- Email: [email protected]
An unusual occurrence of light hydrocarbon gases in a well offshore Northeast Palawan Island, the Philippines
Precambrian Research, 30 (1985) 179--188
179
Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
AN UNUSUAL OCCURRENCE OF L I G H T H Y D R O C A R B O N GASES IN A WELL O F F S H O R E N O R T H E A S T PALAWAN ISLAND, THE PHILIPPINES
ZVI SOFER Cities Service Oil and Gas Corporation, Box 3908, Tulsa, Oklahoma 74102 (U.S.A.)
(Received October 19, 1983; revision accepted February 22, 1985)
ABSTRACT Sofer, Z., 1985. An unusual occurrence of light hydrocarbon gases in a well offshore Northeast Palawan Island, the Philippines. Precambrian Res., 30: 179--188. Sediments from a well offshore Northeast Palawan Island, the Philippines, appear to contain light hydrocarbons of possible abiogenic origin. This observation is based on stable carbon isotope ratios, organic petrography, and chemical and geological considerations.
INTRODUCTION Large accumulations of h y d r o c a r b o n gases are derived from biological material either by thermal degradation of t hat material when deeply buried, or by the action of methanogenic bacteria on t hat material during early burial. Gold (1979) h y p o t h e s i z e d t ha t h y d r o c a r b o n gases of abiogenic origin are very a b u n d a n t and, therefore, could play an i m p o r t a n t role as a source of energy. The occurrence of abiogenic gas was also the subject of a study by an ad-hoc c o m m i t t e e of the National A cadem y of Sciences (USA) and a summary of its r e p o r t appeared in the November, 1980 issue of Geotimes. The major conclusion o f this c o m m i t t e e was t hat a specific locality for a commercial accumulation of such gas is n o t y e t known. Stable carbon isotope ratios are believed to be useful in distinguishing between biogenic and abiogenic derived h y d r o c a r b o n gases. Organic-derived m e t h a n e yields carbon isotope ratios generally m ore negative than --25%0 (relative to the PDB standard), while the associated ethane, propane, and butane usually show progressively m or e positive isotope compositions (Erdman and Morris, 1974). Methane gases from geothermal areas have isotopic compositions ranging m os t l y f r om --20 to --300/00 (Faure, 1977; Fuex, 1977) although --16°/00 has also been r e p o r t e d (Lyon, 1974). Galimov (1975) r e p o r t e d carbon isotope compositions of abiogenic m e t h a n e t rapped in gas inclusions in igneous rocks to be in the range o f - - 1 2 t o - - 3 % 0 ,
0301--9268/85/$03.30
© 1985 Elsevier Science Publishers B.V.
180
whereas the associated ethane and propane showed an inverted relationship, i.e., they were progressively more negative. Yuen et al. (1984) also observed an inverted relationship in the carbon isotopic composition of abiogenic light hydrocarbon gases in the Murchison meteorite. This paper describes sediments in a well which, based on stable carbon isotope ratios, organic petrography, and chemical and geological considerations, appear to contain uncommercial quantities of hydrocarbon gases of an abiogenic origin. The well (Dumaran no. 1) was drilled in an area of no known gas or oil production to a total depth of 2200 m (offshore Northeast Palawan Island, the Philippines); the b o t t o m 100 m penetrated an igneous intrusion (serpentinite); the 950--2100 m depth interval was uniformly composed of fine-grained, low permeability sandstone deposited in a marine environment. The 950 m depth probably represents the Cretaceous--Tertiary boundary and is marked by an unconformity. The 690--950 m interval contains sediments that, for the most part, are the same as those in the 950--2100 m interval, except that carbonate shell fragments are more abundant. No sediment samples were available for analysis above 690 m. EXPERIMENTAL Composite well cuttings representing drilling intervals of 15--30 m were tightly sealed in metal cans shortly after emerging at the surface. Water and bacteriocide were added to the cuttings leaving a b o u t 20% of the can space empty. Quantitative chemical analysis of the light hydrocarbon gases from methane through the butanes (C,--C4 ) released by the cuttings into the head space was c o n d u c t e d using gas chromatography. Most cans had very little h y d r o c a r b o n gases accumulated in the head space. After the cans were opened, similar analyses were c o n d u c t e d on gases released by milling a portion of the cuttings (3 g) in a sealed container. The a m o u n t of gas per gram of sediment released by milling was a b o u t two orders of magnitude larger than the a m o u n t released into the can head space. (However the absoTABLE I Light hydrocarbon analysis of headspace and milled cutting gas (Hydrocarbon concentrations expressed as ppm, volume of gas to volume of cuttings)* Sample Depth (m) 689 689-701 701-716 716-731 731-747 747-762 762-777 808-823 823-838 838-853
CI 1,023 2,592 14,583 4,352 8,209 2,558 8,195 66,139 74,143 121,418
C2
C~
i-C 4
n-C 4
% Organic Carbon
23.3 26.7 95.9 31.7 32.0 32.3 50.6 267.4 280.1 419.0
2.1 3.7 13.~ 2.3 3.3 6.0 4.7 25.0 33.(I 51.0
.O .D 5.7 .0 .0 .0 .0 3.3 6.7 11.7
.0 .0 .0 .0 .0 .0 .0 2.3 10.7 14.0
.10 .08 .15 ,13 .11 .09 .II .16 .21 .16
Gas** m m C ~ Wetness ~ o,L: ~, I%) .27 .85 2.52 .87 1.92 .75 1.93 10.64 9.09 ]9.52
2.43 1.16 .78 .78 .43 1.48 .67 .45 .44 .41
*** ~ 13CH (% ~)
181
TABLE I (continued) Sample
Depth (m)
C1
867-884 65,258 884-899 7%043 914-929 18,587 945-960 81,595 960-975 12,610 I021-I036 31,345 I036-1051 II,913 1051-1067 14,139 1082-1097 8,148 1097-1112 8,640 1112-1127 14,124 1127-1143 10,961 1143-1158 18,762 1158-1173 13,846 1173-1189 7,181 1189-1204 10,803 1204-1219 11,703 1219-1234 10,979 1234-1250 9,722 1250-1265 12,261 1265-1280 11,692 1280-1295 8,371 1295-1311 10,781 1311-1326 10,910 1326-1341 3,980 1341-1387 15,964 1432-1448 I0,238 1463-1478 18,199 1494-1509 II,195 1539-1544 6,312 1570-1585 I0,282 1585-1600 II,442 1600-1615 12,333 1615-1631 11,804 1631-1646 11,721 1646-1661 8,806 1676-1707 10,712 1707-1737 10,276 1737-1768 7,524 1768-1798 8,975 1798-1827 1,071 1827-1859 8,274 1859-1890 14,133 1890-1920 6,886 1920-1950 9,462 1951-1981 5,351 1981-2012 6,981 2012-2042 26,952 2042-2073 8,251 2073-2103 4,913 2103-2134 12,219 2134-2164 3,755 2164-2195 7,596
C2
198.0 382.0 247.6 399.3 57.9 195.3 172.5 149.8 103.6 107.9 238.0 160.0 269.3 154.9 121.0 213.1 160.8 158.9 136.8 229.7 204.2 144,3 169.0 159.3 74.1 232.7 179.5 264.0 149.5 I17.1 227.8 187.4 234.6 204.3 207.5 205.0 141,4 216.2 204.8 168.1 21.1 152.1 291.9 136.2 178.4 95.9 138.5 530.9 145.6 82.2 204.4 72.3 83.8
C3
i-C 4
9.7 47.3 79,3 76.7 4.0 35.6 54.9 46.9 27.8 32.4 75.8 45.7 81.3 43.9 35.2 62.2 37.9 44.4 37.0 69.2 57.3 44.4 44.4 39.8 22.4 60.0 59.0 7g.fl 44.5 41.6 75.8 53.3 15.3 58.0 58.4 62.2 41.7 59.6 75.2 56.4 7.2 47.4 102.6 46.9 56.6 27.3 41.5 118.3 43.8 23.1 57.5 24.8 20.4
.0 12.0 23.0 22.N .0 ll.O 15.0 10.6 6.3 7.3 20.0 11.3 21.0 10.7 7.0 17.6 13.3 11.7 9.0 17.3 16.0 ll.O 12.3 .7 a.3 16.0 16.3 29.7 ll.O 10.8 54.3 14.5 19.6 15.7 16.0 19.3 I0.9 16.7 17.4 14.6 .0 11.9 29.3 12.0 14.2 7.0 7.8 44.5 11.5 2.3 16.0 3.7 .0
n-C4
.0 10.3 24.3 26.7 .0 9.3 21.5 16.1 7.7 I0.0 30.0 14.5 31.4 14.3 10.4 21.5 11.1 13.7 10.9 24.1 21.0 15.0 .0 11.7 6.6 21.0 21.3 31.7 17.2 14.1 5.0 21.3 27.3 22.0 19.7 23.6 14.8 21.3 22.8 21.7 .I 16.4 37.4 15.6 21.9 8.0 13.0 34,1 14.3 3.0 20.5 7.2 1.7
% Organic
C m_ m ~
Carbon
90RG C
.16 .16 .15 .18 .17 .21 .17 .19 .18 .18 .17 .20 .18 .16 .17 .20 .18 .18 .17 .18 .19 .17 .17 .19 .18 .18 .17 .19 .18 .20 .19 .ll .20 .18 .18 .17 .17 .17 .17 .17 .18 .17 .18 .17 .22 .16 .18 .19 .14 .14 .14 .14 .14
I0.47 II.32 3.31 II.72 1.91 3.87 1.88 1.97 1.20 1.27 2.24 1.46 2.79 2.29 1.14 1.47 1.73 1.63 1.52 1.85 1.66 1.33 1.69
1.53 .60 2.37 1.63 2.56 1.66 .86 1.49 1.81 1.66 1.77 1.76 1.43 1.68 1.65 1.24 1.44 .16 1.32 2.15 1.11 1.17 .90 I.~5 3.83 1.59 .94 2.35 .73 1.42
Gas** Wetness
(%)
.32 .64 1.97 .64 .49 .80 2.17 1.56 1.75 1.79 2.51 2.07 2.10 1.59 2.36 2.83 1.87 2.04 1.95 2.70 2.49 2.50 2.05 1.90 2.63 2.02 2.63 2.13 1.95 2.83 3.41 2.36 2.35 2.48 2.51 3.40 1.91 2.96 4.08 2.83 2.59 2.68 3.16 2.97 2.79 2.52 2.80 2.63 2.54 2.20 2.38 2.80 1.38
*** ~ 13CH4
{°Ioo)
-16.74 -16.85
-14.57
*Gas analyses were performed o n a Hewlett-Packard 5 8 4 0 gas chromatograph e q u i p p e d w i t h a flame i o n i z a t i o n detector and a 6 ft single c o l u m n p a c k e d w i t h n-octane o n Poracil C 1 0 0 / 1 2 0 mesh. Total C 2 --C4 **Gas Wetness
x 100 Total C 1 ---C 4
* * * I s o t o p e analyses were performed as described o n Table II and on h e a d s p a c e gases only.
182
lute amount of gas was of sufficient quantity for isotope analysis only in the head space.) Such behavior reflects the impermeable nature of the cuttings and suggests that the loss of gas in the canning process may have been small or negligible. Total organic carbon (wt. %) was determined for all of the samples, and selected samples were also analyzed using microscopic techniques for kerogen type and thermal maturity. The carbon isotopic composition of the methane in the head space of three cans was determined. In addition, gas samples recovered in two drill-stem tests (DST nos. 1 and 2) which represent the great majority of the gas recovered from the well, were analyzed for both chemical and carbon isotope composition ¢fnr details see Table II). RESULTS AND DISCUSSION
The combined results of the chemical analysis of the head space and the milled cuttings gases are presented in Table I along with the isotopic composition of the methane from the head space of the three canned cuttings. In addition, Table I contains gas wetness values, the wt. % of organic carbon in the sediment, and the ratio of the combined weight of C, - C 4 compounds to the weight of organic carbon in mg g-1. Table II contains the chemical and isotope analyses of the gases from the two drill-stem tests (DST nos. 1 and 2). Depth profiles of some of the parameters presented in Table I are graphically displayed in Fig. 1. This t y p e of data presentation has been TABLE II Light hydrocarbon analysis Name
DST 1 DST 2
Depth (m)
1649--1658 1399--1417
Mole (%) a
~ 13C (0/0° ) b
Gas c Wetness
C,
C3
C3
C,,+ C O 2
N:
CI
C2
(%)
87.2 87.2
0.5 0.5
0.1 0.1
Tr Tr
12.2 12.0
--16.2 --16.2
--22.5 --19.9
0.7 0.7
-0.2
a Gas analyses were done on a Hewlett-Packard 5840 gas chrotnotograph equipped with a thermal conductivity detector, three columns, and three switching valves to achieve a good separation between nitrogen and oxygen. b Carbon dioxide for stable carbon isotopes of individual gas components was prepared by gas chromatographic separation, then combusting each component in an attached 1000°C combustion tube and collecting the products in separate cold traps. Isotopic analyses were performed on a double-collecting Micromaas 602D mass spectrometer. Results are expressed in the usual parts per mil (°/o,) notation, relative to the PDB standard (Craig, 1953). The accuracy of the isotope results depends somewhat on the sample size (most of the inaccuracy is introduced by the separation and combustion steps). Individual components present in the combusted mixture in quantities larger than 0.I cc S T P have a standard deviation (Io) better than -+0.3%o. Individual components smaller than 0.05 cc S T P s h o w a standard deviation of + 1.0°0. c See Table I for definition.
183 C1-C4 100
HYDROCARBONS (VOLUME PPM) ,iooo 10000 100000 1000000
MILLI(~RAMS GRAMS ORGANIC t
GAS WETNESS (VOLUME %) 1 2 3 4
0
(~I-(~4 CARBON 2
3
750
j
lOOO
~1250 W
2"
3EL ~ - - = = J.-_~_
:=DST#1--=-=
>
1500----"
:~::_
__ := . . . . . . . . . . .
=~DST#2
-_: 16.2
~
S
\
19 9(ET)
J
/ .16.8 .16~2 •~2.5(ET]
~
175o 14.6
200o
2::,"
.~, 0
A
1
2
3
B
4
5
I
2
C
Fig. 1, Depth profiles of hydrocarbon data, Dumaran no. 1.
advocated by Kubler (1979) and is routinely used in the evaluation of gas source rocks and reservoirs. Figure 1 generally represents simple and, yet, very unique depth profiles for the different parameters presented. Except for an interval between 800 and 950 m, the C1 --C4 hydrocarbon concentrations fluctuate over a narrow range throughout the well (Fig. 1A); between 800 and 950 m the C1--C4 concentration is about seven times larger than the rest of the well. Gas wetnesses are very low throughout the well (Fig. 1B), (4% is the highest value observed). Between 950 and 1200 m wetness values increase from 0.5% to 2.5%, at which point they remain essentially constant throughout the rest of the well. Sediments throughout the well contain very low amounts of organic carbon, averaging 0.17 + 0.03% by weight (Table I). Because of the relatively constant values of organic carbon, the depth profile of the weight ratio of C1--C4 c o m p o u n d s to organic carbon (mg g - ' ) (Fig. 1 C ) is very similar to the pattern described by the C~ --C4 (volume ppm) profile (Fig. 1A). The weight ratio of C1--C4 hydrocarbons to organic carbon for sediments in the studied interval ranges between 1.0 and 20.0 and is relatively high compared to ratios observed in sediments of other wells analyzed by this laboratory and other laboratories (Snowdon and McCrossan, 1973; Kubler, 1979). More c o m m o n values are between 0.0 and 1.0. These high values, combined with low wetness values and other criteria (i.e., the slope of the plot of C1--C4 versus TOC) established by S n o w d o n and McCrossan (1973) suggest that the studied interval is one into which gas has migrated.
184
Visual kerogen analyses aimed at understanding the type and degree of thermal maturity of the kerogen in this well were conducted on 15 sidewall cores. Four samples, including one representing the igneous intrusion, did not contain enough organic matter for any analyses. The rest of the samples contained very small amounts of kerogen which was composed of 85--100% vitrinite and inertinite. Moderate amounts of reworked vitrinite, as well as moderate amounts of coked vitrinite, were observed in almost all samples. Except for one shallow sample, vitrinite reflectance showed a wide range of values corresponding mostly to high maturity levels (Fig. 2). Because of the very large scatter in the reflectivity values, it is difficult to use visual judgment to select vitrinite macerals that represent the indigenous population of the well. Vitrinite anisotropy increases with thermal maturation and may account for some of the spread in the data. However, the range in the values is larger than observed in other wells (Hunt, 1979). The wide range of reflectivity values strongly suggests that most of the vitrinite macerals are recycled and therefore have no gas generation potential. The poor gas generating quality of the kerogen therefore supports the conclusion that the gas in this well has migrated from elsewhere. The chemical composition and the carbon isotope composition of methane and ethane from the two drill-stem tests and the carbon isotope composition of methane from the head space of the three cans of cuttings (given in Fig. 1A and Tables I and II) show little variation with depth, suggesting that, within the interval they represent, the gases have a c o m m o n .
ii
.
.
.
.
I
ii
i
I
I igll tlgaql
n
i D| i 1100
i
i
it
il
J
i ]lll
I
Ill{
i
I i
I
1500 ii
lshl I L4
I L5
i a.5
I 0.7
I I lUlL0 %
J 1.0 RANDOM
i.
I
~lll
I 2.0
t
l 3.0
4.0
5.0
REFLECTANCE
Fig. 2. Vitrinite reflectivity (% Ro) of individual maeerals versus depth in the Dumaran no. 1 well.
185
source and have experienced similar geochemical processes. Also, the isotopic composition of the methane is indicative of a source with a very high degree of thermal maturity (Stahl and Carey, 1975). Lastly, the isotopic composition of the ethane from DST nos. 1 and 2 shows an inversion of the normal relationship between methane and ethane (Fig. 3), i.e., the ethane from DST nos. 1 and 2 is isotopically more negative than the methane. The normal isotopic relationship results from the thermal degradation of kerogen, while the inverse relationship is consistent with ethane being synthesized from methane (Yuen et al., 1984). Oxidations processes (by bacteria at low temperatures and in the presence of fresh water) can result in 13C-enriched methane (Ormland and Des Marais, 1983). However, it has not yet been demonstrated that such a process could also result in an inverted, isotopic relationship between methane and ethane.
-10
GAS INCLUSIONS IN IGNEOUS ROCKS
-20 DST #1
-30
U WYOMING
O0 -40
-50
-S0
/ I 1
1 2
L 3
I 4
N U M B E R OF C A R B O N A T O M S I N G A S MOLECULE
Fig. 3. Isotope composition of gaseous hydrocarbons.
186 In addition, water samples obtained with DST no. 2 show sea-water salinities. The inverted isotopic relationship between the ethane and methane could also be explained in terms of a loss of the gas generated in early stages of sediment maturation and accumulation of only the very late gas generated, probably due to a late formation of a proper cap rock (Smith et al., 1971). Such a rare inversion was also observed by Fuex (1977) in a natural gas from Quebec. Since no organic source was encountered in the Dumaran well, the presence of such source can be proven only by drilling more wells in the area. However, the gas data indicate that an organic source for the gas is n o t the only possible explanation; the other possibility, i.e., an abiogenic origin, is discussed in the following paragraph. The nitrogen content of the DST gases is higher than normally found in natural gases. The source of the nitrogen could be the organic matter in the sediments, but because nitrogen is believed to be generated at early stages of source maturation (Hunt, 1979), its presence would not be expected in association with hydrocarbons that represent late increments of gas generation. Therefore, the presence of nitrogen suggests a different source, most likely the igneous intrusion underlying the sedimentary section. Such a hypothesis is not only consistent with the high concentration of nitrogen, but can also explain the carbon isotopic character of the methane and ethane in the gases. As seen in Fig. 3, the inverted relation between the isotopic composition of methane, ethane, etc., is not only observed in cases of late increments of gas generation but also in gas inclusions of igneous origin and in meteorites. Isotopically, igneous gas inclusions are closer in composition to the gases reported in this well than the late increments of gas generation observed in a gas sample from Quebec (Fuex, 1977), hence, supporting the hypothesis of an abiogenic origin. Also, the chemical composition of the Quebec gas showed no nitrogen (Fuex, 1977), while the gas inclusions showed up to 10% nitrogen (Galimov, 1975). Many geothermal gases are composed of large fractions of CO2 which is notably absent in this well. Yet, as Bray and Foster (1980) suggested, there are numerous chemical reactions that could occur between CO2 and sediments, and could be responsible for the removal of the CO2 from the gas mixture in this well. Finally, Freund et al. (1982) showed that olivine crystals release substantial amounts (25--100p g g-t ) of light hydrocarbon gases (as well as 1--2 mg CO: g-1 rock) when heated to 550°C. Weathered olivine is the source of the serpentinite in the Dumaran no. 1 well. Weathering could have also provided the necessary mechanism for the release of the hydrocarbon gases trapped in the olivine. Conjectures on the origin and occurrence o f abiogenic light hydrocarbon gases The chemical reactions responsible for the generation of the hydrocarbon gases in igneous environments are n o t clear. One possibility is that the
187
Dumaran well gases are of a juvenile origin and represent a residual degassing of the mantle. Deines (1980) suggested that methane is part of the mantle gases. This is an interesting hypothesis because the measured 513C of the methane could have then contributed to the isotopic composition of the methane in the Precambrian atmosphere. The presence of small quantities of ethane, propane, etc., in this case is explained by the formation and combination of methyl radicals. Another possible generation mechanism is the hydrolysis of metal carbides that might form in molten rocks. No support is available for this possibility since metal carbides have not yet been found as natural minerals. Freund et al. (1982) showed that atomic carbon dissolved in MgO can generate CO2 and light h y d r o c a r b o n gases (C1--C4 and others) when heated in the presence of trace water to temperatures of 450--1000°C. The atomic carbon in this system reacts with the lattice oxygen to give CO: and with hydrogen, derived from O H - , to give the hydrocarbons. They also suggest that similar reactions in olivine and basalts are responsible for the presence of hydrocarbon gases in certain minerals. Theoretical calculations by Galimov (1975), and experimental data by Des Marais et al. (1981), showed that ethane and propane are depleted in the heavy carbon isotopes relative to methane in abiogenic hydrocarbon gases. The well which produced this gas was a p o o r commercial producer for an offshore operation and was plugged and abandoned. Yet, the gases can serve as possible examples of the chemical and isotopic character of igneous abiogenic hydrocarbons. The question whether such gases can become a significant energy source is difficult to answer at this stage, although exploration for natural gas of biological origin in sedimentary basins offers much more promise for success. The chemical and isotopic character exhibited by the gas in this well has been, so far, rarely observed in nature, and one conc].usion is that h y d r o c a r b o n gases of an abiogenic igneous origin cannot be considered as playing an important role as an energy source. ACKNOWLEDGMENTS
I thank Craig Schiefelbein for his help in analyzing the samples, Cities Service Oil and Gas Corporation for supporting this investigation, and the Bureau of Energy Development, Republic of the Philippines for granting permission to publish this paper.
REFERENCES Bray, E.E. and Foster, W.R., 1980. A process for primary migration of oil. Am. Assoc. Pet. Geol. Bull., 64: 107--114. Craig, H., 1953. The geochemistry of the stable isotopes of carbon. Geochim. Cosmochim. Acta, 3: 53--92. Des Marais, D.J., Donchin, J.H., Nebring, N.L. and Truesdell, A.H., 1981. Molecular carbon isotopic evidence for the origin of geothermal hydrocarbons. Nature, 292: 826--828.
188
Dienes, P., 1980. The carbon isotopic composition of diamonds: relationshipto diamond shape, color, occurrence and vapor composition. Geochim. Cosmochim. Acta, 44: 943--962. Erdman, J.G. and Morris, D.A., 1974. Geochemical correlation of petroleum. Am. Assoc. Pet. Geol. Bull., 58: 2326--2337. Faure, G., 1977. In: Principles of Isotope Geology. Wileyand Son, New York, 389 pp. Freund, F., Knobe], R., Wengeler, H., Kathrein, H., Oberheuser, G., Schaffer, K. and Schaffer, R.G., 1982. Organic compounds in the early atmosphere formed abiotically from atomic carbon. Geol. Rundsch., 71: 1.-21. Fuex, A.N., 1977. The use of stable carbon isotopes in hydrocarbon exploration. J. Geochem. Explor., 7: 155--188. Galimov, E.M., 1975. In: Carbon Isotopes in Oil-Gas Geology. Translation by U.S. Department of Commerce, National Technical Information Service, N75-27563, pp. 359--361. Gold, T., 1979. Terrestrial sources of carbon and earthquake outgassing. J. Pet. Geol., 1: 3--19. Hunt, J.M., 1979. In: Petroleum Geochemistry and Geology. W.H. Freeman and Company, San Francisco, pp. 164--166 and 467--469. Kubler, B., 1979. In: United Nations ESCAP, CCOP/TP6: Generation and Maturation of Hydrocarbons in Sedimentary Basins, pp. 81--97. Lyon, G.L., 1974. Geothermal gases. In: I.R. Kaplan (Editor), Natural Gases in Marine Sediments, Plenum Press, New York, pp. 141--150. Ormland, R.S. and Des Marais, D.J., 1983. Distribution abundance and carbon isotopic composition of gaseous hydrocarbons in Big Soda Lake, Nevada: an alkaline, meromiclic lake. Geochim. Cosmochim. Aeta, 47: 2107--2114. Smith, J.E., Erdman, J.G., and Morris, D.A., 1971. Migration, accumulation and retention of petroleum in the earth. Proc. 8th World Pet. Congr., 2: 13--26. Snowdon, R.L. and McCrossan, R.G., 1973. Identification of petroleum source rocks using hydrocarbon gas an organic carbon content. Can. Geol. Surv., Pap. 72--36, 12 pp. Stahl, W.J. and Carey, B.D., Jr., 1975. Source-rock identification by isotope analyses of natural gases from fields in the Val Verde and Delaware Basin, West Texas. Chem. Geol., 16: 257--267. Yuen, G., Blair, N., Des Marais, D.J. and Chung, S., 1984. Carbon isotope composition of low molecular weight hydrocarbons and monocarboxylic acids from Murchison meteorite. Nature, 307 : 252--254.
179
Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
AN UNUSUAL OCCURRENCE OF L I G H T H Y D R O C A R B O N GASES IN A WELL O F F S H O R E N O R T H E A S T PALAWAN ISLAND, THE PHILIPPINES
ZVI SOFER Cities Service Oil and Gas Corporation, Box 3908, Tulsa, Oklahoma 74102 (U.S.A.)
(Received October 19, 1983; revision accepted February 22, 1985)
ABSTRACT Sofer, Z., 1985. An unusual occurrence of light hydrocarbon gases in a well offshore Northeast Palawan Island, the Philippines. Precambrian Res., 30: 179--188. Sediments from a well offshore Northeast Palawan Island, the Philippines, appear to contain light hydrocarbons of possible abiogenic origin. This observation is based on stable carbon isotope ratios, organic petrography, and chemical and geological considerations.
INTRODUCTION Large accumulations of h y d r o c a r b o n gases are derived from biological material either by thermal degradation of t hat material when deeply buried, or by the action of methanogenic bacteria on t hat material during early burial. Gold (1979) h y p o t h e s i z e d t ha t h y d r o c a r b o n gases of abiogenic origin are very a b u n d a n t and, therefore, could play an i m p o r t a n t role as a source of energy. The occurrence of abiogenic gas was also the subject of a study by an ad-hoc c o m m i t t e e of the National A cadem y of Sciences (USA) and a summary of its r e p o r t appeared in the November, 1980 issue of Geotimes. The major conclusion o f this c o m m i t t e e was t hat a specific locality for a commercial accumulation of such gas is n o t y e t known. Stable carbon isotope ratios are believed to be useful in distinguishing between biogenic and abiogenic derived h y d r o c a r b o n gases. Organic-derived m e t h a n e yields carbon isotope ratios generally m ore negative than --25%0 (relative to the PDB standard), while the associated ethane, propane, and butane usually show progressively m or e positive isotope compositions (Erdman and Morris, 1974). Methane gases from geothermal areas have isotopic compositions ranging m os t l y f r om --20 to --300/00 (Faure, 1977; Fuex, 1977) although --16°/00 has also been r e p o r t e d (Lyon, 1974). Galimov (1975) r e p o r t e d carbon isotope compositions of abiogenic m e t h a n e t rapped in gas inclusions in igneous rocks to be in the range o f - - 1 2 t o - - 3 % 0 ,
0301--9268/85/$03.30
© 1985 Elsevier Science Publishers B.V.
180
whereas the associated ethane and propane showed an inverted relationship, i.e., they were progressively more negative. Yuen et al. (1984) also observed an inverted relationship in the carbon isotopic composition of abiogenic light hydrocarbon gases in the Murchison meteorite. This paper describes sediments in a well which, based on stable carbon isotope ratios, organic petrography, and chemical and geological considerations, appear to contain uncommercial quantities of hydrocarbon gases of an abiogenic origin. The well (Dumaran no. 1) was drilled in an area of no known gas or oil production to a total depth of 2200 m (offshore Northeast Palawan Island, the Philippines); the b o t t o m 100 m penetrated an igneous intrusion (serpentinite); the 950--2100 m depth interval was uniformly composed of fine-grained, low permeability sandstone deposited in a marine environment. The 950 m depth probably represents the Cretaceous--Tertiary boundary and is marked by an unconformity. The 690--950 m interval contains sediments that, for the most part, are the same as those in the 950--2100 m interval, except that carbonate shell fragments are more abundant. No sediment samples were available for analysis above 690 m. EXPERIMENTAL Composite well cuttings representing drilling intervals of 15--30 m were tightly sealed in metal cans shortly after emerging at the surface. Water and bacteriocide were added to the cuttings leaving a b o u t 20% of the can space empty. Quantitative chemical analysis of the light hydrocarbon gases from methane through the butanes (C,--C4 ) released by the cuttings into the head space was c o n d u c t e d using gas chromatography. Most cans had very little h y d r o c a r b o n gases accumulated in the head space. After the cans were opened, similar analyses were c o n d u c t e d on gases released by milling a portion of the cuttings (3 g) in a sealed container. The a m o u n t of gas per gram of sediment released by milling was a b o u t two orders of magnitude larger than the a m o u n t released into the can head space. (However the absoTABLE I Light hydrocarbon analysis of headspace and milled cutting gas (Hydrocarbon concentrations expressed as ppm, volume of gas to volume of cuttings)* Sample Depth (m) 689 689-701 701-716 716-731 731-747 747-762 762-777 808-823 823-838 838-853
CI 1,023 2,592 14,583 4,352 8,209 2,558 8,195 66,139 74,143 121,418
C2
C~
i-C 4
n-C 4
% Organic Carbon
23.3 26.7 95.9 31.7 32.0 32.3 50.6 267.4 280.1 419.0
2.1 3.7 13.~ 2.3 3.3 6.0 4.7 25.0 33.(I 51.0
.O .D 5.7 .0 .0 .0 .0 3.3 6.7 11.7
.0 .0 .0 .0 .0 .0 .0 2.3 10.7 14.0
.10 .08 .15 ,13 .11 .09 .II .16 .21 .16
Gas** m m C ~ Wetness ~ o,L: ~, I%) .27 .85 2.52 .87 1.92 .75 1.93 10.64 9.09 ]9.52
2.43 1.16 .78 .78 .43 1.48 .67 .45 .44 .41
*** ~ 13CH (% ~)
181
TABLE I (continued) Sample
Depth (m)
C1
867-884 65,258 884-899 7%043 914-929 18,587 945-960 81,595 960-975 12,610 I021-I036 31,345 I036-1051 II,913 1051-1067 14,139 1082-1097 8,148 1097-1112 8,640 1112-1127 14,124 1127-1143 10,961 1143-1158 18,762 1158-1173 13,846 1173-1189 7,181 1189-1204 10,803 1204-1219 11,703 1219-1234 10,979 1234-1250 9,722 1250-1265 12,261 1265-1280 11,692 1280-1295 8,371 1295-1311 10,781 1311-1326 10,910 1326-1341 3,980 1341-1387 15,964 1432-1448 I0,238 1463-1478 18,199 1494-1509 II,195 1539-1544 6,312 1570-1585 I0,282 1585-1600 II,442 1600-1615 12,333 1615-1631 11,804 1631-1646 11,721 1646-1661 8,806 1676-1707 10,712 1707-1737 10,276 1737-1768 7,524 1768-1798 8,975 1798-1827 1,071 1827-1859 8,274 1859-1890 14,133 1890-1920 6,886 1920-1950 9,462 1951-1981 5,351 1981-2012 6,981 2012-2042 26,952 2042-2073 8,251 2073-2103 4,913 2103-2134 12,219 2134-2164 3,755 2164-2195 7,596
C2
198.0 382.0 247.6 399.3 57.9 195.3 172.5 149.8 103.6 107.9 238.0 160.0 269.3 154.9 121.0 213.1 160.8 158.9 136.8 229.7 204.2 144,3 169.0 159.3 74.1 232.7 179.5 264.0 149.5 I17.1 227.8 187.4 234.6 204.3 207.5 205.0 141,4 216.2 204.8 168.1 21.1 152.1 291.9 136.2 178.4 95.9 138.5 530.9 145.6 82.2 204.4 72.3 83.8
C3
i-C 4
9.7 47.3 79,3 76.7 4.0 35.6 54.9 46.9 27.8 32.4 75.8 45.7 81.3 43.9 35.2 62.2 37.9 44.4 37.0 69.2 57.3 44.4 44.4 39.8 22.4 60.0 59.0 7g.fl 44.5 41.6 75.8 53.3 15.3 58.0 58.4 62.2 41.7 59.6 75.2 56.4 7.2 47.4 102.6 46.9 56.6 27.3 41.5 118.3 43.8 23.1 57.5 24.8 20.4
.0 12.0 23.0 22.N .0 ll.O 15.0 10.6 6.3 7.3 20.0 11.3 21.0 10.7 7.0 17.6 13.3 11.7 9.0 17.3 16.0 ll.O 12.3 .7 a.3 16.0 16.3 29.7 ll.O 10.8 54.3 14.5 19.6 15.7 16.0 19.3 I0.9 16.7 17.4 14.6 .0 11.9 29.3 12.0 14.2 7.0 7.8 44.5 11.5 2.3 16.0 3.7 .0
n-C4
.0 10.3 24.3 26.7 .0 9.3 21.5 16.1 7.7 I0.0 30.0 14.5 31.4 14.3 10.4 21.5 11.1 13.7 10.9 24.1 21.0 15.0 .0 11.7 6.6 21.0 21.3 31.7 17.2 14.1 5.0 21.3 27.3 22.0 19.7 23.6 14.8 21.3 22.8 21.7 .I 16.4 37.4 15.6 21.9 8.0 13.0 34,1 14.3 3.0 20.5 7.2 1.7
% Organic
C m_ m ~
Carbon
90RG C
.16 .16 .15 .18 .17 .21 .17 .19 .18 .18 .17 .20 .18 .16 .17 .20 .18 .18 .17 .18 .19 .17 .17 .19 .18 .18 .17 .19 .18 .20 .19 .ll .20 .18 .18 .17 .17 .17 .17 .17 .18 .17 .18 .17 .22 .16 .18 .19 .14 .14 .14 .14 .14
I0.47 II.32 3.31 II.72 1.91 3.87 1.88 1.97 1.20 1.27 2.24 1.46 2.79 2.29 1.14 1.47 1.73 1.63 1.52 1.85 1.66 1.33 1.69
1.53 .60 2.37 1.63 2.56 1.66 .86 1.49 1.81 1.66 1.77 1.76 1.43 1.68 1.65 1.24 1.44 .16 1.32 2.15 1.11 1.17 .90 I.~5 3.83 1.59 .94 2.35 .73 1.42
Gas** Wetness
(%)
.32 .64 1.97 .64 .49 .80 2.17 1.56 1.75 1.79 2.51 2.07 2.10 1.59 2.36 2.83 1.87 2.04 1.95 2.70 2.49 2.50 2.05 1.90 2.63 2.02 2.63 2.13 1.95 2.83 3.41 2.36 2.35 2.48 2.51 3.40 1.91 2.96 4.08 2.83 2.59 2.68 3.16 2.97 2.79 2.52 2.80 2.63 2.54 2.20 2.38 2.80 1.38
*** ~ 13CH4
{°Ioo)
-16.74 -16.85
-14.57
*Gas analyses were performed o n a Hewlett-Packard 5 8 4 0 gas chromatograph e q u i p p e d w i t h a flame i o n i z a t i o n detector and a 6 ft single c o l u m n p a c k e d w i t h n-octane o n Poracil C 1 0 0 / 1 2 0 mesh. Total C 2 --C4 **Gas Wetness
x 100 Total C 1 ---C 4
* * * I s o t o p e analyses were performed as described o n Table II and on h e a d s p a c e gases only.
182
lute amount of gas was of sufficient quantity for isotope analysis only in the head space.) Such behavior reflects the impermeable nature of the cuttings and suggests that the loss of gas in the canning process may have been small or negligible. Total organic carbon (wt. %) was determined for all of the samples, and selected samples were also analyzed using microscopic techniques for kerogen type and thermal maturity. The carbon isotopic composition of the methane in the head space of three cans was determined. In addition, gas samples recovered in two drill-stem tests (DST nos. 1 and 2) which represent the great majority of the gas recovered from the well, were analyzed for both chemical and carbon isotope composition ¢fnr details see Table II). RESULTS AND DISCUSSION
The combined results of the chemical analysis of the head space and the milled cuttings gases are presented in Table I along with the isotopic composition of the methane from the head space of the three canned cuttings. In addition, Table I contains gas wetness values, the wt. % of organic carbon in the sediment, and the ratio of the combined weight of C, - C 4 compounds to the weight of organic carbon in mg g-1. Table II contains the chemical and isotope analyses of the gases from the two drill-stem tests (DST nos. 1 and 2). Depth profiles of some of the parameters presented in Table I are graphically displayed in Fig. 1. This t y p e of data presentation has been TABLE II Light hydrocarbon analysis Name
DST 1 DST 2
Depth (m)
1649--1658 1399--1417
Mole (%) a
~ 13C (0/0° ) b
Gas c Wetness
C,
C3
C3
C,,+ C O 2
N:
CI
C2
(%)
87.2 87.2
0.5 0.5
0.1 0.1
Tr Tr
12.2 12.0
--16.2 --16.2
--22.5 --19.9
0.7 0.7
-0.2
a Gas analyses were done on a Hewlett-Packard 5840 gas chrotnotograph equipped with a thermal conductivity detector, three columns, and three switching valves to achieve a good separation between nitrogen and oxygen. b Carbon dioxide for stable carbon isotopes of individual gas components was prepared by gas chromatographic separation, then combusting each component in an attached 1000°C combustion tube and collecting the products in separate cold traps. Isotopic analyses were performed on a double-collecting Micromaas 602D mass spectrometer. Results are expressed in the usual parts per mil (°/o,) notation, relative to the PDB standard (Craig, 1953). The accuracy of the isotope results depends somewhat on the sample size (most of the inaccuracy is introduced by the separation and combustion steps). Individual components present in the combusted mixture in quantities larger than 0.I cc S T P have a standard deviation (Io) better than -+0.3%o. Individual components smaller than 0.05 cc S T P s h o w a standard deviation of + 1.0°0. c See Table I for definition.
183 C1-C4 100
HYDROCARBONS (VOLUME PPM) ,iooo 10000 100000 1000000
MILLI(~RAMS GRAMS ORGANIC t
GAS WETNESS (VOLUME %) 1 2 3 4
0
(~I-(~4 CARBON 2
3
750
j
lOOO
~1250 W
2"
3EL ~ - - = = J.-_~_
:=DST#1--=-=
>
1500----"
:~::_
__ := . . . . . . . . . . .
=~DST#2
-_: 16.2
~
S
\
19 9(ET)
J
/ .16.8 .16~2 •~2.5(ET]
~
175o 14.6
200o
2::,"
.~, 0
A
1
2
3
B
4
5
I
2
C
Fig. 1, Depth profiles of hydrocarbon data, Dumaran no. 1.
advocated by Kubler (1979) and is routinely used in the evaluation of gas source rocks and reservoirs. Figure 1 generally represents simple and, yet, very unique depth profiles for the different parameters presented. Except for an interval between 800 and 950 m, the C1 --C4 hydrocarbon concentrations fluctuate over a narrow range throughout the well (Fig. 1A); between 800 and 950 m the C1--C4 concentration is about seven times larger than the rest of the well. Gas wetnesses are very low throughout the well (Fig. 1B), (4% is the highest value observed). Between 950 and 1200 m wetness values increase from 0.5% to 2.5%, at which point they remain essentially constant throughout the rest of the well. Sediments throughout the well contain very low amounts of organic carbon, averaging 0.17 + 0.03% by weight (Table I). Because of the relatively constant values of organic carbon, the depth profile of the weight ratio of C1--C4 c o m p o u n d s to organic carbon (mg g - ' ) (Fig. 1 C ) is very similar to the pattern described by the C~ --C4 (volume ppm) profile (Fig. 1A). The weight ratio of C1--C4 hydrocarbons to organic carbon for sediments in the studied interval ranges between 1.0 and 20.0 and is relatively high compared to ratios observed in sediments of other wells analyzed by this laboratory and other laboratories (Snowdon and McCrossan, 1973; Kubler, 1979). More c o m m o n values are between 0.0 and 1.0. These high values, combined with low wetness values and other criteria (i.e., the slope of the plot of C1--C4 versus TOC) established by S n o w d o n and McCrossan (1973) suggest that the studied interval is one into which gas has migrated.
184
Visual kerogen analyses aimed at understanding the type and degree of thermal maturity of the kerogen in this well were conducted on 15 sidewall cores. Four samples, including one representing the igneous intrusion, did not contain enough organic matter for any analyses. The rest of the samples contained very small amounts of kerogen which was composed of 85--100% vitrinite and inertinite. Moderate amounts of reworked vitrinite, as well as moderate amounts of coked vitrinite, were observed in almost all samples. Except for one shallow sample, vitrinite reflectance showed a wide range of values corresponding mostly to high maturity levels (Fig. 2). Because of the very large scatter in the reflectivity values, it is difficult to use visual judgment to select vitrinite macerals that represent the indigenous population of the well. Vitrinite anisotropy increases with thermal maturation and may account for some of the spread in the data. However, the range in the values is larger than observed in other wells (Hunt, 1979). The wide range of reflectivity values strongly suggests that most of the vitrinite macerals are recycled and therefore have no gas generation potential. The poor gas generating quality of the kerogen therefore supports the conclusion that the gas in this well has migrated from elsewhere. The chemical composition and the carbon isotope composition of methane and ethane from the two drill-stem tests and the carbon isotope composition of methane from the head space of the three cans of cuttings (given in Fig. 1A and Tables I and II) show little variation with depth, suggesting that, within the interval they represent, the gases have a c o m m o n .
ii
.
.
.
.
I
ii
i
I
I igll tlgaql
n
i D| i 1100
i
i
it
il
J
i ]lll
I
Ill{
i
I i
I
1500 ii
lshl I L4
I L5
i a.5
I 0.7
I I lUlL0 %
J 1.0 RANDOM
i.
I
~lll
I 2.0
t
l 3.0
4.0
5.0
REFLECTANCE
Fig. 2. Vitrinite reflectivity (% Ro) of individual maeerals versus depth in the Dumaran no. 1 well.
185
source and have experienced similar geochemical processes. Also, the isotopic composition of the methane is indicative of a source with a very high degree of thermal maturity (Stahl and Carey, 1975). Lastly, the isotopic composition of the ethane from DST nos. 1 and 2 shows an inversion of the normal relationship between methane and ethane (Fig. 3), i.e., the ethane from DST nos. 1 and 2 is isotopically more negative than the methane. The normal isotopic relationship results from the thermal degradation of kerogen, while the inverse relationship is consistent with ethane being synthesized from methane (Yuen et al., 1984). Oxidations processes (by bacteria at low temperatures and in the presence of fresh water) can result in 13C-enriched methane (Ormland and Des Marais, 1983). However, it has not yet been demonstrated that such a process could also result in an inverted, isotopic relationship between methane and ethane.
-10
GAS INCLUSIONS IN IGNEOUS ROCKS
-20 DST #1
-30
U WYOMING
O0 -40
-50
-S0
/ I 1
1 2
L 3
I 4
N U M B E R OF C A R B O N A T O M S I N G A S MOLECULE
Fig. 3. Isotope composition of gaseous hydrocarbons.
186 In addition, water samples obtained with DST no. 2 show sea-water salinities. The inverted isotopic relationship between the ethane and methane could also be explained in terms of a loss of the gas generated in early stages of sediment maturation and accumulation of only the very late gas generated, probably due to a late formation of a proper cap rock (Smith et al., 1971). Such a rare inversion was also observed by Fuex (1977) in a natural gas from Quebec. Since no organic source was encountered in the Dumaran well, the presence of such source can be proven only by drilling more wells in the area. However, the gas data indicate that an organic source for the gas is n o t the only possible explanation; the other possibility, i.e., an abiogenic origin, is discussed in the following paragraph. The nitrogen content of the DST gases is higher than normally found in natural gases. The source of the nitrogen could be the organic matter in the sediments, but because nitrogen is believed to be generated at early stages of source maturation (Hunt, 1979), its presence would not be expected in association with hydrocarbons that represent late increments of gas generation. Therefore, the presence of nitrogen suggests a different source, most likely the igneous intrusion underlying the sedimentary section. Such a hypothesis is not only consistent with the high concentration of nitrogen, but can also explain the carbon isotopic character of the methane and ethane in the gases. As seen in Fig. 3, the inverted relation between the isotopic composition of methane, ethane, etc., is not only observed in cases of late increments of gas generation but also in gas inclusions of igneous origin and in meteorites. Isotopically, igneous gas inclusions are closer in composition to the gases reported in this well than the late increments of gas generation observed in a gas sample from Quebec (Fuex, 1977), hence, supporting the hypothesis of an abiogenic origin. Also, the chemical composition of the Quebec gas showed no nitrogen (Fuex, 1977), while the gas inclusions showed up to 10% nitrogen (Galimov, 1975). Many geothermal gases are composed of large fractions of CO2 which is notably absent in this well. Yet, as Bray and Foster (1980) suggested, there are numerous chemical reactions that could occur between CO2 and sediments, and could be responsible for the removal of the CO2 from the gas mixture in this well. Finally, Freund et al. (1982) showed that olivine crystals release substantial amounts (25--100p g g-t ) of light hydrocarbon gases (as well as 1--2 mg CO: g-1 rock) when heated to 550°C. Weathered olivine is the source of the serpentinite in the Dumaran no. 1 well. Weathering could have also provided the necessary mechanism for the release of the hydrocarbon gases trapped in the olivine. Conjectures on the origin and occurrence o f abiogenic light hydrocarbon gases The chemical reactions responsible for the generation of the hydrocarbon gases in igneous environments are n o t clear. One possibility is that the
187
Dumaran well gases are of a juvenile origin and represent a residual degassing of the mantle. Deines (1980) suggested that methane is part of the mantle gases. This is an interesting hypothesis because the measured 513C of the methane could have then contributed to the isotopic composition of the methane in the Precambrian atmosphere. The presence of small quantities of ethane, propane, etc., in this case is explained by the formation and combination of methyl radicals. Another possible generation mechanism is the hydrolysis of metal carbides that might form in molten rocks. No support is available for this possibility since metal carbides have not yet been found as natural minerals. Freund et al. (1982) showed that atomic carbon dissolved in MgO can generate CO2 and light h y d r o c a r b o n gases (C1--C4 and others) when heated in the presence of trace water to temperatures of 450--1000°C. The atomic carbon in this system reacts with the lattice oxygen to give CO: and with hydrogen, derived from O H - , to give the hydrocarbons. They also suggest that similar reactions in olivine and basalts are responsible for the presence of hydrocarbon gases in certain minerals. Theoretical calculations by Galimov (1975), and experimental data by Des Marais et al. (1981), showed that ethane and propane are depleted in the heavy carbon isotopes relative to methane in abiogenic hydrocarbon gases. The well which produced this gas was a p o o r commercial producer for an offshore operation and was plugged and abandoned. Yet, the gases can serve as possible examples of the chemical and isotopic character of igneous abiogenic hydrocarbons. The question whether such gases can become a significant energy source is difficult to answer at this stage, although exploration for natural gas of biological origin in sedimentary basins offers much more promise for success. The chemical and isotopic character exhibited by the gas in this well has been, so far, rarely observed in nature, and one conc].usion is that h y d r o c a r b o n gases of an abiogenic igneous origin cannot be considered as playing an important role as an energy source. ACKNOWLEDGMENTS
I thank Craig Schiefelbein for his help in analyzing the samples, Cities Service Oil and Gas Corporation for supporting this investigation, and the Bureau of Energy Development, Republic of the Philippines for granting permission to publish this paper.
REFERENCES Bray, E.E. and Foster, W.R., 1980. A process for primary migration of oil. Am. Assoc. Pet. Geol. Bull., 64: 107--114. Craig, H., 1953. The geochemistry of the stable isotopes of carbon. Geochim. Cosmochim. Acta, 3: 53--92. Des Marais, D.J., Donchin, J.H., Nebring, N.L. and Truesdell, A.H., 1981. Molecular carbon isotopic evidence for the origin of geothermal hydrocarbons. Nature, 292: 826--828.
188
Dienes, P., 1980. The carbon isotopic composition of diamonds: relationshipto diamond shape, color, occurrence and vapor composition. Geochim. Cosmochim. Acta, 44: 943--962. Erdman, J.G. and Morris, D.A., 1974. Geochemical correlation of petroleum. Am. Assoc. Pet. Geol. Bull., 58: 2326--2337. Faure, G., 1977. In: Principles of Isotope Geology. Wileyand Son, New York, 389 pp. Freund, F., Knobe], R., Wengeler, H., Kathrein, H., Oberheuser, G., Schaffer, K. and Schaffer, R.G., 1982. Organic compounds in the early atmosphere formed abiotically from atomic carbon. Geol. Rundsch., 71: 1.-21. Fuex, A.N., 1977. The use of stable carbon isotopes in hydrocarbon exploration. J. Geochem. Explor., 7: 155--188. Galimov, E.M., 1975. In: Carbon Isotopes in Oil-Gas Geology. Translation by U.S. Department of Commerce, National Technical Information Service, N75-27563, pp. 359--361. Gold, T., 1979. Terrestrial sources of carbon and earthquake outgassing. J. Pet. Geol., 1: 3--19. Hunt, J.M., 1979. In: Petroleum Geochemistry and Geology. W.H. Freeman and Company, San Francisco, pp. 164--166 and 467--469. Kubler, B., 1979. In: United Nations ESCAP, CCOP/TP6: Generation and Maturation of Hydrocarbons in Sedimentary Basins, pp. 81--97. Lyon, G.L., 1974. Geothermal gases. In: I.R. Kaplan (Editor), Natural Gases in Marine Sediments, Plenum Press, New York, pp. 141--150. Ormland, R.S. and Des Marais, D.J., 1983. Distribution abundance and carbon isotopic composition of gaseous hydrocarbons in Big Soda Lake, Nevada: an alkaline, meromiclic lake. Geochim. Cosmochim. Aeta, 47: 2107--2114. Smith, J.E., Erdman, J.G., and Morris, D.A., 1971. Migration, accumulation and retention of petroleum in the earth. Proc. 8th World Pet. Congr., 2: 13--26. Snowdon, R.L. and McCrossan, R.G., 1973. Identification of petroleum source rocks using hydrocarbon gas an organic carbon content. Can. Geol. Surv., Pap. 72--36, 12 pp. Stahl, W.J. and Carey, B.D., Jr., 1975. Source-rock identification by isotope analyses of natural gases from fields in the Val Verde and Delaware Basin, West Texas. Chem. Geol., 16: 257--267. Yuen, G., Blair, N., Des Marais, D.J. and Chung, S., 1984. Carbon isotope composition of low molecular weight hydrocarbons and monocarboxylic acids from Murchison meteorite. Nature, 307 : 252--254.