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The hydrogen and carbon isotopic composition of methane from natural gases of various origins
Grorhmica CICosmochrmwa Arm Vol. 44,pp.649lo661 0 PergamonPress Ltd1980. Printed inGreat Britain
The hydrogen and carbon isotopic composition of methane from natural gases of various origins MARTIN SCHOELL
Bundesanstalt fir Geowissenschaften (Received
I
August
und Rohstoffe, Stilleweg 2, 3000 Hannover 51, West Germany 1979; accepted
it1revised form 19 December
1979)
Abstract-The
deuterium concentrations (6D vs SMOW) of biogenic methanes from world-wide occurrences range from - 180 to -2N&, and were found to be depleted in deuterium by approx. 16&& compared to the deuterium concentration of their associated waters. Theoretical considerations support this relationship to be the result of bacterial transformation of CO1 to methane and is therefore indicative of the biogenic origin of methane. Thermogenic gases with high C,, concentrations (wet gases associated with crude oil) have D/H ratios from -260 to - 150%”with deuterium contents tending to increase with decreasing wetness. Dry gases which are not associated with petroleum are more enriched in deuterium (- I80 to - 1307&Jand show an increase in deuterium with increasing rank of the source beds as it is similarly known for carbon- 13. Many dry gases in young sedimentary basins were found to contain significant amounts of C,, hydrocarbons. These gases cannot be grouped with either the biogenic or thermogenic gases and their methane is concluded to be of mixed biogenic and thermochemical origin. Using a 613C/6D diagrammatic display of the isotope data of methanes the various genetic groups of natural gases can be defined more clearly.
INTRODUCTION THE ORIGIN of
natural gas IS closely related to the diagenetic and thermal alteration of organic matter. Bacterial processes produce methane during early stages of diagenesis whereas increasing thermal maturation of organic matter produces both methane and C2 through C4 hydrocarbons (which will be herein summarized as C, + hydrocarbons). Whereas the literature contains an abundance of data on carbon isotopes in methanes from natural gases, little is known on the variation of their deuterium concentrations. Early work of CLOUDet al. (1958) showed hydrogen isotope fractionations to exist when CH, is formed by microbial processes. SCHIEGLand VOGEL (1970) and LYON (1974) reported D/H variations in natural gas methanes of up to 60’&. NAKAI et al. (1974) detected a relation between the deuterium concentration of both biogenic methane and associated waters. SCHOELL (1977) reported on D/H ratios in natural gas methanes from a subalpine basin and found D/H ratios of methanes to indicate thermal and biogenic methane occurring in this area. CHUNG (1976) and SACKETT (1978) presented data which showed appreciable D/H fractionations in methane derived during pyrolysis of organic matter. The aim of this paper is to investigate systematically the hydrogen isotopic composition of natural gas methanes and to compare these with their carbon isotopic composition. The main interest is to examine whether the various genetic groups of methanes have characteristic deuterium concentrations as it is similarly known for their carbon isotopic compositions. The combined use of both isotope parameters should
then be of high diagnostic value in identifying the origin of natural gases. The genetic types of investigated methanes are displayed in Fig. 1 which is a generally accepted model for the origin of natural gases (S~KOLOV et al., 1972; KARSTEVet al., 1971; TIS.WT et al., 1974; Dow, 1977; TISOT and WELTE,1978). The gases which are indicated in Fig. 1 may be called primary gases in contrast to those which have undergone secondary processes like migration, mixing, biodegradation, absorption, etc. This paper deals mainly with methanes from these primary gases.
649
EXPERIMENTAL Oxidation
PROCEDURES
of methane
Gas samples were prepared for mass spectrometry in a vacuum line similar to that described by GUNTER and MUSGRAVE(1971). The methane was separated by gaschromatography from all other gaseous constituents and was transferred on line with the carrier gas (helium) to the oxidation line where it was oxidized by copper oxide (880°C). The resulting water was trapped together with CO1 at liquid nitrogen temperature. Carbon dioxide was subsequently separated from H,O at dry ice methanol temperatures and the remaining water was reduced to H, by passing it over hot uranium (BIGELEISEN et al.,1952).The hydrogen gas was trapped in a sample container with activated charcoal at liquid nitrogen temperatures. The accuracy of this flow line was checked by a closed system oxidation. Standard methane was catalytically oxidized in a closed reaction vessel by means of a red hot platinum wire. The reaction products were completely recovered and reacted as described above. The deuterium concentrations found were identical within limits of error.
650
M. SCHOELL
CLASSIFICATION
OF NATURAL
GASES
hydrocarbons generated c3
*) vpes
of kerogen
lTtssor
et al.
1971)
II : marine-sapropelrc 111: terrestfial-humic
Fig. 1. Classification
of natural
gases and abbreviations for genetic groups in this paper.
tion
Mass spectrometr~
of gases which are discussed
:
Carbon and hydrogen isotope analyses were carried out on VG 903 and VG 602 mass spectrometers respectively. The isotopic composition is reported in the usual ¬a-
6=
R, smpie - Rrtsndard R s,andsrd
. looo&,).
Table I. Hydrogen and carbon isotopic composition of methanes from natural gases of various origins and chemical compositions. The results are grouped according to their origin as discussed in the text. The standard for 6’“C and 6D values is PDB and SMOW respectively
No.
Country
0 I 2 3 4 5 6 7 8 9
U.S.A. U.S.A. U.S.A. U.S.A. U.S.A. U.S.A. Phil Phil Germany Germany Germany Germany Germany Germany Germany Germany Germany Germany Germany Germany Germany Germany Germany Italy Italy Italy Italy Italy Italy Italy Italy
I0 II 12 I3 I4 15 16 I7 18 I9 20 21 22 23 24 25 26 27 28 29 30
Locality Glacial drift Glacial drift Glacial drift Glacial drift Glacial drift Glacial drift IO Margot 31 Margot Lala La Brake marsh gas Inzenham 05 Inzenham 06 Inzenham 08 Inzenham 09 Inzenham W 1 Inzenham W2 Inzenham W3 Inzenham W4 Schmidhausen Cl Breitbrunn Muehldorf-S C 10 Ampfing 27 Weitermuehle C IO Weitermuehle C6 Caviaga 16 Collecchio 7 TresigaIIo I2 Port0 Corsini Porto Garibaldi 5 Ravenna Mare A 15 Selva 9 Piadena Ovest 25
Depth (ml -~~ ~ -~~ ~~ 900 ~ ~ ~ 1000 2000 1300 I 100 1000 I IOU 1400 200
Age
Quater. Quater. Quater. Quater. Quater. Quater. Quater. Quater. Quater. Mioce. Mioce. Mioce. Mioce. Mioce. Mioce. Mioce. Mioce. Mioce. Oligo. Oligo. Oligo. Mioc. Oligo. Plioc. Quater. 1300 Quater. 3OCKl Plioc. 2500 Plioc. 1500 Plioc. 1300 Plioc 2500 Plioc.
_
-
73.91 87.47
~ -
99.23 99.25 99.28
0.31 0.22 0.20
~
98.85
0.06
98.75 98.80 97.64 99.32 98.72 98.76 98.55 97.49 96.34 99.39 97.84 98.83 97.95 99.37 99.14 98.30
0.16 0.19 0.15 0.29 0.50 0.14 0.05 0.17 2.95 0.04 0.09 0.09 0.08 0.08 0.t 1 0.34
~ -~
-75.7 -82.7 -91.0 - 72.8 -73.0 - 76.0 -76.2 - 76.2 -68.8 - 70.0 -67.6 -68.1 -68.9 -69.5 -72.2 -71.8 -71.1 -71.6 -67.7 - 64.0 - 65.6 -67.8 - 66.4 - 55.3 - 76.3 -71.4 - 72.7 - 73.3 - 70.7 - 69.2 -64.2
-277 -245 -226 - 239 -234 -239 - 207 -216 - 244 -198 -192 -192 -194 - 205 - 207 -213 - 202 -213 - 192 -216 -201 - 207 - 198 -200 -187 -189 -179 - 181 - 179 -182 -189
-85 -47 -40 -50
-2 -18 -39 -56 -62
-24 -46 -12 -22 -32
Hydrogen and carbon isotopic composition of methane Table l-continued No.
country
31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 5.5 56 57 58 59 60 61 62 63 64 6.5 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89
Italy Italy Italy Italy Italv Germany Germany Germany Germany Germany Germany Germany Germany Germany Germany Germany Germany Germany Germany Germany Germany Germany Germany Germany n.r. n.r. nr. nr. n.r. %y
Egipt Egypt
U.S.A. U.S.A. U.S.A. U.S.A. U.S.A. U.S.A. U.S.A. U.S.A. U.S.A. U.S.A. U.S.A. U.S.A. U.S.A. U.S.A. U.S.A. Germany Germany Germany Germany Germany Germany Germany Germany
Locality Imola 10 Brugherio 13 Cornegliano 2 Bagnolo Mella 8 Len0 1 Weitermuehle 5 Isen 1 Oedgassen 1 Schnaupping 2 Hohenlinden 2 Albaching 5 Moosach 7 Moosach IA Eltze-Hard. 5 Loeningen 15 Loeningen 26 Wald 3 ~ullendorf 15 ~ullendo~ 15 Fronhofen 101 Fronhofen 3 Fronhofen 7 Moenchsrot Kirchdorf 3 illmensee 1 North Sea North Sea North Sea North Sea North Sea North Sea Cortemaggiore 73 Cremona Sud 9 Malossa I I i.n.r. i.n.r. Wyatt Sheffield Rojo Caballo Rojo Caballo Rojo Caballo Gomez Grey Ranch-H Mi Vida East Quito Grey Ranch-S East Quito Mi Vida East Quito Grey Ranch Grey Ranch Kalie Z 4 Wielen Z 4 Frenswegen 5 Getelo Z 1 Ratzel Z 4 Oythe Z 2 Wielen Z 4 Dalum Z 5
Depth (m) 1500 I100 1400 1200 1400 1800 1900 2200 2200 2100 2500 2800 2900 2500 1100 1100 1000 1000
C &
Ace
Plioc. Plioc. Plioc. Plioc. Plioc. Eocene Eocene Eocene Eocene Eocene Eocene Eocene Eocene U. trias. M. juras. U. juras. U. trias. U. trias. 1100 U. trias. 1000 U. trias. 1700 M. juras. 1700 L. juras. 800 Oligo. 1700 M. iuras. U. &as. i.n.r. i.n.r. i.n.r. i.n.r. i.n.r. i.n.r. i.n.r. i.n.r. i.n.r. i.n.r. i.n.r. i.n.r. 1500 Plioc. 1300 Plioc. 5500 L. juras. 2500 Mioce. 3700 U. cretae. 2200 Carbon. 2400 Carbon. Carbon. Carbon. Carbon. Carbon. Carbon. 4900 Silur. Carbon. Carbon. Carbon. Silur. Silur. Devon. Carbon. 3300 Carbon. 2300 Carbon. 2300 Carbon. 2500 Carbon. 2700 Carbon. 4000 Carbon. 2300 Carbon. 3400 Carbon.
1.0 0.8 1.1 0.9 1‘9 2.4 1.9 1.9 2.0 0.9 2.0 1.9 2.0 1.4 1.9 1.3 0.6 2.1 0.9 0.8 1.9 0.6 2.0
97.19 98.40 99.41 98.71 97.30 96.64 97.21 94.38 96.62 95.26 92.42 88.36 95.10 65.80 89.90 91.33 48.32 89.01 71.77 97.15 82.11 75.72 67.71 92.98 84.13 57.30 17.46 63.95 69.24 53.61 92.29 93.79 89.97 76.3 1 62.18 80.16 88.77 91.35 92.57 97.79 96.72 57.08 93.90 98.18 91.15 94.40 94.35 98.03 95.97 60.08 92.49 92.10 92.94 91.90 89.93 87.28 92.61 86.06
0.28 0.08 0.29 0.16 0.25
1.94 1.48 4.43 2.19 2.73 4.94 7.75 4.32 28.47 9.93 7.88 2.07 3.98 15.20 2.46 14.76 13.00 21.05 4.62 13.56 28.87 79.73 28.36 24.44 38.31 6.19 4.92 7.80 20.98 31.81 15.33 9.64 6.91 6.35 0.22 :: 0:45 0.35 7.89 0.72 0.49 0.40 I .49 1.18 2.32 5.68 1.22 5.35 6.10 0.73 4.92 0.44
- 58.2 - 62.4 -61.8 -60.8 - 59.3 -56.9 -58.9 - 53.4 -57.5 -54.7 - 52.6 -53.6 -58.0 -47.3 -44.6 -44.4 -44.7 -44.1 -35.1 -47.4 -42.5 - 36.0 -37.1 -36.2 -36.5 -44.8 -50.4 - 50.6 -50.7 -47.2 -51.3 -44.3 -46.9 -36.2 -46.3 -45.3 - 43.0 - 42.0 -41.9 -42.1 - 37.4 - 35.6 - 35.9 - 36.0 - 36.9 -40.5 - 36.9 - 36.4 - 37.5 - 37.5 - 37.5 -27.5 - 29.2 - 25.9 - 29.7 - 29.2 - 24.2 - 29.2 - 25.4
-210 - 198 - 204 - 194 - 200 -203 - 203 -201 - 199 -201 - 202 -206 - 206 - 203 - 203 -201 -214 -217 -173 -236 -217 -185 -182 -164 -188 -197 -232 -231 -245 - 229 - 248 -165 - 182 -153 -222 -256 -177 - 161 -154 -151 -141 -142 - 141 -146 -136 -148 -145 -144 -138 -150 -142 -143 -153 -139 -154 -155 -133 -149 -138
-27 -19 -
Biogenic methanes de$cient of C 2+ hydrocarbons (group ‘B’)-Nos O-5: Glacial drift gases from Illinois, U.S.A. Nos 6-8: Marsh gases from different areas [6, 7 Philippines, 8 Germany (group B(t))]. Shallow dry gases from subalpine Tertiary basins (group ‘B’ and ‘M’jNos 9-22: S-Germany [group B(m)]. NOS 23-35: PO Valley basin, N. Italy [group B(m); No. 31 group ‘M’]. Nos 3643: Natural gases from basinal Tertiary, Molasse basin, S. Germany, with significant C2+ contents (group ‘M). Natural gases associated with crude oils (group TjNos 4446: N.W. Germany. Nos 47-55: S. Germany, Molasse Basin. Nos 56-61: North Sea. Nos 62-64: N. Italy. Nos 65566: Egypt. Low C2 + thermogenic gases (group ‘T7”)---Nos 67-81: Natural gases from the Delaware-Val Verde basin, Texas [group TT(m)]. Nos 82-89: Natural gases from N.W. Germany ‘coal gas’ [group lT(h)].
652
M. SCHOELL
R is the ‘3C”*C and D/H ratio respectively. GD-values are given relaive to the SMOW standard (CRAIG, 1961) and &r3C-values relative to the PDB standard (CRAIG. 1957). Duplicate analyses were performed on all gases for which 6D-values are reported in Table 1. The overall reproducibility for the values given is + I.7”&,for 6D and +O. I’:,,, for 6°C. Mean values and long term reproducibiSties (Irr standard deviation of the single determination) of D/H measurements were calculated for two laboratorystandard gases. Dry gas standard (St. CH,) - 124.8 i: 2.6 Wet gas standard (E 245) - 175.6 + 2.6.
where : H = W, 0 = M = S= 1= 2=
moles of hydrogen, denotes the water or organic matter, methane, isotpic composition, initial composition, composition after reaction.
The following equation is derived for the relation between the isotopic composition of the methane, the organic matter and the water:
Gas chromatograpllj
Routine CC techniques were applied to determine the concentration of C, through CQ hydrocarbons. For the purpose of this paper only C, and C~+-con~ntrations are given whereby c L+ = (I - C,/ZCn). loO(“/;,).
Municipal raw sewage was used to investigate bacterial in the laboratory. Four plastic bottles were filled with 1I. of sewage sludge, as it is used in fermentors. The methane
sludge was spiked with different amounts of pure D,O. The samples were activated to produce methane in a thermostated bath (40°C). The production of gas was brought to completeness, all gas was collected and an aliquot of this gas was used for isotopic analysis.
where X is the ratio AHo/AHw; AH, = Ho, - Ho, and AH, = H,, - HW2, i.e. X is the ratio of the hydrogen used from organic vs the hydrogen used from the water reservoir. C is a constant. The isotopic composition of organic material in natural sedimentary environments does not vary systematically. However. the deuterium concentrations in natural waters vary considerably. Precipitation in continental environments is depleted in deuterium (6D _ -20 to -9(r),,,,) whereas marine waters and interstitial waters are comparatively enriched (6D Q 0 to - 2@~~,,,). A simplified version of eqn (2) is therefore applicable to natural environments, i.e. 6, = rn.6, + f,
BIOGENIC
(3)
METHANE
where Bacterial production of methane is a widespread phenomenon in nature. It has been described as
occurring in various environments such as marine sediments (CLAYPOOL and KAPLAN, 1974), lake sediments (CAPPENBERG, 1974; WJNFREYet at., 1977). salt marshes (KING and WIEBE,1977) and glacial till deposits (COLEMAY,1976). Methane in shallow dry gas deposits has also been described as being of biogenic origin (NAKAI. 1960; COLOMBOet al., 1969; RICE, 1975; BELYAVEVef nf., 1977; FUEX, 1977).
WOLIN (1976) and CAPPENBERG (1976) have summarized various types of enzymatic reactions in methane producing bacterial food chains which can be divided into two groups:
(i) Reduction of CO,: these reactions comprise two steps. firstly production of Co, and Hz by nonmethanogens and secondly reduction of CO, with H, by methanogens. (ii) Hydrogenation of CHI radicals, i.e. a complete transfer of the CH3 radical to CH,. In either of these reactions the hydrogen in the CH, molecule is derived from principically two sources: organic matter and water hydrogen. The following mass balance equation can be written: Hw,&,
+ Hor60, = H,&
+ Ho2602 + Hwzhw2
(I)
1
‘n= x+i
and
X
C, = Kg
bo, t-c.
The percentage of methane hydrogen which is derived from water is found by the expression: Hw = lOO~m(SJ. Equation (3) is a linear relationship between dN and dw where the slope l/(X + I) gives information on how much hydrogen in the methane molecule is derived from organic matter or water. A slope of 0.25 should be excepted if methane is produced by the radical hydrogenation reaction [X = 3; CH, l/(X + 1) = 0.251. For the CO2 reduction all hydrogen in the CH, molecule could stem from water hydrogen; then a slope of 1 should result in a graphical display of eqn (3). The slope of the dn,0/i&HJ relationship varies between 0.25 and 1 if methane is derived from both reaction types. Experimental
productiott
of methane
Theoretical considerations have demonstrated that there should be a positive correlation between the D-concentration of water and methane if hydrogen from the water is used in the enzymatic synthesis of methane. This was investigated in simple laboratory experiments where CH4 was produced from municipal sewage sludge. The deuterium concentrations of the water were artificially increased using D,O. The results of these experiments are illustrated in Fig. 2.
Hydrogen and carbon isotopic composition of methane I -100 -
I
I
t +200
I +300
I
Natural Methanes
653
Experimentally produced CHL
7 8
(sewage sludge) v 3
Q 0
-300 -
I 0
-f.400 -100
I +lOO
6D +o
[%ol
Fig. 2. Hydrogen isotopic composition of methane which was bacterially produced from raw sewage. The associated water was spiked with different amounts of deuterium-oxide. The D,O-tracer experiment shows firstly that there is an immediate response to the’ change of the environmental water composition. The D,O spiked aliquots revealed deuterium rich methanes. A linear correlation between water and methane deuterium concentration is observed (Fig. 3). aDme,,,anec O.GD,,,,,
- 323%,
This relationship is experimental drogen from water is incorporated reactions which produce methane. indicates that approx. 40% of the methane is derived from water. Natural
gas methanes
ofbiogerlic
evidence that hyin the enzymatic The slope of 0.4 hydrogen in the
origin
The methanes analysed here for their deuterium concentrations are from areas which have already been described as producing biogenic gas. For many of the gases it was also possible to collect their associated waters:
They originate from layers of glacial drift deposits and are dissolved in groundwater. The respective waters were sampled from the same well. (2) Marsh gases from various localities: One sample of a marsh gas from N.W. Germany has been described by STAHL (1968) and was derived from holocene sands. 14C dating revealed an apparent age of 5000yr BP. Marsh gases from the Philippines were sampled from gas emerging in swamps. (3) Methanes from natural gases of the subalpine basin in S. Germany (SCHOELL,1977), and the PO valley in N. Italy (SCHOELLand NEGLIAin preparation). Both sets of gases are typical samples of ‘shallow dry gas deposits’. Gases with C,+-contents below 0.5% have been selected. In S. Germany waters from producing water wells were sampled. The waters of the Italian gases were obtained from the separators at the well sites.
The results of these analyses are listed in Table 1 and shown on Fig. 3. The variation of 6D in biogenic (1) Glacial drift gases, Illinois, U.S.A. : These gases methanes is considerable: - 175 to -250%,. Marsh have been described in detail by COLEMAN (1976).gas methane and methanes from glacial drift gases, which are both from terrestrial environments are depleted in deuterium as compared to other biogenic gases (tertiary basins, Cariacho Trench) which are derived’ from marine environments. enwonmenr Figure 4 shows that the environmental control of 0 marine the methane D/H ratios is again due to the correlation of the respective water D/H ratios. The correlation is almost identical to that which was found for Japanese natural gases (NAKAIet al., 1974), i.e.
I
-300
-250
6D Fig. 3. D/H
ratio
-200 CH/,
frequency distribution biogenic origin.
-150
6D,,,e,hrne = Qv,,,,
[%ol of metdanes
of
-
(160 + lW&,.
The collection of data from world wide occurrences suggests that this relationship is characteristic for all natural biogenic gases. The slope of 1 in this relationship suggests that 1OO0/O of the hydrogen in these
M. SCHOELL
654
-180
1’
lill
-80
I
I
I
-LO
I
0
6D,o [%*I Fig. 4. Hydrogen isotopic compositjon of biogenic methanes and their associated waters from various areas as compared with cogenetic methane/water pairs from natural gas deposits in Japan. 1, Japan (NAKAI et al., 1974); 2, marsh gas, N. Germany; 3, glacial drift gases, Illinois, U.S.A.; 4, Cariaco trench [aD(H,O) estimated]; 5, natural gas deposits, N. Italy; 6, natural gas deposits, S. Germany (SCHOELL, 1977 and this paper).
methanes is derived from their associated waters. Another explanation could be, that during the enzymatic reactions of the methane synthesis interreactive groups are in isotopic equilibrium with water. If CH* is produced by the reduction of CO,, hydrogen is used which has been formed from intermediate substances like pyruvate, formate, hypoxanthine and reduced pyridine nucleotides. In all cases interreactive groups are formed to which hydrogen is only weakly bound [NAP (P), pyruvate etc.]. It is well known that this type of hydrogen readily exchanges isotopically with environmental water (EPSTEINet al., 1976). It should therefore be expected that hydrogen which is formed in bacterial food chains has isotopitally exchanged with environmental water hydrogen. This would imply that during CO2 reduction of methane only ‘equilibrate hydrogen enters the CH4 molecule. In the CH, radical hydrogenation reaction only one hydrogen has exchanged with the environmental water and three atoms are derived from the organic matter. show that the considerations The above relationship in natural biogenic 6D,,,,,,,-6D,,,,,gases is indicative that these gases pr~omin~tly formed by the bacterial reduction of CO,
CO2 + 4Hz -+ CH4 + 2H20. The CHJ hydrogenation by methanogens, which has been found to be important in Lake Vechten sediments (CAP~NB~~, 1976) is obviously of minor importance for the formation of natural biogenic gases.
SACKFTT(personal communication) reported a 6D value of -3Iq&, for a Missisippi swamp gas. This value does not fit the above 6Dc,,&D,, relationship for natural methanes. A very unlikely 6D,, of - ISO?&,would be necessary to fit this relationship. A fermentation process similar to that which was investigated with the sewage sludge rather than CO, reduction could explain this particular isotopic composition of the swamp gas. Figure 5 shows the relationship between the hydrogen and the carbon isotopic composition of methanes from shallow dry gases. The 613C/6D boundaries given in this figure by stippled lines are those which are found to be limiting for ‘pure’ biogenic gases such as glacial drift gases (%HOELL and COLF.MAN,in prep aration), i.e. 6D and ai3C values more negative than - 150 and - 64’?&,respectively. The -64’& boundary is suggested if all proved samples of biogenic methanes are considered for which secondary effects like bacterial oxidation are unlikely. Almost all published data of biogenic methanes which are deficient of saturated CZ+ hydrocarbons plot within this field (NAKAI et al., 1974; COLEMAN,1976; LYON, 1974). Variations in 6D in biogenic methane are predominantly due to environmental changes. A differentiation of *terrestrial’ biogenic gases B(t)(GD < - 19C$&) and ‘marine’ biogenic gases ((~)(~D > -WY&) is suggested (see Table I, Fig. 1 and Fig. 11). The gases from the various basins are well recognized by their isotopic pattern (Fig. 5).
Methanes of mixed oriyin (Group ‘M’) and dj~re~ltiutjuff j&n biagenic r~~tha~es
their
Some of the gases from young subalpine basins-do neither fit to the biogenic gases nor to the thermogenie gases (see below). These gases occur often at basinal reservoir rocks in the respective basins. These gases are listed in Table I, Nos 31 and 36-43 and are summarized as group ‘M’ (Figs 5 and 11). All these gases have in common significant amounts of C,, hydrocarbons (up to 4.9%). C, + hydrocarbons in such quantities are generally not of biogenic origin (JANEZIG, 1979); abiogenic processes have therefore to be assumed to be operative. SCHOELL(1979) showed that the contents of CZ+ hydrocarbons is correlated with an increase of i3C in the methanes (6’3C values -6Q, to - SY&). This suggests that methane of nonbiogenic origin is produced together with ethane and is mixed with these gases. The SD values in this group scatter little around - I?Off’,,,,, i.e. the admixed methane of nonbiogenic origin should be around - 200‘& THERMOGENIC GASES Thermal alteration of organic matter is a gradual process which produces liquid and gaseous compounds of continuously changing composition. It is therefore impossibIe to define clearly cut groups of natural gases. This paper follows the differentiation of
655
Hydrogen and carbon isotopic composition of methane
-250
-90
-80
-70
-SO
-50
Fig. 5. D/H and i’C/i*C ratios of natural gas methanes from late tertiary subalpine basins (S. Germany and N. Italy) compared with methanes of biogenic origin. The box given in stippled lines gives the variation of pure biogenic gases (‘3’). 1, Glacial drift gases; 2, marsh gas, N. Germany; 3, marsh gas, Philippines; 4, natural gases, S. Germany; 5, natural gases, N. Italy atone); 6, gases from the eocene lithothamnia limestone, S. Germany; 7, Cariaco trench (LYON,1974); 8, natural gases, Japan (NAKAIet al., 1974). thermogenic gases which was proposed by FUEX (1977) and STAHL (1977): (i) gas associated with oil generation (T’), and (ii) dry gas (‘TT’) from either terrigenic (humic) or marine sapropelic matter r’I.T(h)’ and “IT(m) see Fig. 11.
Gas methanes from associated
high CZ+ gases
which are
with crude oils (group ‘T)
Gases from various typical oil producing areas were selected (see Table 1). (1) North Sea. Field data are not released for these gases. The gases were all brought to the laboratory together with oils and were associated with oils in situ. One sample was degassed from the crude oil in the laboratory. All gases have high Cj?+ contents. (2) N.IV. Germany. 3 samples are derived from triassic and jurassic source rocks of the Lower Saxonian basin and represent typical associated gases of mesozoic source rocks in N.W. Germany (STAHL, 1979). (3) S. Germany. These gases are derived from the western part of the Molasse Basin and have already been described by SCHOELL(1977). The gases are from mesozoic’ and tertiary reservoir rocks. The source rocks are not identified. (4) N. Italy. Three samples have been collected from classical oil/gas deposits from N. Italy which have already been described (MINUCCI, 1959; LucCHETHet Uf., 1969). (5) Egypt. Two samples have been analyzed which were delivered from oil drill&s where the gas was still associated with the crude. Further data are not released.
Figures 6.1 through 6.3 give the results which were obtained for the methanes of the associated gases. The variations of the D/H ratios are remarkable and comprise a range of up to IO@/&,.The SD values scatter even within geographically closely related samples nearly as much as all gases of this group. For example the gases from S. Germany are from very closely grouped reservoirs and display a range of approx. 7(J& in 6D values. The same is observed for the North Sea gases. In general deuterium in the methane is depleted with increasing CZ+ values or wetness of the gas. One of the most D-depleted methanes was found in the sample which was degassed from the oil, As shown in Fig. 6.3 there is also a gross correlation of the carbon-13 and deuterium concentrations in the methanes of group ‘T’. The relation of 6D over 613C is approx. 1: IO. It should be noted, that the D/H variations in these methanes are qualitatively comparable to the variations which were observed in laboratory pyrolysis experiments (SACKETT,1978). In Fig. 7 the experimental fractionation trendlines have been drawn by assuming that hydrocarbons of the mean isotopic composition of crude oils (SCHOUL and REDDING, 1978) of kerogens (REDDING, 1978) were the parent materials to all these gases. The similarity of the 13C and D composition of group T methanes and pyrolysis methanes suggests that closed system pyrolysis bears a rather close resemblance to gas foxing processes in nature. The free radical mechanism which has been quoted by SACKETT(1978) as a possible mechanism for D/H fractionations in experimentally produced methane
M.
656
SCHOELL
rrn- North Sea --
-20 -
---
I [2J
\,
-140
. \
NW-Germany I QJ South Germany i GJ North Italy : ~ Egypt
~- rr '
\<"~-~11i 1m l 1 ~) ~~~~c~g~~~E ~~~Ge II
,.
~'~>:. ~,
r-o' ~
~
'0' ~
@]
<:t
(f;j
o
o
-40
)
MARINE SOURCE ROCK
:'\. .
1
~ '.. £:~\
:r:
0
",,0
1m
1
~ .." ..~?::\:..
-180
~
<:t
:r:
Natural Gases USA
TT
\»..
'_ •• ~@j
-30
00
;':'.
. -:'::"
-220 .:.'.:'
-50
...:....
..........~:: . ,..:.. ',@ .
....
\.::'.~
- 260
o
o
50
10
c2 +[%J
30 C2 +
Fig. 6.1
[%]
Fig. 6.2
1T1ml
-150
.. .. ...
-250
-50
- 40
-30
-20
3 13CCH4 [%oJ Fig. 6.3 Fig. 6. Variation of the D/H and '3C/'2C isotopic composition in methane from natural gases in relation to the molecular composition in wet gases. The fields TT(h) and TT(m) denote the composition of low C 2 + thermogenic gases. (See Fig. I and text below.) 6.1, oD/C 2 +; 6.2, 0' :iC;C2+; and 6.3, OD/O '3 C relationship respectively.
50
Hydrogen and carbon isotopic composition of methane
657
The D/H-isotope analyses on these methanes revealed that their hydrogen isotopic compositions are also dependent on the maturity of their source rocks (Fig. 8). The deuterium concentrations are obviously only related to the maturity of the source material whereas the ‘~C~on~ntration is dependent on the maturity and the type of source organic matter. The calculation of the GD/Ro-relationship gives the following equation 6D = 35.5 log R. - 1527& whereas the ~i3C/R~-reIationship following two equations
is described by the
8.6 log R, - 28?&
TT(h):
613C =
TT(m):
613C = 14.8 log R, - 41”&,.
In trying to understand the increase of deuterium in methane with maturity various processes could account for the fractionation of hydrogen isotopes. (1) Isotopic exchange between methane and associated water: with respect to the mass balance hydro-
Fig. 7.
from closed system laboratory experiments at 400 and 500°C (SACKF.TT,2978).The
,
t
/
1.0
2.0
30
, 1.0
/ 2.0
! 30
parent material is assumed to have the mean isotopic composition of crude oils (SCHOELLand REDDING,1978) and kerogen (REDDING, 1978).
should therefore aiso be taken into consideration natural gas formation.
for
Methanes from low Cz+ gases [groups ‘TT(h)’ ‘T T(m)‘]
and
Two groups of low C2+ or dry gases have been selected to investigate the D/H fractionation patterns of their methanes, i.e. N.W. German coal gases and gases from the Delaware-Val Verde basin (Table 1). These gases have already been investigated in detail with respect to the fractionation of carbon isotopes of their methanes in relation to the maturity of their source rocks (STAHL,1975; STAHLand CAREY, 1975). ~arboniferous coal-measures of the N.W. German Basin are the source rocks of the N.W. German coal gases which thereby represent gases from humic organic matter (type III kerogens). These gases are denoted as group ITT(h)‘. Background information on the origin of these gases is given in BOIGK etal. (t976), ~AR~NSTEIN et af. (1971) and STAHL and KOCH (1974). The source rocks in the Delaware Val Verde Basin are Silurian through Carboniferous in age and are described to be of marine origin; hence they should represent kerogens of type II of the Tissot classification. These gases are denoted as group ‘TT(m)‘. Both sets of gases are from source organic matter of varying maturity (vitrinite reflectance R,). The ‘3C-concentration in the methane was found to increase with increasing maturity (STAHL, 1975; STAHLand CAREY, 1975).
-140
-160
01
-50 ’ 0.4
R, [%I Fig. 8. D/H and “C/“C ratios in natural gases in relation to the maturity of their source rocks. Gases from the Val Verde Delaware Basin are from marine source rocks and methanes from coal gases (N.W. Germany) are from humic source rocks. RO-values (mean reflectance of vitrinite under oil)are adopted from S~~~~(l975)and STAHL and CAREY (1976). I,Gases in carboniferous reservoir rocks, N.W. Germany [TT(h)]; 2, dry gases in Paleozoic reservoir rocks from the Delaware Val Verde basin. Texas.
658
M.
-125
SCHOELL
rr1m~
\
-150
IiJ7l -175
L!...:J
•
-45
-40
-30
-35
-25
-20
o13CCHt
[%0]
Fig. 9. D/H ratios in thermogenic methanes from marine and humic source rocks plotted versus their 13C/ '2 C ratios. The arrows give the pathway of increasing maturity of the source rocks.
gen which is bound in associated waters (oil field brines) is mostly in excess of the hydrogen in methane. At isotopic equilibrium between CH 4 and H 20, for example at temperatures around 2()()OC, the methanes should be 65%0 depleted in deuterium as compared to the cogenetic water (BOTIINGA, 1969). Most oilfield brines and waters associated with natural gas deposits lie within the range (- 25 ± 15)%0. Methanes in equilibrium with these waters should range in their bD-values from -75 to -105%0. However, the range of the carboniferous gases is -133 to -177. Lower temperatures should result in even more positive bD values. The presented data thus show that hydrogen isotopic equilibrium has not been achieved in the methanes from dry gases. Moreover the results of laboratory exchange experiments (Kopp, in preparation) do not support H 2 0-CH 4 hydrogen
exchange reactions to be responsible for DiH vanation of natural gas methanes. (2) Fractionation due to bond energies in precursor organic matter: SACKETI et al. (1970) have pointed out that the "strength of terminal carbon-<:arbon bonds in parent organic materials are the primary factors that determine the isotopic composition of all methane occurrences". For CH 2 D terminal groups the carbon--earbon bond should be stronger than for CH 3 terminal groups. One should therefore expect increasing cleavage of C-CH 2 D bonds with increasing thermal stress i.e. the deuterium concentration in CH 4 should increase with maturity. This model could explain satisfactorily the correlated increase of both heavy isotopes in the methane; it does not explain why the hydrogen isotopic fractionation is the same for both humic (i.e. predominantly aromatic) and lip-
Marsh & Glacial Drift - Terrestrial
II I I
Marine Sediments
Biogenic Methane
DDB
II I I II
III
"B
{t} "
"B (m) "
Molasse Basin oS-Germany
1
YOUNG
DD
TERTIARY
BASINS I
II
II I I
I
11111
I II I I I II
II I II I I I
N-Italy
High C2• Gases
II
Thermo genic
I 11111 I II
Methane II I
f)l)TDI)
- 250
-200
I III I
"r
""'j
'''o~,,~~,
Basin"TT(m)" Low
N-German Coal Gases"TT(h)"
-150
Fig. 10. Summary of the variation of D/H ratios in natural methanes.
C2. GASES
"TT"
Hydrogen and carbon isotopic composition of methane o
'(j(.
659
-100
B~m~
-\---------
-150
-200
O'-<::r----'
o
o
-250
o
-90
-70
-50
-30
-10
Fig. II. Hydrogen and carbon isotopic composition of methanes from natural gases. The plot of ,, 13 C vs "D values facilitates the differentiation of genetic groups and the identification of secondary processes such as mixing.
tinitic (i.e. predominantly aliphatic) precursor materials. For C-isotopes CHUNG and SACKETT (1978) have explained this effect as being due to the more aromatic structure of humic organic matter. Methane from aromatic matter is always less depleted in the heavy isotope compared to its parent material than CH 4 from aliphatic materials. This 'structural effect' may also be operative for hydrogen, but it could be immeasurable with the present accuracy of OjH ratio measurements. Since both isotopic species of the methane molecule are fractionated in favour of the heavy isotope during metamorphism of organic matter, the combination of both in a (j13Cj(jO plot reveals maturation pathways for methanes from dry gases which are indicated in Fig. 9. The calculation of maturity trendlines for marine and terrestrial source rocks revealed the following equations: TT(m): TT(h):
3.2(j 13 C - 25%0 (jO = 3.8(j 13 C - 40"100'
(jO
=
Natural dry gases from mixed source rocks should plot in this figure in the field between the evolution lines of pure marine or terrestrial source rocks. Methanes from low C2+ gases exhibit a lower scatter in OjH ratios (~25%0) than methanes from high C2+ gases which are associated with crude oils. By this, high C 2 + gas methanes which are mostly associated with oil may be differentiated from dry (nonassociated) gas methanes (Fig. II).
types of natural methanes and demonstrate that the different methanes are more easily differentiated on a (jO- 13 C plot, than by using only the carbon isotope ratios. Variations within the genetic groups 'T' and 'TT' given in Fig. II are caused by kinetic processes during the formation of the gases. Their isotopic composition may be altered by secondary processes. For example in group 'M' the variations are most likely caused by mixing processes. Future investigations should concentrate on these secondary processes like mixing, migration and biodegradation which should also reveal typical isotopic patterns. Carbon-13 and deuterium isotope analyses will be most useful in this respect since secondary processes affect both isotopic species-and methane is a highly mobile molecule. Acknowledgements-The following oil companies generously supplied samples for this investigation: AGIP San Donato Milanese, Brigitta Elwerath Betriebsflihrungsgesellschaft MbH, Deminex, Deutsche Schachtabau und Tiefbohrgesellschaft, Deutsche Texaco, Mobil Oil AG Deutschland, Preussag AG, Tenneco Oil Company. I am grateful to Dr W. STAHL who offered to include here the Delaware Val Verde data. The analytical work was performed by U. RINOW, S. RESKI and O. AMIT. The drawings are the skilful work of 1. KENKLIES. An early versioll of this manuscript was read by D. WAPLES by whose comments it was greatly improved. I had great benefit from discussion with W. M. SACKETT, I. WENDT and W. STAHL. I. R. KAPLAN, W. SACKETT and B. DURAND reviewed this manuscript. Financial support was given by the Bundesministerium fUr Forschung und Technologie through Grant ET 3003 A.
Isotopic characterization of genetic types of natural methanes and future work
REFERENCES
Figures 10 and II summarize the OjH ratios on natural methanes. They illustrate the various genetic
BARTENSTEIN H., TEICHMULLER M. and TEICHMULLER R. (1971) Die Umwandlung der organischen Substanz im
660
M.
SCHOELL
Dach des Bramscher Mass&. Fortschr. Geal. Rheinl. a. on the origin or methane in hydrothermal gases. GeoWest& 18, 501-538. chfm. Cosmochim. Acta 35, 1I _3I 18. BELYAYEV S. S., LAURINAVICHUS K. S. and GAYTAN V. I. HWD A. and GUTJAHRC. C. M. (1975) Organic metamor(1977) Modern microbioiogicaI formation of methane in phism and the generation of petrofeum. Am. Assoc. Pet. the Quaternary and Pliocene rocks of the Caspian Geol. Bull. 59,986-996. depression. Geochem. Inc. 14, 172- 176. JANEZI~G. G. (1979) Role of biogenic light hydrocarbon generation in near-surface prospecting. Program AAPGBIGELEISEN J., PERLMANM. L. and Po~%R H. C. (1952) SEPM Annual Convention. Houston, Texas. Conversion of hydrogenic materials to hydrogen for isoKARTSEVA. A., VA~S~E~~CH N. B., GEODEKIAN A. A., NERUtopic analysis. Anal. Chem. 24, 1356. BOIGK H., HAGEMANN W. W., STAHLW. and WOLLANKE, ~HEVS. G. and S~KOLOVX. A. (1971) The principal stage in the formation of petroleum. PTOC. 8fh World Pet. G. (1976) I~topenphysikaIische Untersuchungen zur Congr. 2, 3-t 1. Hetkunft und Migration des Stickstoffs nordwestKING G. M. and WIEBEW. J. (1977) Methane release from deutscher Erdgase aus Oberkarbon und Rotliegend. soils of a Georgia salt marsh. Geochim. Cosmochim. Acta Erdoel K&e, Erdgas, Petrochem. 29, 103-l 12. BOTTINGAY. (1969) Calculated fractionation factors for 42. 343-348. _ carbon and hydrogen isotope exchange in the system LUCCHETTI L., ARI~~TI A., BONGIOKNI D., CAKISSIMO L.. D’AGOSTINO O., FANCELLIR., MOTTA V. and PARENTIC. calcite-carbon dioxide-graphite--methane---hydrogen-(1969) ttalia (Geologia e ricerca petrolifera) Pianura water vapor. Geochim. Cosr?lochim. .4cta 33,49--64. CAPPENBERG Th. I% (1974) Interrelations between sulfatePadano-Veneta. In E~~cic~o~edja de1 Petrolio e def Gas reducing and methane-producing bacteria in bottom Naturaie, pp. 358-439. Editore Carlo Colombo. deposits of a fresh water lake--I. Field observations. LYON G. L. (1974) Isotopic analysis of gas from the CarAtltonie van Leeuwenhoek; J. Microbial. Serol. 40, iaco Trench sediments. In Natural Gases in Marine Sedi285-295. ments (ed. I. R. Kaplan), pp. 9 l-97. Plenum Press. CAPPENBERG Th. E. (1976) Methanogenesis in the bottom MINU~~‘~G. (1959) Produzione di gas naturale dalle deposits of a small stratifying lake. In Microhinl Produc‘Argille Scagliose’ di caste1 dell’ Alpi. In J Giacimenti riots and ~ti~i~utjo~t OJ Gases (eds H. G. Schlegel, G. Gassij&i de[/ Europa Oecfde~ltafe. Accademia Nationale. Gottschalk and N. Pfennig), pp. 125- 134. Goitze. Dei Lincei, Roma. 407.-4. CHUNG I-I. M. (1976) Isotope fractionation during the NAKAI N. (1960) Carbon isotope fractionation of natural maturation of organic matter. Ph.D. Dissertation. Texas gas in Japan. Earth Sci. Nugoya Univ. 8, 174- 180. A & M University, College Station, Texas. NAKAIN., YOSHII>A Y. and ANI~ON. (1974) Isotopic studies CHUNG H. M. and SACKER W. M. (1978) Carbon isotope on oil and natural gas fields in Japan. Chikyu Kagaku effects during the pyrolytic formation of methane from 7/8,87-98. coal. In Short papers qf the Fourth ftliernat~onal ConferPATTEISKY K. and TEICHM~LLEK M. (1960) Inkohiungsverence, G~oc~~ra~~ology, Cosrnoc~rotIol~~~, Isotope Geology lauf, 1nkohIungsmaBst~be und Kiassifikation der Kohten auf Grund der Vitrit-Analysen. Brennsr. Chem. 41, 79-84, f%% fed. R. E. Zartmann), United States, Geol. Survey 97.- 104, 133-m 137. Open’File Report 78-701.’ CLAYPOOLG. E. and KAPLANI. R. (1974) The orirrin and PINE M. J. and BARKERI-1.A. (iY55) The Pathway of Hydistribution of methane in marine ‘sediments. In ?jlatural drogen in the acetate fermentation. Studies on the MethGases ill Marirze Sediments (ed. I. R. Kaplan), pp. 99-l 39. ane Fermentation 71, 644-648. Plenum. REDDING C. (1978) Hydrogen and carbon isotopes in coals CLOUD P. E., FRIEDMAN I., SISLERF. D. and DIBELERV. H. and kerogens. In Short papers of‘rhe Fourth ~~ter~~ufjo~~af (1958) Microbial fractionation of hydrogen isotopes. Confiwn~r. ~e~~cf~ronolo~~~, Cosmochronology, Isotope Science 127, 1394-I 395. Geology 197X (ed. R. E. Zartman), United States Geol. COLEMAND. D. (1976) Isotopic characterization of Illinois Survey Open File Report 7R -701, 348. natural gas. Ph.D. Dissertation, University of Illinois, RICE D. D. (I 975) Significance of Origins of Natural Gases Urbana,Champaign. of Montana Plains. Am. Ass. Petr. Geol. Bull. 59, 921. COLEMAN U.. GAZZARRINIF., CONFIANTINIR., TONGIORGI R~SENFELD W. D. and SILVFRMAN S. R. (1959) Carbon isoE. and CAFLI~CHL. (1969) Carbon isotopic study of hytope fractionation in bacterial production of methane. drocarbons in Italian natural gases. In Advances in OrScience 130, 165% 1659. garlic Geochemistry 1968 (eds P. A. Schenk and I. HaveSUXETT W. M., NAKAPARSKIN S. and DALRYMPLE (1970) naar), pp. 499-516. Pergamon Press. Carbon isotope effects in methane production by therCRAG H. (1957) Isotopic standards for carbon and oxygen mal cracking. In Adrarws i,l Organic Geochemistry (eds and correction factors for mass-spectrometric analysis of G. D. Hobson and G. C. Speers). pp. 37. 53. Pergamon carbon dioxide. Geochim. C’osmochim. Acta 12, 133-149. Press. CRAIG H. (1961) Standard for reporting concentrations of SA~KETTW. M. (197X) Carbon and hydrogen isotope deuterium and oxygen-i8 in natural waters. Science 133, effects during the thermocatalytic production of hydro1833-1834. carbons in laboratory simulation experiments. Geocbim. Dow W. G. (1977) Kerogen studies and geological interCosmochim. Acta 42. 571-580. pretations. J. Geochem. EYplor. 7, 79-99. SCHIEGLW. E. and VOGELJ. C. (1970) Deuterium content EPSTEIN S., YAPP C. J. and HAU J. H. (1976) The determiof organic matter. Earth Plawr. Sci. Lett. 7, 307-3 13. nation of the D/H ratio of non-exchangeable hydrogen SCHOELL M. (1977) Die Erdgase der siiddeutschen ‘3C/‘ZC_ in cellulose extract from aquatic and land plants. Earth D/Hund Molasse- - Anwendung von Planet. Sci. Left. 30, 241-251. Isotopenanalysen zur KIPrung ihrer Enstehung. ErdiieI EVANSC. R., ROGERSM. A. and BAKLEY N. J. (1971) EvoluErdgas 2.93, 31 i-327. tion and alteration of petroleum in Western Canada. SCHOELLM. (1979) Dual origin of natural gases in Subalnine Basins. Program AAPG-SEPM Annual ConvenChem. Geol. 8, t 47-I 70. F~:Ex A. N. (1977) The use of stable carbon isotopes in iion, Houston. 1979, 15% hydrocarbon exploration. J. Geochem. Esplor. 7. SCHOELLM. and REUI)INC;C. (197X) Hydrogen isotopic composition of selected crude oils and their fractions. In IS’;-19xx. GALIMOVE. M. and IVLEVA. A. (1970) Theoretical and Short papers of thr Fourth Irlternational Coj!ference, experimental study of isotope effects in natural gaseous ~;e~~~~roffo~og~, ros,tzttcilronolog~, isotope Geology f 97,‘i hydrocarbons. Isof. Titles 1, 73-97. (ed. R. E. Zartmann), United States Geol. Survey Open File Report 78-701. pp. 384-385. GLWTER B. D. and MUSGRAVEB. C. (1971) New evidence
Hydrogen and carbon isotopic composition of methane
661
STAHLW. and KOCH J. (1974) ‘3C/‘2C-Verhaltnis nordof biological ao tivity in the formation of natural gas deposits. Am. C/tern. deutscher Erdgase, Reifemerkmal ihrer MuttersubstanSot. Div. Pet. Chem. Prepr. 21, 3, 416. zen. Erdiiel Kohle. Erdaas. Petrochem. 21. 623. S~KOLOV V. A., TICHOMOLOVA ‘I”. V. and CHERIMYSINOVSTAHLW. and CAREYB.-D.’ (I 975) Source rock identification by isotope analyses of natural gases from fields in 0. A. (1972) The composition and distribution of the Val Verde and Delaware Basins, West Texas. Chem.. gaseous hydrocarbons in dependence of depth, as the Geol. 16, 257-267. ~on~quen~ of their generation and Mi~ation. In TEICHM~~LLERM. (1971) Anwendung kohlepetrograAduunces in Organic Geoche~stry 1971 (edi H. R. v. phischer Methoden bei der Erdol- und ErdgasprospekGaertner and H. Wehner). DD. . . 479-486. Peraamon Press. tion. Erdliel Kohle 24, 69-76. STAHL W. J. (1968) Kohlenstoff-Isotopen&alysen zur TI~OT B., DURANDB., ESPITALIEJ. and COMBAZA. (1974) Kllrung der Herkunft nordwestdeutscher Erdgase. DisInfluence of nature and diagenesis of organic matter in sertation, Technische Hochschule Clausthal, 107 pp. formation of petroleum. Am. Assoc. Pet. Geol. Bulf. 58, STAHL W. J. (1974) Carbon isotope fractionations in 499-506. natural gases. Nature 251, 134-135. TI~~OTB. P. and WELTED. H. (1978) Petro~~m F~rn?ation STAHLW. (1975)Kohlenstoff-Isoto~nv~r~ltnisse an Erdand Occurrence. Springer Verlag. gasen, Reifekennzeichen ihrer Muttersubstanzen. Erdiiel WINFREYM. R., NELSOND. R., KLENCKISS. C. and ZEIKUS Kohle, E&as, Petrochem. 28, 188-191. J. B. (1977) Association of hydrogen metabolism with STAHLW. J. (I 977) Carbon and nitrogen isotopes in hydromethanogenesis in Lake Mendota sediments. Appl. Encarbon research and exploration. Chem. Geof. 20, oiron. Microhiol. 312-3 18. 121-149. WOLINM. J. (1976) Interaction between H2 producing and STAHL W. J. (1979) Reifeabhlngigkeit der Kohlenstoffmethane producing species. in ~~~cro~ja~Productioff and Isoto~nverh~ltni~ des Methans von Erdolgasen aus Ut~~j~atjo~of Gases (eds H. G. Schlegel, G. Gottschalk Norddeutschland. Erdiiel Kohl& Erdgas, Petro~hem. 31, and N. Pfennig), 141-150. Goltze. 51s517. SILVERMAN S. R. (1976) The importance
The hydrogen and carbon isotopic composition of methane from natural gases of various origins MARTIN SCHOELL
Bundesanstalt fir Geowissenschaften (Received
I
August
und Rohstoffe, Stilleweg 2, 3000 Hannover 51, West Germany 1979; accepted
it1revised form 19 December
1979)
Abstract-The
deuterium concentrations (6D vs SMOW) of biogenic methanes from world-wide occurrences range from - 180 to -2N&, and were found to be depleted in deuterium by approx. 16&& compared to the deuterium concentration of their associated waters. Theoretical considerations support this relationship to be the result of bacterial transformation of CO1 to methane and is therefore indicative of the biogenic origin of methane. Thermogenic gases with high C,, concentrations (wet gases associated with crude oil) have D/H ratios from -260 to - 150%”with deuterium contents tending to increase with decreasing wetness. Dry gases which are not associated with petroleum are more enriched in deuterium (- I80 to - 1307&Jand show an increase in deuterium with increasing rank of the source beds as it is similarly known for carbon- 13. Many dry gases in young sedimentary basins were found to contain significant amounts of C,, hydrocarbons. These gases cannot be grouped with either the biogenic or thermogenic gases and their methane is concluded to be of mixed biogenic and thermochemical origin. Using a 613C/6D diagrammatic display of the isotope data of methanes the various genetic groups of natural gases can be defined more clearly.
INTRODUCTION THE ORIGIN of
natural gas IS closely related to the diagenetic and thermal alteration of organic matter. Bacterial processes produce methane during early stages of diagenesis whereas increasing thermal maturation of organic matter produces both methane and C2 through C4 hydrocarbons (which will be herein summarized as C, + hydrocarbons). Whereas the literature contains an abundance of data on carbon isotopes in methanes from natural gases, little is known on the variation of their deuterium concentrations. Early work of CLOUDet al. (1958) showed hydrogen isotope fractionations to exist when CH, is formed by microbial processes. SCHIEGLand VOGEL (1970) and LYON (1974) reported D/H variations in natural gas methanes of up to 60’&. NAKAI et al. (1974) detected a relation between the deuterium concentration of both biogenic methane and associated waters. SCHOELL (1977) reported on D/H ratios in natural gas methanes from a subalpine basin and found D/H ratios of methanes to indicate thermal and biogenic methane occurring in this area. CHUNG (1976) and SACKETT (1978) presented data which showed appreciable D/H fractionations in methane derived during pyrolysis of organic matter. The aim of this paper is to investigate systematically the hydrogen isotopic composition of natural gas methanes and to compare these with their carbon isotopic composition. The main interest is to examine whether the various genetic groups of methanes have characteristic deuterium concentrations as it is similarly known for their carbon isotopic compositions. The combined use of both isotope parameters should
then be of high diagnostic value in identifying the origin of natural gases. The genetic types of investigated methanes are displayed in Fig. 1 which is a generally accepted model for the origin of natural gases (S~KOLOV et al., 1972; KARSTEVet al., 1971; TIS.WT et al., 1974; Dow, 1977; TISOT and WELTE,1978). The gases which are indicated in Fig. 1 may be called primary gases in contrast to those which have undergone secondary processes like migration, mixing, biodegradation, absorption, etc. This paper deals mainly with methanes from these primary gases.
649
EXPERIMENTAL Oxidation
PROCEDURES
of methane
Gas samples were prepared for mass spectrometry in a vacuum line similar to that described by GUNTER and MUSGRAVE(1971). The methane was separated by gaschromatography from all other gaseous constituents and was transferred on line with the carrier gas (helium) to the oxidation line where it was oxidized by copper oxide (880°C). The resulting water was trapped together with CO1 at liquid nitrogen temperature. Carbon dioxide was subsequently separated from H,O at dry ice methanol temperatures and the remaining water was reduced to H, by passing it over hot uranium (BIGELEISEN et al.,1952).The hydrogen gas was trapped in a sample container with activated charcoal at liquid nitrogen temperatures. The accuracy of this flow line was checked by a closed system oxidation. Standard methane was catalytically oxidized in a closed reaction vessel by means of a red hot platinum wire. The reaction products were completely recovered and reacted as described above. The deuterium concentrations found were identical within limits of error.
650
M. SCHOELL
CLASSIFICATION
OF NATURAL
GASES
hydrocarbons generated c3
*) vpes
of kerogen
lTtssor
et al.
1971)
II : marine-sapropelrc 111: terrestfial-humic
Fig. 1. Classification
of natural
gases and abbreviations for genetic groups in this paper.
tion
Mass spectrometr~
of gases which are discussed
:
Carbon and hydrogen isotope analyses were carried out on VG 903 and VG 602 mass spectrometers respectively. The isotopic composition is reported in the usual ¬a-
6=
R, smpie - Rrtsndard R s,andsrd
. looo&,).
Table I. Hydrogen and carbon isotopic composition of methanes from natural gases of various origins and chemical compositions. The results are grouped according to their origin as discussed in the text. The standard for 6’“C and 6D values is PDB and SMOW respectively
No.
Country
0 I 2 3 4 5 6 7 8 9
U.S.A. U.S.A. U.S.A. U.S.A. U.S.A. U.S.A. Phil Phil Germany Germany Germany Germany Germany Germany Germany Germany Germany Germany Germany Germany Germany Germany Germany Italy Italy Italy Italy Italy Italy Italy Italy
I0 II 12 I3 I4 15 16 I7 18 I9 20 21 22 23 24 25 26 27 28 29 30
Locality Glacial drift Glacial drift Glacial drift Glacial drift Glacial drift Glacial drift IO Margot 31 Margot Lala La Brake marsh gas Inzenham 05 Inzenham 06 Inzenham 08 Inzenham 09 Inzenham W 1 Inzenham W2 Inzenham W3 Inzenham W4 Schmidhausen Cl Breitbrunn Muehldorf-S C 10 Ampfing 27 Weitermuehle C IO Weitermuehle C6 Caviaga 16 Collecchio 7 TresigaIIo I2 Port0 Corsini Porto Garibaldi 5 Ravenna Mare A 15 Selva 9 Piadena Ovest 25
Depth (ml -~~ ~ -~~ ~~ 900 ~ ~ ~ 1000 2000 1300 I 100 1000 I IOU 1400 200
Age
Quater. Quater. Quater. Quater. Quater. Quater. Quater. Quater. Quater. Mioce. Mioce. Mioce. Mioce. Mioce. Mioce. Mioce. Mioce. Mioce. Oligo. Oligo. Oligo. Mioc. Oligo. Plioc. Quater. 1300 Quater. 3OCKl Plioc. 2500 Plioc. 1500 Plioc. 1300 Plioc 2500 Plioc.
_
-
73.91 87.47
~ -
99.23 99.25 99.28
0.31 0.22 0.20
~
98.85
0.06
98.75 98.80 97.64 99.32 98.72 98.76 98.55 97.49 96.34 99.39 97.84 98.83 97.95 99.37 99.14 98.30
0.16 0.19 0.15 0.29 0.50 0.14 0.05 0.17 2.95 0.04 0.09 0.09 0.08 0.08 0.t 1 0.34
~ -~
-75.7 -82.7 -91.0 - 72.8 -73.0 - 76.0 -76.2 - 76.2 -68.8 - 70.0 -67.6 -68.1 -68.9 -69.5 -72.2 -71.8 -71.1 -71.6 -67.7 - 64.0 - 65.6 -67.8 - 66.4 - 55.3 - 76.3 -71.4 - 72.7 - 73.3 - 70.7 - 69.2 -64.2
-277 -245 -226 - 239 -234 -239 - 207 -216 - 244 -198 -192 -192 -194 - 205 - 207 -213 - 202 -213 - 192 -216 -201 - 207 - 198 -200 -187 -189 -179 - 181 - 179 -182 -189
-85 -47 -40 -50
-2 -18 -39 -56 -62
-24 -46 -12 -22 -32
Hydrogen and carbon isotopic composition of methane Table l-continued No.
country
31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 5.5 56 57 58 59 60 61 62 63 64 6.5 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89
Italy Italy Italy Italy Italv Germany Germany Germany Germany Germany Germany Germany Germany Germany Germany Germany Germany Germany Germany Germany Germany Germany Germany Germany n.r. n.r. nr. nr. n.r. %y
Egipt Egypt
U.S.A. U.S.A. U.S.A. U.S.A. U.S.A. U.S.A. U.S.A. U.S.A. U.S.A. U.S.A. U.S.A. U.S.A. U.S.A. U.S.A. U.S.A. Germany Germany Germany Germany Germany Germany Germany Germany
Locality Imola 10 Brugherio 13 Cornegliano 2 Bagnolo Mella 8 Len0 1 Weitermuehle 5 Isen 1 Oedgassen 1 Schnaupping 2 Hohenlinden 2 Albaching 5 Moosach 7 Moosach IA Eltze-Hard. 5 Loeningen 15 Loeningen 26 Wald 3 ~ullendorf 15 ~ullendo~ 15 Fronhofen 101 Fronhofen 3 Fronhofen 7 Moenchsrot Kirchdorf 3 illmensee 1 North Sea North Sea North Sea North Sea North Sea North Sea Cortemaggiore 73 Cremona Sud 9 Malossa I I i.n.r. i.n.r. Wyatt Sheffield Rojo Caballo Rojo Caballo Rojo Caballo Gomez Grey Ranch-H Mi Vida East Quito Grey Ranch-S East Quito Mi Vida East Quito Grey Ranch Grey Ranch Kalie Z 4 Wielen Z 4 Frenswegen 5 Getelo Z 1 Ratzel Z 4 Oythe Z 2 Wielen Z 4 Dalum Z 5
Depth (m) 1500 I100 1400 1200 1400 1800 1900 2200 2200 2100 2500 2800 2900 2500 1100 1100 1000 1000
C &
Ace
Plioc. Plioc. Plioc. Plioc. Plioc. Eocene Eocene Eocene Eocene Eocene Eocene Eocene Eocene U. trias. M. juras. U. juras. U. trias. U. trias. 1100 U. trias. 1000 U. trias. 1700 M. juras. 1700 L. juras. 800 Oligo. 1700 M. iuras. U. &as. i.n.r. i.n.r. i.n.r. i.n.r. i.n.r. i.n.r. i.n.r. i.n.r. i.n.r. i.n.r. i.n.r. i.n.r. 1500 Plioc. 1300 Plioc. 5500 L. juras. 2500 Mioce. 3700 U. cretae. 2200 Carbon. 2400 Carbon. Carbon. Carbon. Carbon. Carbon. Carbon. 4900 Silur. Carbon. Carbon. Carbon. Silur. Silur. Devon. Carbon. 3300 Carbon. 2300 Carbon. 2300 Carbon. 2500 Carbon. 2700 Carbon. 4000 Carbon. 2300 Carbon. 3400 Carbon.
1.0 0.8 1.1 0.9 1‘9 2.4 1.9 1.9 2.0 0.9 2.0 1.9 2.0 1.4 1.9 1.3 0.6 2.1 0.9 0.8 1.9 0.6 2.0
97.19 98.40 99.41 98.71 97.30 96.64 97.21 94.38 96.62 95.26 92.42 88.36 95.10 65.80 89.90 91.33 48.32 89.01 71.77 97.15 82.11 75.72 67.71 92.98 84.13 57.30 17.46 63.95 69.24 53.61 92.29 93.79 89.97 76.3 1 62.18 80.16 88.77 91.35 92.57 97.79 96.72 57.08 93.90 98.18 91.15 94.40 94.35 98.03 95.97 60.08 92.49 92.10 92.94 91.90 89.93 87.28 92.61 86.06
0.28 0.08 0.29 0.16 0.25
1.94 1.48 4.43 2.19 2.73 4.94 7.75 4.32 28.47 9.93 7.88 2.07 3.98 15.20 2.46 14.76 13.00 21.05 4.62 13.56 28.87 79.73 28.36 24.44 38.31 6.19 4.92 7.80 20.98 31.81 15.33 9.64 6.91 6.35 0.22 :: 0:45 0.35 7.89 0.72 0.49 0.40 I .49 1.18 2.32 5.68 1.22 5.35 6.10 0.73 4.92 0.44
- 58.2 - 62.4 -61.8 -60.8 - 59.3 -56.9 -58.9 - 53.4 -57.5 -54.7 - 52.6 -53.6 -58.0 -47.3 -44.6 -44.4 -44.7 -44.1 -35.1 -47.4 -42.5 - 36.0 -37.1 -36.2 -36.5 -44.8 -50.4 - 50.6 -50.7 -47.2 -51.3 -44.3 -46.9 -36.2 -46.3 -45.3 - 43.0 - 42.0 -41.9 -42.1 - 37.4 - 35.6 - 35.9 - 36.0 - 36.9 -40.5 - 36.9 - 36.4 - 37.5 - 37.5 - 37.5 -27.5 - 29.2 - 25.9 - 29.7 - 29.2 - 24.2 - 29.2 - 25.4
-210 - 198 - 204 - 194 - 200 -203 - 203 -201 - 199 -201 - 202 -206 - 206 - 203 - 203 -201 -214 -217 -173 -236 -217 -185 -182 -164 -188 -197 -232 -231 -245 - 229 - 248 -165 - 182 -153 -222 -256 -177 - 161 -154 -151 -141 -142 - 141 -146 -136 -148 -145 -144 -138 -150 -142 -143 -153 -139 -154 -155 -133 -149 -138
-27 -19 -
Biogenic methanes de$cient of C 2+ hydrocarbons (group ‘B’)-Nos O-5: Glacial drift gases from Illinois, U.S.A. Nos 6-8: Marsh gases from different areas [6, 7 Philippines, 8 Germany (group B(t))]. Shallow dry gases from subalpine Tertiary basins (group ‘B’ and ‘M’jNos 9-22: S-Germany [group B(m)]. NOS 23-35: PO Valley basin, N. Italy [group B(m); No. 31 group ‘M’]. Nos 3643: Natural gases from basinal Tertiary, Molasse basin, S. Germany, with significant C2+ contents (group ‘M). Natural gases associated with crude oils (group TjNos 4446: N.W. Germany. Nos 47-55: S. Germany, Molasse Basin. Nos 56-61: North Sea. Nos 62-64: N. Italy. Nos 65566: Egypt. Low C2 + thermogenic gases (group ‘T7”)---Nos 67-81: Natural gases from the Delaware-Val Verde basin, Texas [group TT(m)]. Nos 82-89: Natural gases from N.W. Germany ‘coal gas’ [group lT(h)].
652
M. SCHOELL
R is the ‘3C”*C and D/H ratio respectively. GD-values are given relaive to the SMOW standard (CRAIG, 1961) and &r3C-values relative to the PDB standard (CRAIG. 1957). Duplicate analyses were performed on all gases for which 6D-values are reported in Table 1. The overall reproducibility for the values given is + I.7”&,for 6D and +O. I’:,,, for 6°C. Mean values and long term reproducibiSties (Irr standard deviation of the single determination) of D/H measurements were calculated for two laboratorystandard gases. Dry gas standard (St. CH,) - 124.8 i: 2.6 Wet gas standard (E 245) - 175.6 + 2.6.
where : H = W, 0 = M = S= 1= 2=
moles of hydrogen, denotes the water or organic matter, methane, isotpic composition, initial composition, composition after reaction.
The following equation is derived for the relation between the isotopic composition of the methane, the organic matter and the water:
Gas chromatograpllj
Routine CC techniques were applied to determine the concentration of C, through CQ hydrocarbons. For the purpose of this paper only C, and C~+-con~ntrations are given whereby c L+ = (I - C,/ZCn). loO(“/;,).
Municipal raw sewage was used to investigate bacterial in the laboratory. Four plastic bottles were filled with 1I. of sewage sludge, as it is used in fermentors. The methane
sludge was spiked with different amounts of pure D,O. The samples were activated to produce methane in a thermostated bath (40°C). The production of gas was brought to completeness, all gas was collected and an aliquot of this gas was used for isotopic analysis.
where X is the ratio AHo/AHw; AH, = Ho, - Ho, and AH, = H,, - HW2, i.e. X is the ratio of the hydrogen used from organic vs the hydrogen used from the water reservoir. C is a constant. The isotopic composition of organic material in natural sedimentary environments does not vary systematically. However. the deuterium concentrations in natural waters vary considerably. Precipitation in continental environments is depleted in deuterium (6D _ -20 to -9(r),,,,) whereas marine waters and interstitial waters are comparatively enriched (6D Q 0 to - 2@~~,,,). A simplified version of eqn (2) is therefore applicable to natural environments, i.e. 6, = rn.6, + f,
BIOGENIC
(3)
METHANE
where Bacterial production of methane is a widespread phenomenon in nature. It has been described as
occurring in various environments such as marine sediments (CLAYPOOL and KAPLAN, 1974), lake sediments (CAPPENBERG, 1974; WJNFREYet at., 1977). salt marshes (KING and WIEBE,1977) and glacial till deposits (COLEMAY,1976). Methane in shallow dry gas deposits has also been described as being of biogenic origin (NAKAI. 1960; COLOMBOet al., 1969; RICE, 1975; BELYAVEVef nf., 1977; FUEX, 1977).
WOLIN (1976) and CAPPENBERG (1976) have summarized various types of enzymatic reactions in methane producing bacterial food chains which can be divided into two groups:
(i) Reduction of CO,: these reactions comprise two steps. firstly production of Co, and Hz by nonmethanogens and secondly reduction of CO, with H, by methanogens. (ii) Hydrogenation of CHI radicals, i.e. a complete transfer of the CH3 radical to CH,. In either of these reactions the hydrogen in the CH, molecule is derived from principically two sources: organic matter and water hydrogen. The following mass balance equation can be written: Hw,&,
+ Hor60, = H,&
+ Ho2602 + Hwzhw2
(I)
1
‘n= x+i
and
X
C, = Kg
bo, t-c.
The percentage of methane hydrogen which is derived from water is found by the expression: Hw = lOO~m(SJ. Equation (3) is a linear relationship between dN and dw where the slope l/(X + I) gives information on how much hydrogen in the methane molecule is derived from organic matter or water. A slope of 0.25 should be excepted if methane is produced by the radical hydrogenation reaction [X = 3; CH, l/(X + 1) = 0.251. For the CO2 reduction all hydrogen in the CH, molecule could stem from water hydrogen; then a slope of 1 should result in a graphical display of eqn (3). The slope of the dn,0/i&HJ relationship varies between 0.25 and 1 if methane is derived from both reaction types. Experimental
productiott
of methane
Theoretical considerations have demonstrated that there should be a positive correlation between the D-concentration of water and methane if hydrogen from the water is used in the enzymatic synthesis of methane. This was investigated in simple laboratory experiments where CH4 was produced from municipal sewage sludge. The deuterium concentrations of the water were artificially increased using D,O. The results of these experiments are illustrated in Fig. 2.
Hydrogen and carbon isotopic composition of methane I -100 -
I
I
t +200
I +300
I
Natural Methanes
653
Experimentally produced CHL
7 8
(sewage sludge) v 3
Q 0
-300 -
I 0
-f.400 -100
I +lOO
6D +o
[%ol
Fig. 2. Hydrogen isotopic composition of methane which was bacterially produced from raw sewage. The associated water was spiked with different amounts of deuterium-oxide. The D,O-tracer experiment shows firstly that there is an immediate response to the’ change of the environmental water composition. The D,O spiked aliquots revealed deuterium rich methanes. A linear correlation between water and methane deuterium concentration is observed (Fig. 3). aDme,,,anec O.GD,,,,,
- 323%,
This relationship is experimental drogen from water is incorporated reactions which produce methane. indicates that approx. 40% of the methane is derived from water. Natural
gas methanes
ofbiogerlic
evidence that hyin the enzymatic The slope of 0.4 hydrogen in the
origin
The methanes analysed here for their deuterium concentrations are from areas which have already been described as producing biogenic gas. For many of the gases it was also possible to collect their associated waters:
They originate from layers of glacial drift deposits and are dissolved in groundwater. The respective waters were sampled from the same well. (2) Marsh gases from various localities: One sample of a marsh gas from N.W. Germany has been described by STAHL (1968) and was derived from holocene sands. 14C dating revealed an apparent age of 5000yr BP. Marsh gases from the Philippines were sampled from gas emerging in swamps. (3) Methanes from natural gases of the subalpine basin in S. Germany (SCHOELL,1977), and the PO valley in N. Italy (SCHOELLand NEGLIAin preparation). Both sets of gases are typical samples of ‘shallow dry gas deposits’. Gases with C,+-contents below 0.5% have been selected. In S. Germany waters from producing water wells were sampled. The waters of the Italian gases were obtained from the separators at the well sites.
The results of these analyses are listed in Table 1 and shown on Fig. 3. The variation of 6D in biogenic (1) Glacial drift gases, Illinois, U.S.A. : These gases methanes is considerable: - 175 to -250%,. Marsh have been described in detail by COLEMAN (1976).gas methane and methanes from glacial drift gases, which are both from terrestrial environments are depleted in deuterium as compared to other biogenic gases (tertiary basins, Cariacho Trench) which are derived’ from marine environments. enwonmenr Figure 4 shows that the environmental control of 0 marine the methane D/H ratios is again due to the correlation of the respective water D/H ratios. The correlation is almost identical to that which was found for Japanese natural gases (NAKAIet al., 1974), i.e.
I
-300
-250
6D Fig. 3. D/H
ratio
-200 CH/,
frequency distribution biogenic origin.
-150
6D,,,e,hrne = Qv,,,,
[%ol of metdanes
of
-
(160 + lW&,.
The collection of data from world wide occurrences suggests that this relationship is characteristic for all natural biogenic gases. The slope of 1 in this relationship suggests that 1OO0/O of the hydrogen in these
M. SCHOELL
654
-180
1’
lill
-80
I
I
I
-LO
I
0
6D,o [%*I Fig. 4. Hydrogen isotopic compositjon of biogenic methanes and their associated waters from various areas as compared with cogenetic methane/water pairs from natural gas deposits in Japan. 1, Japan (NAKAI et al., 1974); 2, marsh gas, N. Germany; 3, glacial drift gases, Illinois, U.S.A.; 4, Cariaco trench [aD(H,O) estimated]; 5, natural gas deposits, N. Italy; 6, natural gas deposits, S. Germany (SCHOELL, 1977 and this paper).
methanes is derived from their associated waters. Another explanation could be, that during the enzymatic reactions of the methane synthesis interreactive groups are in isotopic equilibrium with water. If CH* is produced by the reduction of CO,, hydrogen is used which has been formed from intermediate substances like pyruvate, formate, hypoxanthine and reduced pyridine nucleotides. In all cases interreactive groups are formed to which hydrogen is only weakly bound [NAP (P), pyruvate etc.]. It is well known that this type of hydrogen readily exchanges isotopically with environmental water (EPSTEINet al., 1976). It should therefore be expected that hydrogen which is formed in bacterial food chains has isotopitally exchanged with environmental water hydrogen. This would imply that during CO2 reduction of methane only ‘equilibrate hydrogen enters the CH4 molecule. In the CH, radical hydrogenation reaction only one hydrogen has exchanged with the environmental water and three atoms are derived from the organic matter. show that the considerations The above relationship in natural biogenic 6D,,,,,,,-6D,,,,,gases is indicative that these gases pr~omin~tly formed by the bacterial reduction of CO,
CO2 + 4Hz -+ CH4 + 2H20. The CHJ hydrogenation by methanogens, which has been found to be important in Lake Vechten sediments (CAP~NB~~, 1976) is obviously of minor importance for the formation of natural biogenic gases.
SACKFTT(personal communication) reported a 6D value of -3Iq&, for a Missisippi swamp gas. This value does not fit the above 6Dc,,&D,, relationship for natural methanes. A very unlikely 6D,, of - ISO?&,would be necessary to fit this relationship. A fermentation process similar to that which was investigated with the sewage sludge rather than CO, reduction could explain this particular isotopic composition of the swamp gas. Figure 5 shows the relationship between the hydrogen and the carbon isotopic composition of methanes from shallow dry gases. The 613C/6D boundaries given in this figure by stippled lines are those which are found to be limiting for ‘pure’ biogenic gases such as glacial drift gases (%HOELL and COLF.MAN,in prep aration), i.e. 6D and ai3C values more negative than - 150 and - 64’?&,respectively. The -64’& boundary is suggested if all proved samples of biogenic methanes are considered for which secondary effects like bacterial oxidation are unlikely. Almost all published data of biogenic methanes which are deficient of saturated CZ+ hydrocarbons plot within this field (NAKAI et al., 1974; COLEMAN,1976; LYON, 1974). Variations in 6D in biogenic methane are predominantly due to environmental changes. A differentiation of *terrestrial’ biogenic gases B(t)(GD < - 19C$&) and ‘marine’ biogenic gases ((~)(~D > -WY&) is suggested (see Table I, Fig. 1 and Fig. 11). The gases from the various basins are well recognized by their isotopic pattern (Fig. 5).
Methanes of mixed oriyin (Group ‘M’) and dj~re~ltiutjuff j&n biagenic r~~tha~es
their
Some of the gases from young subalpine basins-do neither fit to the biogenic gases nor to the thermogenie gases (see below). These gases occur often at basinal reservoir rocks in the respective basins. These gases are listed in Table I, Nos 31 and 36-43 and are summarized as group ‘M’ (Figs 5 and 11). All these gases have in common significant amounts of C,, hydrocarbons (up to 4.9%). C, + hydrocarbons in such quantities are generally not of biogenic origin (JANEZIG, 1979); abiogenic processes have therefore to be assumed to be operative. SCHOELL(1979) showed that the contents of CZ+ hydrocarbons is correlated with an increase of i3C in the methanes (6’3C values -6Q, to - SY&). This suggests that methane of nonbiogenic origin is produced together with ethane and is mixed with these gases. The SD values in this group scatter little around - I?Off’,,,,, i.e. the admixed methane of nonbiogenic origin should be around - 200‘& THERMOGENIC GASES Thermal alteration of organic matter is a gradual process which produces liquid and gaseous compounds of continuously changing composition. It is therefore impossibIe to define clearly cut groups of natural gases. This paper follows the differentiation of
655
Hydrogen and carbon isotopic composition of methane
-250
-90
-80
-70
-SO
-50
Fig. 5. D/H and i’C/i*C ratios of natural gas methanes from late tertiary subalpine basins (S. Germany and N. Italy) compared with methanes of biogenic origin. The box given in stippled lines gives the variation of pure biogenic gases (‘3’). 1, Glacial drift gases; 2, marsh gas, N. Germany; 3, marsh gas, Philippines; 4, natural gases, S. Germany; 5, natural gases, N. Italy atone); 6, gases from the eocene lithothamnia limestone, S. Germany; 7, Cariaco trench (LYON,1974); 8, natural gases, Japan (NAKAIet al., 1974). thermogenic gases which was proposed by FUEX (1977) and STAHL (1977): (i) gas associated with oil generation (T’), and (ii) dry gas (‘TT’) from either terrigenic (humic) or marine sapropelic matter r’I.T(h)’ and “IT(m) see Fig. 11.
Gas methanes from associated
high CZ+ gases
which are
with crude oils (group ‘T)
Gases from various typical oil producing areas were selected (see Table 1). (1) North Sea. Field data are not released for these gases. The gases were all brought to the laboratory together with oils and were associated with oils in situ. One sample was degassed from the crude oil in the laboratory. All gases have high Cj?+ contents. (2) N.IV. Germany. 3 samples are derived from triassic and jurassic source rocks of the Lower Saxonian basin and represent typical associated gases of mesozoic source rocks in N.W. Germany (STAHL, 1979). (3) S. Germany. These gases are derived from the western part of the Molasse Basin and have already been described by SCHOELL(1977). The gases are from mesozoic’ and tertiary reservoir rocks. The source rocks are not identified. (4) N. Italy. Three samples have been collected from classical oil/gas deposits from N. Italy which have already been described (MINUCCI, 1959; LucCHETHet Uf., 1969). (5) Egypt. Two samples have been analyzed which were delivered from oil drill&s where the gas was still associated with the crude. Further data are not released.
Figures 6.1 through 6.3 give the results which were obtained for the methanes of the associated gases. The variations of the D/H ratios are remarkable and comprise a range of up to IO@/&,.The SD values scatter even within geographically closely related samples nearly as much as all gases of this group. For example the gases from S. Germany are from very closely grouped reservoirs and display a range of approx. 7(J& in 6D values. The same is observed for the North Sea gases. In general deuterium in the methane is depleted with increasing CZ+ values or wetness of the gas. One of the most D-depleted methanes was found in the sample which was degassed from the oil, As shown in Fig. 6.3 there is also a gross correlation of the carbon-13 and deuterium concentrations in the methanes of group ‘T’. The relation of 6D over 613C is approx. 1: IO. It should be noted, that the D/H variations in these methanes are qualitatively comparable to the variations which were observed in laboratory pyrolysis experiments (SACKETT,1978). In Fig. 7 the experimental fractionation trendlines have been drawn by assuming that hydrocarbons of the mean isotopic composition of crude oils (SCHOUL and REDDING, 1978) of kerogens (REDDING, 1978) were the parent materials to all these gases. The similarity of the 13C and D composition of group T methanes and pyrolysis methanes suggests that closed system pyrolysis bears a rather close resemblance to gas foxing processes in nature. The free radical mechanism which has been quoted by SACKETT(1978) as a possible mechanism for D/H fractionations in experimentally produced methane
M.
656
SCHOELL
rrn- North Sea --
-20 -
---
I [2J
\,
-140
. \
NW-Germany I QJ South Germany i GJ North Italy : ~ Egypt
~- rr '
\<"~-~11i 1m l 1 ~) ~~~~c~g~~~E ~~~Ge II
,.
~'~>:. ~,
r-o' ~
~
'0' ~
@]
<:t
(f;j
o
o
-40
)
MARINE SOURCE ROCK
:'\. .
1
~ '.. £:~\
:r:
0
",,0
1m
1
~ .." ..~?::\:..
-180
~
<:t
:r:
Natural Gases USA
TT
\»..
'_ •• ~@j
-30
00
;':'.
. -:'::"
-220 .:.'.:'
-50
...:....
..........~:: . ,..:.. ',@ .
....
\.::'.~
- 260
o
o
50
10
c2 +[%J
30 C2 +
Fig. 6.1
[%]
Fig. 6.2
1T1ml
-150
.. .. ...
-250
-50
- 40
-30
-20
3 13CCH4 [%oJ Fig. 6.3 Fig. 6. Variation of the D/H and '3C/'2C isotopic composition in methane from natural gases in relation to the molecular composition in wet gases. The fields TT(h) and TT(m) denote the composition of low C 2 + thermogenic gases. (See Fig. I and text below.) 6.1, oD/C 2 +; 6.2, 0' :iC;C2+; and 6.3, OD/O '3 C relationship respectively.
50
Hydrogen and carbon isotopic composition of methane
657
The D/H-isotope analyses on these methanes revealed that their hydrogen isotopic compositions are also dependent on the maturity of their source rocks (Fig. 8). The deuterium concentrations are obviously only related to the maturity of the source material whereas the ‘~C~on~ntration is dependent on the maturity and the type of source organic matter. The calculation of the GD/Ro-relationship gives the following equation 6D = 35.5 log R. - 1527& whereas the ~i3C/R~-reIationship following two equations
is described by the
8.6 log R, - 28?&
TT(h):
613C =
TT(m):
613C = 14.8 log R, - 41”&,.
In trying to understand the increase of deuterium in methane with maturity various processes could account for the fractionation of hydrogen isotopes. (1) Isotopic exchange between methane and associated water: with respect to the mass balance hydro-
Fig. 7.
from closed system laboratory experiments at 400 and 500°C (SACKF.TT,2978).The
,
t
/
1.0
2.0
30
, 1.0
/ 2.0
! 30
parent material is assumed to have the mean isotopic composition of crude oils (SCHOELLand REDDING,1978) and kerogen (REDDING, 1978).
should therefore aiso be taken into consideration natural gas formation.
for
Methanes from low Cz+ gases [groups ‘TT(h)’ ‘T T(m)‘]
and
Two groups of low C2+ or dry gases have been selected to investigate the D/H fractionation patterns of their methanes, i.e. N.W. German coal gases and gases from the Delaware-Val Verde basin (Table 1). These gases have already been investigated in detail with respect to the fractionation of carbon isotopes of their methanes in relation to the maturity of their source rocks (STAHL,1975; STAHLand CAREY, 1975). ~arboniferous coal-measures of the N.W. German Basin are the source rocks of the N.W. German coal gases which thereby represent gases from humic organic matter (type III kerogens). These gases are denoted as group ITT(h)‘. Background information on the origin of these gases is given in BOIGK etal. (t976), ~AR~NSTEIN et af. (1971) and STAHL and KOCH (1974). The source rocks in the Delaware Val Verde Basin are Silurian through Carboniferous in age and are described to be of marine origin; hence they should represent kerogens of type II of the Tissot classification. These gases are denoted as group ‘TT(m)‘. Both sets of gases are from source organic matter of varying maturity (vitrinite reflectance R,). The ‘3C-concentration in the methane was found to increase with increasing maturity (STAHL, 1975; STAHLand CAREY, 1975).
-140
-160
01
-50 ’ 0.4
R, [%I Fig. 8. D/H and “C/“C ratios in natural gases in relation to the maturity of their source rocks. Gases from the Val Verde Delaware Basin are from marine source rocks and methanes from coal gases (N.W. Germany) are from humic source rocks. RO-values (mean reflectance of vitrinite under oil)are adopted from S~~~~(l975)and STAHL and CAREY (1976). I,Gases in carboniferous reservoir rocks, N.W. Germany [TT(h)]; 2, dry gases in Paleozoic reservoir rocks from the Delaware Val Verde basin. Texas.
658
M.
-125
SCHOELL
rr1m~
\
-150
IiJ7l -175
L!...:J
•
-45
-40
-30
-35
-25
-20
o13CCHt
[%0]
Fig. 9. D/H ratios in thermogenic methanes from marine and humic source rocks plotted versus their 13C/ '2 C ratios. The arrows give the pathway of increasing maturity of the source rocks.
gen which is bound in associated waters (oil field brines) is mostly in excess of the hydrogen in methane. At isotopic equilibrium between CH 4 and H 20, for example at temperatures around 2()()OC, the methanes should be 65%0 depleted in deuterium as compared to the cogenetic water (BOTIINGA, 1969). Most oilfield brines and waters associated with natural gas deposits lie within the range (- 25 ± 15)%0. Methanes in equilibrium with these waters should range in their bD-values from -75 to -105%0. However, the range of the carboniferous gases is -133 to -177. Lower temperatures should result in even more positive bD values. The presented data thus show that hydrogen isotopic equilibrium has not been achieved in the methanes from dry gases. Moreover the results of laboratory exchange experiments (Kopp, in preparation) do not support H 2 0-CH 4 hydrogen
exchange reactions to be responsible for DiH vanation of natural gas methanes. (2) Fractionation due to bond energies in precursor organic matter: SACKETI et al. (1970) have pointed out that the "strength of terminal carbon-<:arbon bonds in parent organic materials are the primary factors that determine the isotopic composition of all methane occurrences". For CH 2 D terminal groups the carbon--earbon bond should be stronger than for CH 3 terminal groups. One should therefore expect increasing cleavage of C-CH 2 D bonds with increasing thermal stress i.e. the deuterium concentration in CH 4 should increase with maturity. This model could explain satisfactorily the correlated increase of both heavy isotopes in the methane; it does not explain why the hydrogen isotopic fractionation is the same for both humic (i.e. predominantly aromatic) and lip-
Marsh & Glacial Drift - Terrestrial
II I I
Marine Sediments
Biogenic Methane
DDB
II I I II
III
"B
{t} "
"B (m) "
Molasse Basin oS-Germany
1
YOUNG
DD
TERTIARY
BASINS I
II
II I I
I
11111
I II I I I II
II I II I I I
N-Italy
High C2• Gases
II
Thermo genic
I 11111 I II
Methane II I
f)l)TDI)
- 250
-200
I III I
"r
""'j
'''o~,,~~,
Basin"TT(m)" Low
N-German Coal Gases"TT(h)"
-150
Fig. 10. Summary of the variation of D/H ratios in natural methanes.
C2. GASES
"TT"
Hydrogen and carbon isotopic composition of methane o
'(j(.
659
-100
B~m~
-\---------
-150
-200
O'-<::r----'
o
o
-250
o
-90
-70
-50
-30
-10
Fig. II. Hydrogen and carbon isotopic composition of methanes from natural gases. The plot of ,, 13 C vs "D values facilitates the differentiation of genetic groups and the identification of secondary processes such as mixing.
tinitic (i.e. predominantly aliphatic) precursor materials. For C-isotopes CHUNG and SACKETT (1978) have explained this effect as being due to the more aromatic structure of humic organic matter. Methane from aromatic matter is always less depleted in the heavy isotope compared to its parent material than CH 4 from aliphatic materials. This 'structural effect' may also be operative for hydrogen, but it could be immeasurable with the present accuracy of OjH ratio measurements. Since both isotopic species of the methane molecule are fractionated in favour of the heavy isotope during metamorphism of organic matter, the combination of both in a (j13Cj(jO plot reveals maturation pathways for methanes from dry gases which are indicated in Fig. 9. The calculation of maturity trendlines for marine and terrestrial source rocks revealed the following equations: TT(m): TT(h):
3.2(j 13 C - 25%0 (jO = 3.8(j 13 C - 40"100'
(jO
=
Natural dry gases from mixed source rocks should plot in this figure in the field between the evolution lines of pure marine or terrestrial source rocks. Methanes from low C2+ gases exhibit a lower scatter in OjH ratios (~25%0) than methanes from high C2+ gases which are associated with crude oils. By this, high C 2 + gas methanes which are mostly associated with oil may be differentiated from dry (nonassociated) gas methanes (Fig. II).
types of natural methanes and demonstrate that the different methanes are more easily differentiated on a (jO- 13 C plot, than by using only the carbon isotope ratios. Variations within the genetic groups 'T' and 'TT' given in Fig. II are caused by kinetic processes during the formation of the gases. Their isotopic composition may be altered by secondary processes. For example in group 'M' the variations are most likely caused by mixing processes. Future investigations should concentrate on these secondary processes like mixing, migration and biodegradation which should also reveal typical isotopic patterns. Carbon-13 and deuterium isotope analyses will be most useful in this respect since secondary processes affect both isotopic species-and methane is a highly mobile molecule. Acknowledgements-The following oil companies generously supplied samples for this investigation: AGIP San Donato Milanese, Brigitta Elwerath Betriebsflihrungsgesellschaft MbH, Deminex, Deutsche Schachtabau und Tiefbohrgesellschaft, Deutsche Texaco, Mobil Oil AG Deutschland, Preussag AG, Tenneco Oil Company. I am grateful to Dr W. STAHL who offered to include here the Delaware Val Verde data. The analytical work was performed by U. RINOW, S. RESKI and O. AMIT. The drawings are the skilful work of 1. KENKLIES. An early versioll of this manuscript was read by D. WAPLES by whose comments it was greatly improved. I had great benefit from discussion with W. M. SACKETT, I. WENDT and W. STAHL. I. R. KAPLAN, W. SACKETT and B. DURAND reviewed this manuscript. Financial support was given by the Bundesministerium fUr Forschung und Technologie through Grant ET 3003 A.
Isotopic characterization of genetic types of natural methanes and future work
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Figures 10 and II summarize the OjH ratios on natural methanes. They illustrate the various genetic
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