at modest flow rates, say 16 milliliters per minute. At lower 
temperatures, say -196.degree. C., oxygen can condense on the beads 89 
interrupting the carrier flow. Carbon dioxide has been found 
quantitatively to be retained on the beads 89 at -160.degree. C. using an 
isopentane-liquid nitrogen slush, for 25 minutes at flow rates of about 
100 milliliters per minute. 
Thus, at air carrier flow rates of 20-30 milliliters per minute, the carbon 
dioxide will be retained for periods of two hours or more. Moreover, at 
-180.degree. C., the retention time of carbon dioxide will undoubtedly 
increase if liquid argon (-180.degree. C.) is used. 
EXPERIMENTAL DATA 
An investigation of isotopic fractionation associated with system 20 of 
FIG. 4, has been undertaken, such investigation utilizing standard 5% 
methane-argon and 10% methane-argon mixtures. Results of isotopic 
examination are as set forth below in Table I. 
TABLE I 
______________________________________ 
Recovery 
.delta..sup.13 C(PDB) 
Sample # 
Std Vol. Inj. 
% .Salinity. 
______________________________________ 
HC-11 5% CH.sub.4 /Ar 
50 100 -39.31 
HC-12 5% CH.sub.4 /Ar 
50 100 -39.27 
HC-13 5% CH.sub.4 /Ar 
100 100 -36.36 
HC-14 5% CH.sub.4 /Ar 
100 100 -39.57 
HC-23 10% CH.sub.4 /N.sub.2 
100 100 -26.07 
HC-24 10% CH.sub.4 /N.sub.2 
100 100 -25.45 
HC-25 10% CH.sub.4 /N.sub.2 
100 100 -25.53 
HC-26 10% CH.sub.4 /N.sub.2 
100 100 -26.73 
Ref-1 10% CH.sub.4 /N.sub.2 
-- 100 -30.50 
Ref-2 10% CH.sub.4 /N.sub.2 
-- 100 -28.11 
Ref-3 10% CH.sub.4 /N.sub.2 
-- 100 -29.13 
______________________________________ 
Samples HC-11 through HC-14 show good agreement and demonstrate a high 
level of precision at two concentration levels of 50 microliters and 100 
microliters of the carbon dioxide (STP). Mean and standard deviations of 
the delta .sup.13 C composition were -39.38.+-.0.13. Reference samples 
have not yet been analyzed. 
Samples HC-23 through HC-26 and reference samples 1-3, provide similar 
results except the delta .sup.13 C distributions were systematically 
"lighter" by a value of -3.57.degree./.degree..degree.. There is, at 
present, no explanation for the systematic bias noted above. 
In a parallel study, sea water saturated with a methane/argon mixture at 
two temperatures (18.degree. C. and 2.degree. C.) was stripped of its 
dissolved methane. The dissolved methane was then condensed to carbon 
dioxide at an air carrier flow rate of 30 milliliters per minute. 
Approximately 30% of the methane was removed from solution in 10 minutes. 
Results of isotopic examination are set forth below with reference to 
Table II. 
TABLE II 
______________________________________ 
Recovery 
.delta..sup.13 C(PDB) 
Sample # 
Std Vol. Inj. 
% .Salinity. 
______________________________________ 
HC-15 5% CH.sub.4 /Ar 
1495.sup.a 
32 -40.16 
HC-19 5% CH.sub.4 /Ar 
2180.sup.a 
30 -39.17 
______________________________________ 
.sup.a = saturation concentrations in 1 liter sea water at 18.degree. and 
2.degree. C. 
Note that samples HC-15 and HC-19 represent partially stripped sea water 
previously equillibrated with the 5% methane/argon mixture, the same 
mixture used to obtain the results in Table I. But also note that the 
results of Table II are not significantly different than those obtained 
for samples HC-11 through HC-14 of Table I which were achieved using the 
same source tank of methane/argon. Of particular interest in Tables I and 
II, is the fact that the fractionation associated with incomplete 
stripping of the samples HC-15 and HC-19 in Table II is less than 
1.degree./.degree..degree.. This is expected since the fractionation 
factor is about 1.03 (or the square root of 17/16). That is to say, for 
small methane stripping efficiences (less than 1%), the resulting carbon 
dioxide will be 3.degree./.degree..degree. lighter than the parent 
methane. At 30% recoveries, the fractionation is within experimental 
error. 
Isotopic fractionation is dependent on vapor pressures of C.sub.12 H.sub.4 
and C.sub.13 H.sub.4, each of which being temperature dependent. At 
temperature extremes likely to be encountered at the surface of the earth 
(-2.degree. C. to 45.degree. C.), isotopic fractionation associated with 
gas extraction should be within experimental error. It is also worth 
noting that success of the present invention does not depend on the 
absolute delta C.sup.13 values provided for either the biogenic or 
thermogenic methane fractions. Relative changes in a local region are the 
focus of interest. 
From the above, it is apparent from the invention as herein before 
described that variations are readily apparent to those skilled in the 
art, and hence the invention is not limited to combinations herein before 
described but should be given the broadest possible interpretation in 
terms of the following claims.