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The isotopic compositions of molecular nitrogen: implications on their origins in natural gas accumulations
Chemical Geology 164 Ž2000. 321–330 www.elsevier.comrlocaterchemgeo
The isotopic compositions of molecular nitrogen: implications on their origins in natural gas accumulations Yuenian Zhu a
a,)
, Buqing Shi a , Chaobin Fang
b
Institute of Petroleum Resources and EnÕironmental Geology, Petroleum UniÕersity, Dongying, 257062 Shandong, China b Exploration and DeÕelopment Scientific Research Institute, China Offshore Oil Nanhai West Cooperation, Zhanjiang, 524057 Guangdong, China Received 28 July 1998; accepted 24 June 1999
Abstract The isotopic compositions of N2 Ž d15 N, ‰, atm. imply the geochemical origins of molecular nitrogen in natural gas accumulations, but the irregular variation of d15 NN2 for most petroliferous basins puzzles scientists. We believe that this is due to multiple origins of N2 which are often mixed together and leads to the irregularities in gas pools. The four petroliferous basins, the West Siberian basins in Russia, the Yinggehai basin of China, the Californian Great Valley basin of USA and the Mid-European basin, have significantly different isotope characteristics of N2 , which may provide insight into the isotope geochemical implications of N2 in natural gas accumulations. N2 with y19‰( d15 NN2 ( y10‰ may be from sedimentary organic matter at the immature and the early mature stage Ž R o ( 0.6%.. Values of y10‰-15 N N2 ( y2‰ may indicate that N2 originates from mature Žincluding high mature. sedimentary organic matter Ž R o f 0.6–2.0%.. Values of y2‰ - d15 NN2 - q1‰ imply that N2 may be from the deep crust or mantle. N2 with d15 NN2 s 0‰ and N2rAr s 38–84 suggest that the atmosphere is the origin. Values of q1‰( d15 NN2 - q4‰ characterize N2 from ammonium clay minerals in shales and mudrocks during metamorphism. d15 N N2 s q4‰ is the feature of N2 from saltpeter in evaporites. Values of q4‰- d15 N N2 ( q18‰ indicate that N2 maybe derived from post-mature sedimentary organic matter Ž R o ) 2.0%.. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Nitrogen; Isotope; Natural gas; Ammonia; Sedimentary organic matter; Clay mineral; Yinggehai basin
1. Introduction Molecular nitrogen ŽN2 . is one of the most common non-hydrocarbon components in gas accumulations ŽZhu, 1994.. N2-rich gas pools have been discovered in many petroliferous basins all over the world, such as Mid-European basin ŽKrooss et al.,
)
Corresponding author. E-mail: [email protected]
1995; Littke et al., 1995., Volga-Ural basin ŽPrasolov et al., 1990., Zhungaer basin ŽWang and Jiang, 1997., Yinggehai basin, Sanshui basin ŽZeng, 1986., Subei basin ŽXu, 1994., Songliao basin ŽHuang, 1995., Great Valley basin ŽJenden et al., 1988a. and Western Interior basin ŽHoering and Moore, 1958; Headlee, 1962; Jenden et al., 1988b.. However, the isotope geochemistry of N2 in petroliferous basins is still poorly known ŽKrouse, 1979; Prasolov et al., 1990; Whiticar, 1994; Xu, 1994; Krooss et al., 1995; Littke et al., 1995; Du et
0009-2541r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 5 4 1 Ž 9 9 . 0 0 1 5 1 - 5
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al., 1996.. We believe that this is due to multiple origins which are often mixed together and lead to irregular variations of d15 N of N2 in gas pools. In order to unravel the molecular nitrogen isotopic compositions in petroliferous basins, the isotope geochemistry of single sources of N2 has to be studied. In order to elucidate the geochemical implications of N2 isotopic compositions in natural accumulations, we chose four basins Ži.e., the West Siberian basin in Russia, the Yinggehai basin of China, the California Great Valley basin of USA and the MidEuropean basin. among the numerous N2-bearing basins in the world. The four basins have significantly different isotopic characteristics of gases, which may provide insight into the isotope geochemistry of N2 in petroliferous basins.
2. Four typical N2-bearing basins 2.1. West Siberian basin The West Siberian basin is a great gas-bearing province formed in Mesozoic time. More than 20 giant and super-giant gas fields have been discovered in the northern part of the basin. Both the two sets of the source rocks and reservoirs are from the Cretaceous ŽDa, 1985; Galimov, 1988; Zhang, 1990.. The Cenomanian reservoirs contain a huge gas accumulations of either bacterial or early thermogenic origin ŽGalimov, 1988; Galimov et al., 1990; Cramer et al., 1998.. The source rock is a coalbearing series of Lower Cenomanian, Abbian and Aphian age with organic matter at the immature and the early stage of its maturation Žvitrinite reflectance coefficient R o - 0.6%.. Sandstones in the Upper Cenomanian are the principal gas reservoirs, which produce dry gas ŽC 1rC 1 – 2 ) 0.99, CH 4 s 99.5– 99.7%, C 2 H 6 s 0.02 – 0.08%, C 3 H 8 s 0.03 – 0.025%.. N2 and CO 2 also exist commonly in these gas reservoirs, which the volume content of N2 Ž0.1–1.2%. being higher than that of CO 2 Ž0.1– 0.3%.. The carbon isotope values of methane are light Ž d13 C s y68–y 46‰, PDB. ŽDa, 1985; Galimov, 1988. and the nitrogen isotope values of N2 are also light Ž d15 N s y19.0–y 10.7‰. with a variability from y16 to y12‰. The associated
3
Her4 He ratios are at 10y8 order of magnitude ŽPrasolov et al., 1990.. 2.2. Yinggehai basin Yinggehai basin is a gas-rich basin formed in Cenozoic times. Three giant gas fields have been discovered in its southern part. The source rock is a neritic and bathyal dark mudstones in the Miocene Meishan formation with mature Žmainly high mature. humic organic matter Ž R o f 0.8–2.0%.. According to the stratigraphy of the Qiongdongnan basin adjacent to Yinggehai basin, other potential source rocks are marine dark mudstones of the Oligocene limnic mudstones and coal-bearing series. The detected natural gas in Yinggehai basin is primarily produced from marine sandstones of the Pliocene Yinggehai formation. Hydrocarbon composition in natural gas varies in a wide range, the contents of CH 4 , C 2 H 6 and C 3 H 8 varying from 6.47 to 90.99%, 0.013 to 2.09%, and 0.002 to 0.55%, respectively, and C 1rC 1 – 5 ratios varying from 0.96 to 1.00. CO 2 Ž0.10–88.93%. and N2 Ž2.93–33.48%. are common in the natural gas, which appear to be inversely correlated to each other. Methane d13 C values vary from y63.14 to y29.08‰, but usually from y40 to y30‰ ŽZhu, 1997., and nitrogen d15 N values vary mainly from y9 to y2‰. 2.3. Californian Great Valley basin The Californian Great Valley basin is rich in oil and gas, which was formed in the Late Mesozoic time. It is separated into two main basins, the Sacramento to the north and the San Joaquin to the south by Stockton arch. The northern San Joaquin basin and the Sacramento basin are dry gas regions, where many gas fields have been discovered ŽJenden et al., 1988a.. Fraciscan metasedimentary complex is the basement of this basin and Cretaceous and Tertiary siltstones, shales and sandstones were deposited above. Gas reservoirs are primarily from the Cretaceous, Miocene and Pliocene ŽJenden et al., 1988a.. The concentrations of methane in natural gas vary from 12 to 96%, C 2q; 0.02 to 5.1%, CO 2 ; 0.001 to 0.6%, and N2 ; 1.0 to 87%. Methane d13 C values vary from y52.4 to y15.2‰, and nitrogen d15 N
Table 1 Chemical and stable isotopic composition of Yinggehai basin gas samples Sample Well number
F1-1-4 Yinggehai F1-1-4 Yinggehai F1-1-4 Yinggehai F1-1-5 Yinggehai F1-1-5 Yinggehai F1-1-7 Yinggehai F1-1-7 Yinggehai F1-1-8 Yinggehai F1-1-9 Yinggehai F1-1-9 Yinggehai F1-1-9 Yinggehai F1-1-Z1 Yinggehai F1-1-Z1 Yinggehai F9-1-1 Yinggehai F9-1-1 Yinggehai F9-1-2 Yinggehai F9-1-2 Yinggehai F9-1-2 Yinggehai F9-1-2 Yinggehai F9-1-2 Yinggehai D0-1-2 Yinggehai D0-1-2 Yinggehai D2-1-1 Ledong D2-1-1 Ledong D2-1-1 Ledong D2-1-2 Ledong D2-1-2 Ledong D2-1-2 Ledong D2-1-2 Ledong D2-1-2 Ledong D2-1-3 Ledong D2-1-3 Ledong D2-1-3 Ledong D2-1-3 Ledong D2-1-3 Yinggehai
Pliocene Pliocene Pliocene Pliocene Pliocene Pliocene Pliocene Pliocene Pliocene Pliocene Pliocene Pliocene Pliocene Pliocene Pliocene Pliocene Pliocene Pliocene Pliocene Pliocene Pliocene Pliocene Quaternary Quaternary Quaternary Quaternary Quaternary Quaternary Quaternary Quaternary Quaternary Quaternary Quaternary Quaternary Pliocene
1277–1293 1230–1340 1360–1375 1322–1326 1386–1410 1358–1386 1403–1425 1342–1358 1369–1405 1318–1325 1425–1433 1975–1990 2245–2265 1467–1473 1494–1502 1470–1505 1760–1800 1872–1893 2095–2135 2216–2230 1056–1065 1217–1220 851–858 1044–1052 1352–1385 392–400 520–530 582–590 901–914 963–985 395–410 579–590 913–938 966–975 1468–1496
70.40 70.69 71.97 71.27 67.32 35.90 40.40 79.64 82.44 73.54 80.28 84.55 84.73 16.94 16.76 65.76 6.47 7.65 6.78 6.77 42.09 46.70 83.48 73.46 63.82 81.10 80.10 80.10 79.00 79.20 87.09 81.93 75.81 79.55 58.97
C 1 r CO 2 C 1 – 5 Ž%.
N2 Ž%. Ar Ž%. N2 rAr He Ž%. d 13 C CH 4 d 13 C CO 2 d 15 N N 2 Ž‰, PDB. Ž‰, PDB. Ž‰, atm.
0.98 0.98 0.98 0.98 0.98 0.96 0.96 0.99 0.98 0.98 0.99 0.98 0.98 0.97 0.98 0.98 1.00 0.99 0.99 0.97 0.97 0.97 0.99 0.98 0.97 1.00 0.99 0.99 0.98 0.98 1.00 0.99 0.99 0.98 0.98
27.70 27.16 26.40 26.15 31.21 5.20 6.00 18.63 15.04 24.04 18.59 11.07 11.76 10.94 11.20 23.69 7.45 5.15 7.02 3.94 4.09 6.93 15.23 23.71 33.48 18.20 18.70 18.50 18.70 18.50 11.48 16.67 16.67 17.76 17.49
0.12 0.22 0.25 0.17 0.21 57.00 51.60 0.35 0.37 0.21 0.35 1.13 1.19 71.39 71.50 9.04 85.90 86.95 84.01 88.93 52.16 44.27 0.17 0.11 0.31 0.10 0.10 0.00 0.10 0.10 0.73 0.07 0.07 0.27 21.46
0.0121 0.1255 0.0100 0.0101 0.0088 0.0460 0.0673 0.1100 0.0460
0.0061 0.0086 0.3700 0.1000 0.0360 0.0520 0.0390 0.0166 0.00086 0.1000 0.1600 0.1700
2308.3 216.4 2640.0 2682.7 3546.6 113.0 82.2 169.4 326.9
1793.4 1302.3 64.0 74.5 143.0 135.0 101.0 246.3 805.8 152.3 148.1 196.9
0.0017 0.0014 0.0015 0.0017 0.0045
0.0012 0.0005
0.0005 0.0005
y38.00 y35.50 y36.50 y35.60 y34.60 y31.80 y31.70 y54.09 y50.32 y41.04 y40.45 y34.14 y34.99 y33.30 y34.00 y35.56 y29.66 y29.68 y30.22 y29.09 y32.80 y31.00 y54.11 y36.04 y32.93 y40.62 y36.36 y37.64 y34.01 y34.14 y63.14 y40.15 y40.15 y35.82 y29.08
y19.90 y20.70 y16.90 y12.52 y3.40 y2.80 y18.35 y14.59 y18.60 y19.24 y7.84 y2.70 y3.80 y5.86 y2.70 y2.89 y3.32 y2.83 y3.90 y3.40 y10.94 y10.44 y8.99
y5.74
y3.2 y5.1 y9.0 y3.4 y8.0 y3.0 y4.1 y1.8 y7.2 y5.0 y3.4 y3.4 y4.1 y8.0 y3.3 y3.3 y4.0 y5.2 y4.4 y3.0 y8.0 y3.1 y6.4 y7.0 y8.0 y5.2 y3.4 y3.3 y3.2 y4.0 y6.0 y5.5 y5.5 y5.4 y4.0
40
Arr 36Ar
305 296 354 333 299 309 317 327 309 297 299 301 304 300 312 406 326 306 311 307 296 295 325 342 346 299 299 298 297 302 295 299 299 299 300
3
Her 4 He Ž=10y7 . 1.66 1.77 3.45 6.79 2.50 0.54 0.50 1.14 1.05 1.53 1.91 2.22 0.53 1.77 4.65 1.85 2.27 1.80 1.89 2.37 1.19 5.06 0.97 0.89 0.89 0.60 0.56 0.60 0.56 0.89 0.62 0.62 0.72 0.63 0.61
Y. Zhu et al.r Chemical Geology 164 (2000) 321–330
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35
Producing Formation Depth range CH 4 Žm. Ž%. formation age
323
324
Y. Zhu et al.r Chemical Geology 164 (2000) 321–330
values vary from q0.9 to q3.5‰ with a main range from q1.0 to 3.0‰ ŽJenden et al., 1988a.. 2.4. Mid-European basin The Mid-European basin is a gas-rich basin formed in Late Paleozoic to Cenozoic times. Lots of N2-rich gas fields have been discovered in its northern part. The source rocks are coal-bearing series in the Upper Carboniferous with post-mature organic matter Ž R o ) 2.0%.. Gas reservoirs are primarily in Permian Rotliegend and Triassic Buntsandstein formations. Main components of natural gas are CH 4 and N2 . The content of N2 is more often greater than 50% in the gas accumulations in the northern part of the basin. The gas occurrence with N2 higher than 50% is consistent with the high maturity of source rocks with vitrinite reflectance values higher than 3.0% ŽKrooss et al., 1995; Littke et al., 1995.. The content of CO 2 is less than 1%. Methane d13 C values are greater than y36‰, and can be as high as y15% ŽPDB.. Molecular nitrogen d15 N values vary from q6.5 to q18.0‰ ŽMaksimov et al., 1975; Boigk et al., 1976; Stahl, 1977; Weinlich, 1991..
3. Experimental and results Thirty-five gas samples were collected at high pressure in steel bottles with stopcocks at both ends from the gas-producing wells in Yinggehai basin, offshore and southwest of Hainan Island, China. In the laboratory, chemical compositions of major, minor and trace components ŽCH 4 , C 2q, CO 2 , N2 , Ar and He. were analyzed using a gas chromatograph system. Peak heights of chemical components in the samples were calibrated against those of the in-house standard gases. Overall, analytical errors were estimated to be - 5% for most samples. Purification of He and Ar was carried out by a two-stage Ti–Zr getter in a special high vacuum stainless steel system. An activated charcoal trap was held at liquid N2 temperature in order to separate Ar from He. A VG-5400 static mass spectrometer was used for the measurements of 3 Her4 He and 40Arr36 Ar ratios. The precision of the measurement was less than 1% for most samples.
Separation of the gas samples into pure gas components ŽCH 4 , CO 2 and N2 . and oxidation of CH 4 to CO 2 were performed in a glass vacuum line using a trap held at liquid N2 temperature, another trap at acetone–liquid N2 sherbet temperature, and a CuO furnace. The d13 C and d15 N were determined by a MAT-251 mass spectrometer ŽFinnigan.. The errors of measurements for d13 C and d15 N for most samples were 0.02 and 0.2%, respectively. Table 1 shows the analytical results of the chemical and isotope composition of Yinggehai basin gas samples, China.
4. Discussion Hoering and Moore Ž1958. discovered the d15 N values of N2 in Oklahoma, Mississippi and Texas to vary from y10.5 to y2.1‰, but to vary from q1.3 to q11.9‰ in Arkansas. They gave no interpretation for the geochemical implications of the two groups of molecular nitrogen d15 N values. Bokhoven and Theeuween Ž1966. measured the molecular nitrogen d15 N values in Dutch gas reservoirs, but they could not affirm the origin of molecular nitrogen. Maksimov et al. Ž1975. studied the molecular nitrogen d15 N values in N2-high gas reservoirs of the MidEuropean basin, which vary from q4.28 to q18‰, and supposed the origin of molecular nitrogen should be derived from deep crystalline rocks. But this inference was denied by Krooss et al. Ž1995. and Littke et al. Ž1995.. Jenden et al. Ž1988a. thought the molecular nitrogen with d15 N values varying from q0.9 to q3.5‰ in gas reservoirs of the Californian Great Valley originated from the Upper Jurassic Fraciscan metasedimentary complex, according to studies on the isotope ratios of carbon, nitrogen and helium. Prasolov et al. Ž1990. discovered that the molecular nitrogen d15 N values in the former USSR could be divided into two groups. One of them has d15 N values of about y10‰, the other about q14‰. The ‘‘light’’ nitrogen originates from organic matter and presumably from ammonia salts via the gaseous ammonia stage. Fig. 1 shows the features of isotopic compositions of the molecular nitrogen in the four N2-bearing basins. It appears that the molecular nitrogen d15 N
Y. Zhu et al.r Chemical Geology 164 (2000) 321–330
325
Fig. 1. Histograms of d15 NN 2 of natural gases from the West Siberian basin ŽPrasolov et al., 1990., Yinggehai basin, Californian Great Valley ŽJenden et al., 1988a. and Mid-European basin ŽBoigk et al., 1976; Weinlich, 1991..
values differentiate among those basins and have multiple central distribution patterns different from the two group patterns suggested by Prasolov et al. Ž1990.. Figs. 2 and 3 show the plots of volume content of molecular nitrogen vs. molecular nitrogen d15 N values, and the N2rAr ratios vs. the molecular nitrogen d15 N values, respectively, which also reflect multiple central distribution patterns. In order to understand the implications of the multiple central distribution patterns of molecular nitrogen d15 N values in the petroliferous basins, it is necessary to introduce the possible origins of molecular nitrogen in subsurface. The possible nitrogen sources include: Ž1. the atmosphere, Ž2. sedimentary organic matter at the immature and the early stage of its maturation Ž R o - 0.6%., Ž3. mature Žincluding high mature. sedimentary organic matter Ž R o f 0.6– 2.0%., Ž4. post-mature sedimentary organic matters Ž R o ) 2.0%., Ž5. ammonium clay minerals in sedimentary rocks, Ž6. radiogenic nuclear reaction, Ž7. nitrogen-bearing salt in evaporites; Ž8. magmatic rocks, and Ž9. the deep crust or mantle degassing.
The molecular nitrogen from the atmosphere is characterized by the d15 N valuess 0‰ and N2rAr ratio s 38–84. This feature is considered to be unchanged in the past 3 billion years ŽSano and Pillinger, 1990.. The atmospheric N2 is not the main source for N2 in natural gas accumulations, which the evidence is that the isotopic compositions of N2 in N2-rich gas reservoirs over the world have no signs of atmospheric characteristics ŽHoering and Moore, 1958; Bokhoven and Theeuween, 1966; Maksimov et al., 1975; Prasolov et al., 1990; Krooss et al., 1995; Littke et al., 1995.. The radiogenic molecular nitrogen can be neglected because of its little contribution to gas pools, and magmatic rocks derived N2 also has little significance for it ŽKrooss et al., 1995. because nitrogen content in magmatite is the lowest in all crust rocks ŽLittke et al., 1995.. Primordial N2 from the deep crust or mantle can migrate along the active deep-setting faults and accumulate ŽKrooss et al., 1995.. The d15 N values of primordial N2 should be similar to that of N2 in the atmosphere, because atmospheric N2 is mainly from
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Y. Zhu et al.r Chemical Geology 164 (2000) 321–330
Fig. 2. Plot of N2 vs. d15 NN 2 of natural gases from the West Siberian basin ŽPrasolov et al., 1990., Yinggehai basin, Californian Great Valley ŽJenden et al., 1988a. and Mid-European basin ŽBoigk et al., 1976; Weinlich, 1991..
Fig. 3. Semilogarithmic plot of N2rAr ratios vs. 15 NN 2 of natural gases from the West Siberian ŽPrasolov et al., 1990., Yinggeha basin, Californian Great Valley ŽJenden et al., 1988a. and Mid-European basin ŽBoigk et al., 1976; Stahl, 1977; Weinlich, 1991..
Y. Zhu et al.r Chemical Geology 164 (2000) 321–330
327
the upper mantle degassing during the Earth early evolution ŽZhang and Zindler, 1989.. Primordial d15 N values of N2 are supposed to vary from y2 to q1‰ by Prasolov et al. Ž1990.. The d15 N values of molecular nitrogen derived from reduction of nitrogen-bearing salt minerals in evaporites are typically equal to q4‰ ŽPrasolov et al., 1987.. Krooss et al. Ž1995. and Littke et al. Ž1995. inferred that sedimentary organic matter and ammonium clay minerals are the main sources of N2 in gas accumulations.
from the sedimentary organic matter at the immature and the early stage of its maturation Ž R o - 0.6%., with d15 N values varying from y19 to y10‰. N2 with the similar d15 N values has also been discovered in the Quaternary of the Qaidamu basin, China. Therefore, the d15 N values of N2 from sedimentary organic matter at the immature and the early stage of its maturation should vary from y19 to y10‰.
4.1. N2 from immature sedimentary organic matter
Although part of sedimentary organic nitrogen has been depleted during the immature and the early stage Ž R o - 0.6%. of its maturation, quite some of organic nitrogen still exists in sedimentary organic matter at the mature stage Ž R o f 0.6–2.0%., such as protein, amino acid, pyride and pyrrole, etc. ŽBaxby et al., 1994., which is the reason why the nitrogen content in sedimentary rocks is the highest in the crustal rocks. During the mature stage, especially in the high mature stage Ž R o f 1.0–2.0%., temperatures are high and approach those necessary to reach the activation energy Ž40 to 55 kcalrmol. to break nitrogen bonds in mature kerogen ŽKrooss et al., 1995.. Except for a small part of NH 3 generated during this stage, which is absorbed by authigenic clay minerals, such as illite and montmorillonite in sedimentary rocks ŽCooper and Evans, 1983; Oh et al., 1988., large volumes of molecular nitrogen are also generated through reactions Ž2. and Ž3. mentioned in Section 4.1. These transformations are the possible reasons of N2-rich gas pools associated with ‘‘red beds’’ and the low content of CO 2 in nature. Via the NH 3 stage, the generated molecular nitrogen is also depleted in heavy nitrogen Ž15 N., but is heavier than that from the sedimentary organic matter at the early stage of its maturation. In general, the molecules with the light isotope would decompose more readily, enriching 14 N in the gas phase relative to the remaining nitrogen-bearing organic matter ŽHoering and Moore, 1958.. The natural gases in the Pliocene Yinggehai formation of Yinggehai basin have the basic feature of an origin from the mature Žincluding high mature. sedimentary organic matter Ž R o f 0.8–2.0%.. 3 Her4 He ratios of the associated helium vary from 5.0 = 10y8 to 6.79 f 10y7 , which suggests that there is
Sedimentary organic matter at the immature and the early stage of its maturation Ž R o - 0.6%. contains proteins and amino acids in high concentration. Proteins can easily form amino acids by hydrolysis, such as COOH–CH 2 –NH 2 , and amino acids are unstable and ammoniated by microorganism. microorganism
CO 2 q H 2 q NH 3
6
COOH–CH 2 –NH 2
q Energy
Ž 1.
The NH 3 in this reaction is unstable. Part of it is absorbed by clay minerals, such as illite. Another part can transform into molecular nitrogen through the following reactions:
™ CH q 4N q 6H O, 2NH q 2Fe O ™ 6FeO q N q 3H O 8NH 3 q CO 2 3
2
4
3
2
2
2
2
Ž 2. Ž 3.
which are taken from the works of Kreular and Schuiling Ž1982. and Getz Ž1987., respectively. Via the NH 3 stage, the N2 from sedimentary organic matter at the early stage of its maturation is depleted in heavy nitrogen ŽMacko et al., 1987; Prasolov et al., 1990.. The molecular nitrogen of natural gas accumulations in the Upper Cretaceous of the West Siberian basin is one from sedimentary organic matter at the immature and the early stage of its maturation. 3 Her4 He ratios of the associated helium Ž10y8 . ŽPrasolov et al., 1990. suggest that no significant N2 from the atmosphere or the deep crust or mantle is present. The d15 N values do not indicate a contribution from nitrogen-bearing salt minerals in evaporites ŽPrasolov et al., 1990.. Thus the molecular nitrogen in the Upper Cretaceous of the West Siberian basin should be derived
4.2. N2 from mature sedimentary organic matter
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little contribution of mantle-derived helium. No trace of magmatism or nitrogen-bearing salt minerals has been discovered up to now in the basin. The molecular nitrogen in the Pliocene Yinggehai formation originated primarily from the mature Žincluding high mature. sedimentary organic matter Ž R o f 0.8–2.0%. in the Miocene Meishan formation, with an isotopic feature of d15 N values varying from y9 to y2‰. The N2 with this isotopic feature has been discovered in the Lower Cretaceous of the West Siberian basin ŽPrasolov et al., 1990. and Western Interior basin ŽHoering and Moore, 1958; Jenden et al., 1988b.. Thus, the range of d15 N values of N2 from mature organic matter should vary from y10 to y2‰. 4.3. N2 from post-mature sedimentary organic matter The geothermal energy at the mature stage of sedimentary organic matter is not high enough to release all organic nitrogen. Therefore, organic nitrogen still exists in post-mature sedimentary organic matter Ž R o ) 2.0%. in high concentration. During the post-mature stage, the geothermal energy is high enough to generate molecular nitrogen through pyrolysis ŽKrooss et al., 1995.. Thus, a large amount of molecular nitrogen can be generated during this stage. Krooss et al. Ž1995. and Littke et al. Ž1995. investigated the N2 generation from post-mature sedimentary organic matter at high temperature by laboratory pyrolysis experiments. They discovered that the peak of N2-rich gas generation was after that of methane-rich gas generation, and using the chemical dynamics method, they concluded that the essential geotemperature to generate N2-high gas ŽN2 ) 50%. from organic matter should be at least 3008C in nature. Natural gases in a reservoir in Permian Rotliegend and Triassic Buntsandstein formations of the MidEuropean basin, especially in its northern part, have typical features of the generation from post-mature sedimentary organic matter. Marine evaporates were deposited in this basin, but there are no saltpeter or other nitrogen-bearing salt minerals in significant amount ŽLittke et al., 1995.. Therefore, the hypothesis of evaporite-derived N2 ŽPrasolov et al., 1990. has been denied. Although the magmatism is obvi-
ous, its contribution to gas accumulation can be neglected. On the contrary, the magma heat should have had accelerated the N2 generation from sedimentary organic matter at high temperature ŽKrooss et al., 1995.. Most of the molecular nitrogen, in Permian Rotliegend and Triassic Buntsandstein formations, should be generated from the Upper Carboniferous post-mature source rocks. The typical feature of molecular nitrogen d15 N values from this type of generation should range from q4 to q18‰. The occurrence of N2 with this origin in a large scale may indicate that it is mainly the post-mature gas reservoired in petroliferous basins ŽKrooss et al., 1995; Littke et al., 1995.. Heavy N2 Ž d15 N f q5– q 18‰. discovered in the rift basin systems of eastern China, American Arkansas and Volga-Ural basin may be of the same origin. 4.4. N2 from ammonium clay minerals . in sedimenThe inorganic fixed nitrogen ŽNHq 4 tary rocks is primarily derived from organic matter. Some part of NH 3 from immature and mature sedimentary organic matter can be absorbed by clay minerals generating during the diagenesis of sedimentary rocks. In other words, ammonium clay min. erals can be formed by fixed nitrogen ŽNHq 4 replacing Kq in clay minerals ŽCooper and Evans, 1983; Oh et al., 1988; Baxby et al., 1994; Krooss et al., 1995; Littke et al., 1995.. The fixed nitrogen in clay minerals shows a high thermal stability and cannot release molecular nitrogen unless at a high temperature. This temperature should be higher than 10008C during laboratory pyrolysis experiment and higher than 5008C under geological conditions according to chemical dynamic calculation ŽWhelan et al., 1988.. Thus, molecular nitrogen can be generated from metamorphism of ammonium clay mineral-bearing sedimentary rocks, with the d15 N value varying from q1.0 to q4.0‰. The natural gases in the Cretaceous and Tertiary of the California Great Valley have complex origins involving mixing of multiple sources, with a narrow range of molecular nitrogen d15 N values from q0.9 to q3.5‰. Jenden et al. Ž1988a. suggested this kind of molecular nitrogen should originate from Upper Jurassic Fraciscan metasedimentary complex. There-
Y. Zhu et al.r Chemical Geology 164 (2000) 321–330
fore, the isotopic composition of N2 from ammonium clay mineral may vary from q1 to q4‰. 5. Conclusions There are multiple origins of N2 in the subsurface and they often mix together in N2-bearing gas reservoirs in petroliferous basins. Therefore, widely ranging d15 N values of N2 have been observed and puzzles scientists. Based on analyzing and contrasting four typical N2-bearing basins, West Siberian basin, Yinggehai basin, Mid-European basin and Great Valley, and the study of the molecular nitrogen’s possible origins and the corresponding isotopic composition features, we can conclude on the isotope geochemistry of molecular nitrogen. Values between y19‰( d15 NN 2 ( y10‰ represents the N2 from sedimentary organic matter at the immature and the early stage of its maturation Ž R o - 0.6%.. N2 with y10‰- d15 NN 2 ( y2‰ originates from mature Žincluding high mature. sedimentary organic matter ŽŽ R f 0.6–2.0%, especially in 1.3–2.0%.. Values of y2‰ - d15 NN 2 - q1‰ may reflect the N2 from the mantle. d15 NN 2 s 0‰ and N2rAr s 38–84 characterizes N2 from atmosphere. Values of q1‰ s d15 NN 2 - q4‰ indicate N2 from metamorphism of ammonium clay mineral-bearing sedimentary rocks. d15 NN 2 s q4‰ is the isotopic composition feature of N2 from saltpeter in evaporites. Values between q4‰ - d15 NN 2 ( q18‰ imply that N2 may originate from post-mature sedimentary organic matter Ž R o ) 2.0%.. The isotopic compositions of N2 from multiple origins appear irregular. Gas pools containing N2-high ŽN2 ) 50%. may be dominantly from post-mature sedimentary organic matter. The occurrence of N2-high gas pools may indicate that this gas is mainly derived from post-mature source rocks in petroliferous basins. N2-rich gas ŽN2 ) 15%. may originate from mature, especially high mature sedimentary organic matter. Natural gases containing N2 could also be formed by metamorphism of ammonium clay mineral-bearing sedimentary rocks. Acknowledgements This work was supported by China National Scientific Fund, the China Petroleum Innovation
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Fund and the Scientific Fund of University of Petroleum, China. [PD]
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The isotopic compositions of molecular nitrogen: implications on their origins in natural gas accumulations Yuenian Zhu a
a,)
, Buqing Shi a , Chaobin Fang
b
Institute of Petroleum Resources and EnÕironmental Geology, Petroleum UniÕersity, Dongying, 257062 Shandong, China b Exploration and DeÕelopment Scientific Research Institute, China Offshore Oil Nanhai West Cooperation, Zhanjiang, 524057 Guangdong, China Received 28 July 1998; accepted 24 June 1999
Abstract The isotopic compositions of N2 Ž d15 N, ‰, atm. imply the geochemical origins of molecular nitrogen in natural gas accumulations, but the irregular variation of d15 NN2 for most petroliferous basins puzzles scientists. We believe that this is due to multiple origins of N2 which are often mixed together and leads to the irregularities in gas pools. The four petroliferous basins, the West Siberian basins in Russia, the Yinggehai basin of China, the Californian Great Valley basin of USA and the Mid-European basin, have significantly different isotope characteristics of N2 , which may provide insight into the isotope geochemical implications of N2 in natural gas accumulations. N2 with y19‰( d15 NN2 ( y10‰ may be from sedimentary organic matter at the immature and the early mature stage Ž R o ( 0.6%.. Values of y10‰-15 N N2 ( y2‰ may indicate that N2 originates from mature Žincluding high mature. sedimentary organic matter Ž R o f 0.6–2.0%.. Values of y2‰ - d15 NN2 - q1‰ imply that N2 may be from the deep crust or mantle. N2 with d15 NN2 s 0‰ and N2rAr s 38–84 suggest that the atmosphere is the origin. Values of q1‰( d15 NN2 - q4‰ characterize N2 from ammonium clay minerals in shales and mudrocks during metamorphism. d15 N N2 s q4‰ is the feature of N2 from saltpeter in evaporites. Values of q4‰- d15 N N2 ( q18‰ indicate that N2 maybe derived from post-mature sedimentary organic matter Ž R o ) 2.0%.. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Nitrogen; Isotope; Natural gas; Ammonia; Sedimentary organic matter; Clay mineral; Yinggehai basin
1. Introduction Molecular nitrogen ŽN2 . is one of the most common non-hydrocarbon components in gas accumulations ŽZhu, 1994.. N2-rich gas pools have been discovered in many petroliferous basins all over the world, such as Mid-European basin ŽKrooss et al.,
)
Corresponding author. E-mail: [email protected]
1995; Littke et al., 1995., Volga-Ural basin ŽPrasolov et al., 1990., Zhungaer basin ŽWang and Jiang, 1997., Yinggehai basin, Sanshui basin ŽZeng, 1986., Subei basin ŽXu, 1994., Songliao basin ŽHuang, 1995., Great Valley basin ŽJenden et al., 1988a. and Western Interior basin ŽHoering and Moore, 1958; Headlee, 1962; Jenden et al., 1988b.. However, the isotope geochemistry of N2 in petroliferous basins is still poorly known ŽKrouse, 1979; Prasolov et al., 1990; Whiticar, 1994; Xu, 1994; Krooss et al., 1995; Littke et al., 1995; Du et
0009-2541r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 5 4 1 Ž 9 9 . 0 0 1 5 1 - 5
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al., 1996.. We believe that this is due to multiple origins which are often mixed together and lead to irregular variations of d15 N of N2 in gas pools. In order to unravel the molecular nitrogen isotopic compositions in petroliferous basins, the isotope geochemistry of single sources of N2 has to be studied. In order to elucidate the geochemical implications of N2 isotopic compositions in natural accumulations, we chose four basins Ži.e., the West Siberian basin in Russia, the Yinggehai basin of China, the California Great Valley basin of USA and the MidEuropean basin. among the numerous N2-bearing basins in the world. The four basins have significantly different isotopic characteristics of gases, which may provide insight into the isotope geochemistry of N2 in petroliferous basins.
2. Four typical N2-bearing basins 2.1. West Siberian basin The West Siberian basin is a great gas-bearing province formed in Mesozoic time. More than 20 giant and super-giant gas fields have been discovered in the northern part of the basin. Both the two sets of the source rocks and reservoirs are from the Cretaceous ŽDa, 1985; Galimov, 1988; Zhang, 1990.. The Cenomanian reservoirs contain a huge gas accumulations of either bacterial or early thermogenic origin ŽGalimov, 1988; Galimov et al., 1990; Cramer et al., 1998.. The source rock is a coalbearing series of Lower Cenomanian, Abbian and Aphian age with organic matter at the immature and the early stage of its maturation Žvitrinite reflectance coefficient R o - 0.6%.. Sandstones in the Upper Cenomanian are the principal gas reservoirs, which produce dry gas ŽC 1rC 1 – 2 ) 0.99, CH 4 s 99.5– 99.7%, C 2 H 6 s 0.02 – 0.08%, C 3 H 8 s 0.03 – 0.025%.. N2 and CO 2 also exist commonly in these gas reservoirs, which the volume content of N2 Ž0.1–1.2%. being higher than that of CO 2 Ž0.1– 0.3%.. The carbon isotope values of methane are light Ž d13 C s y68–y 46‰, PDB. ŽDa, 1985; Galimov, 1988. and the nitrogen isotope values of N2 are also light Ž d15 N s y19.0–y 10.7‰. with a variability from y16 to y12‰. The associated
3
Her4 He ratios are at 10y8 order of magnitude ŽPrasolov et al., 1990.. 2.2. Yinggehai basin Yinggehai basin is a gas-rich basin formed in Cenozoic times. Three giant gas fields have been discovered in its southern part. The source rock is a neritic and bathyal dark mudstones in the Miocene Meishan formation with mature Žmainly high mature. humic organic matter Ž R o f 0.8–2.0%.. According to the stratigraphy of the Qiongdongnan basin adjacent to Yinggehai basin, other potential source rocks are marine dark mudstones of the Oligocene limnic mudstones and coal-bearing series. The detected natural gas in Yinggehai basin is primarily produced from marine sandstones of the Pliocene Yinggehai formation. Hydrocarbon composition in natural gas varies in a wide range, the contents of CH 4 , C 2 H 6 and C 3 H 8 varying from 6.47 to 90.99%, 0.013 to 2.09%, and 0.002 to 0.55%, respectively, and C 1rC 1 – 5 ratios varying from 0.96 to 1.00. CO 2 Ž0.10–88.93%. and N2 Ž2.93–33.48%. are common in the natural gas, which appear to be inversely correlated to each other. Methane d13 C values vary from y63.14 to y29.08‰, but usually from y40 to y30‰ ŽZhu, 1997., and nitrogen d15 N values vary mainly from y9 to y2‰. 2.3. Californian Great Valley basin The Californian Great Valley basin is rich in oil and gas, which was formed in the Late Mesozoic time. It is separated into two main basins, the Sacramento to the north and the San Joaquin to the south by Stockton arch. The northern San Joaquin basin and the Sacramento basin are dry gas regions, where many gas fields have been discovered ŽJenden et al., 1988a.. Fraciscan metasedimentary complex is the basement of this basin and Cretaceous and Tertiary siltstones, shales and sandstones were deposited above. Gas reservoirs are primarily from the Cretaceous, Miocene and Pliocene ŽJenden et al., 1988a.. The concentrations of methane in natural gas vary from 12 to 96%, C 2q; 0.02 to 5.1%, CO 2 ; 0.001 to 0.6%, and N2 ; 1.0 to 87%. Methane d13 C values vary from y52.4 to y15.2‰, and nitrogen d15 N
Table 1 Chemical and stable isotopic composition of Yinggehai basin gas samples Sample Well number
F1-1-4 Yinggehai F1-1-4 Yinggehai F1-1-4 Yinggehai F1-1-5 Yinggehai F1-1-5 Yinggehai F1-1-7 Yinggehai F1-1-7 Yinggehai F1-1-8 Yinggehai F1-1-9 Yinggehai F1-1-9 Yinggehai F1-1-9 Yinggehai F1-1-Z1 Yinggehai F1-1-Z1 Yinggehai F9-1-1 Yinggehai F9-1-1 Yinggehai F9-1-2 Yinggehai F9-1-2 Yinggehai F9-1-2 Yinggehai F9-1-2 Yinggehai F9-1-2 Yinggehai D0-1-2 Yinggehai D0-1-2 Yinggehai D2-1-1 Ledong D2-1-1 Ledong D2-1-1 Ledong D2-1-2 Ledong D2-1-2 Ledong D2-1-2 Ledong D2-1-2 Ledong D2-1-2 Ledong D2-1-3 Ledong D2-1-3 Ledong D2-1-3 Ledong D2-1-3 Ledong D2-1-3 Yinggehai
Pliocene Pliocene Pliocene Pliocene Pliocene Pliocene Pliocene Pliocene Pliocene Pliocene Pliocene Pliocene Pliocene Pliocene Pliocene Pliocene Pliocene Pliocene Pliocene Pliocene Pliocene Pliocene Quaternary Quaternary Quaternary Quaternary Quaternary Quaternary Quaternary Quaternary Quaternary Quaternary Quaternary Quaternary Pliocene
1277–1293 1230–1340 1360–1375 1322–1326 1386–1410 1358–1386 1403–1425 1342–1358 1369–1405 1318–1325 1425–1433 1975–1990 2245–2265 1467–1473 1494–1502 1470–1505 1760–1800 1872–1893 2095–2135 2216–2230 1056–1065 1217–1220 851–858 1044–1052 1352–1385 392–400 520–530 582–590 901–914 963–985 395–410 579–590 913–938 966–975 1468–1496
70.40 70.69 71.97 71.27 67.32 35.90 40.40 79.64 82.44 73.54 80.28 84.55 84.73 16.94 16.76 65.76 6.47 7.65 6.78 6.77 42.09 46.70 83.48 73.46 63.82 81.10 80.10 80.10 79.00 79.20 87.09 81.93 75.81 79.55 58.97
C 1 r CO 2 C 1 – 5 Ž%.
N2 Ž%. Ar Ž%. N2 rAr He Ž%. d 13 C CH 4 d 13 C CO 2 d 15 N N 2 Ž‰, PDB. Ž‰, PDB. Ž‰, atm.
0.98 0.98 0.98 0.98 0.98 0.96 0.96 0.99 0.98 0.98 0.99 0.98 0.98 0.97 0.98 0.98 1.00 0.99 0.99 0.97 0.97 0.97 0.99 0.98 0.97 1.00 0.99 0.99 0.98 0.98 1.00 0.99 0.99 0.98 0.98
27.70 27.16 26.40 26.15 31.21 5.20 6.00 18.63 15.04 24.04 18.59 11.07 11.76 10.94 11.20 23.69 7.45 5.15 7.02 3.94 4.09 6.93 15.23 23.71 33.48 18.20 18.70 18.50 18.70 18.50 11.48 16.67 16.67 17.76 17.49
0.12 0.22 0.25 0.17 0.21 57.00 51.60 0.35 0.37 0.21 0.35 1.13 1.19 71.39 71.50 9.04 85.90 86.95 84.01 88.93 52.16 44.27 0.17 0.11 0.31 0.10 0.10 0.00 0.10 0.10 0.73 0.07 0.07 0.27 21.46
0.0121 0.1255 0.0100 0.0101 0.0088 0.0460 0.0673 0.1100 0.0460
0.0061 0.0086 0.3700 0.1000 0.0360 0.0520 0.0390 0.0166 0.00086 0.1000 0.1600 0.1700
2308.3 216.4 2640.0 2682.7 3546.6 113.0 82.2 169.4 326.9
1793.4 1302.3 64.0 74.5 143.0 135.0 101.0 246.3 805.8 152.3 148.1 196.9
0.0017 0.0014 0.0015 0.0017 0.0045
0.0012 0.0005
0.0005 0.0005
y38.00 y35.50 y36.50 y35.60 y34.60 y31.80 y31.70 y54.09 y50.32 y41.04 y40.45 y34.14 y34.99 y33.30 y34.00 y35.56 y29.66 y29.68 y30.22 y29.09 y32.80 y31.00 y54.11 y36.04 y32.93 y40.62 y36.36 y37.64 y34.01 y34.14 y63.14 y40.15 y40.15 y35.82 y29.08
y19.90 y20.70 y16.90 y12.52 y3.40 y2.80 y18.35 y14.59 y18.60 y19.24 y7.84 y2.70 y3.80 y5.86 y2.70 y2.89 y3.32 y2.83 y3.90 y3.40 y10.94 y10.44 y8.99
y5.74
y3.2 y5.1 y9.0 y3.4 y8.0 y3.0 y4.1 y1.8 y7.2 y5.0 y3.4 y3.4 y4.1 y8.0 y3.3 y3.3 y4.0 y5.2 y4.4 y3.0 y8.0 y3.1 y6.4 y7.0 y8.0 y5.2 y3.4 y3.3 y3.2 y4.0 y6.0 y5.5 y5.5 y5.4 y4.0
40
Arr 36Ar
305 296 354 333 299 309 317 327 309 297 299 301 304 300 312 406 326 306 311 307 296 295 325 342 346 299 299 298 297 302 295 299 299 299 300
3
Her 4 He Ž=10y7 . 1.66 1.77 3.45 6.79 2.50 0.54 0.50 1.14 1.05 1.53 1.91 2.22 0.53 1.77 4.65 1.85 2.27 1.80 1.89 2.37 1.19 5.06 0.97 0.89 0.89 0.60 0.56 0.60 0.56 0.89 0.62 0.62 0.72 0.63 0.61
Y. Zhu et al.r Chemical Geology 164 (2000) 321–330
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35
Producing Formation Depth range CH 4 Žm. Ž%. formation age
323
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values vary from q0.9 to q3.5‰ with a main range from q1.0 to 3.0‰ ŽJenden et al., 1988a.. 2.4. Mid-European basin The Mid-European basin is a gas-rich basin formed in Late Paleozoic to Cenozoic times. Lots of N2-rich gas fields have been discovered in its northern part. The source rocks are coal-bearing series in the Upper Carboniferous with post-mature organic matter Ž R o ) 2.0%.. Gas reservoirs are primarily in Permian Rotliegend and Triassic Buntsandstein formations. Main components of natural gas are CH 4 and N2 . The content of N2 is more often greater than 50% in the gas accumulations in the northern part of the basin. The gas occurrence with N2 higher than 50% is consistent with the high maturity of source rocks with vitrinite reflectance values higher than 3.0% ŽKrooss et al., 1995; Littke et al., 1995.. The content of CO 2 is less than 1%. Methane d13 C values are greater than y36‰, and can be as high as y15% ŽPDB.. Molecular nitrogen d15 N values vary from q6.5 to q18.0‰ ŽMaksimov et al., 1975; Boigk et al., 1976; Stahl, 1977; Weinlich, 1991..
3. Experimental and results Thirty-five gas samples were collected at high pressure in steel bottles with stopcocks at both ends from the gas-producing wells in Yinggehai basin, offshore and southwest of Hainan Island, China. In the laboratory, chemical compositions of major, minor and trace components ŽCH 4 , C 2q, CO 2 , N2 , Ar and He. were analyzed using a gas chromatograph system. Peak heights of chemical components in the samples were calibrated against those of the in-house standard gases. Overall, analytical errors were estimated to be - 5% for most samples. Purification of He and Ar was carried out by a two-stage Ti–Zr getter in a special high vacuum stainless steel system. An activated charcoal trap was held at liquid N2 temperature in order to separate Ar from He. A VG-5400 static mass spectrometer was used for the measurements of 3 Her4 He and 40Arr36 Ar ratios. The precision of the measurement was less than 1% for most samples.
Separation of the gas samples into pure gas components ŽCH 4 , CO 2 and N2 . and oxidation of CH 4 to CO 2 were performed in a glass vacuum line using a trap held at liquid N2 temperature, another trap at acetone–liquid N2 sherbet temperature, and a CuO furnace. The d13 C and d15 N were determined by a MAT-251 mass spectrometer ŽFinnigan.. The errors of measurements for d13 C and d15 N for most samples were 0.02 and 0.2%, respectively. Table 1 shows the analytical results of the chemical and isotope composition of Yinggehai basin gas samples, China.
4. Discussion Hoering and Moore Ž1958. discovered the d15 N values of N2 in Oklahoma, Mississippi and Texas to vary from y10.5 to y2.1‰, but to vary from q1.3 to q11.9‰ in Arkansas. They gave no interpretation for the geochemical implications of the two groups of molecular nitrogen d15 N values. Bokhoven and Theeuween Ž1966. measured the molecular nitrogen d15 N values in Dutch gas reservoirs, but they could not affirm the origin of molecular nitrogen. Maksimov et al. Ž1975. studied the molecular nitrogen d15 N values in N2-high gas reservoirs of the MidEuropean basin, which vary from q4.28 to q18‰, and supposed the origin of molecular nitrogen should be derived from deep crystalline rocks. But this inference was denied by Krooss et al. Ž1995. and Littke et al. Ž1995.. Jenden et al. Ž1988a. thought the molecular nitrogen with d15 N values varying from q0.9 to q3.5‰ in gas reservoirs of the Californian Great Valley originated from the Upper Jurassic Fraciscan metasedimentary complex, according to studies on the isotope ratios of carbon, nitrogen and helium. Prasolov et al. Ž1990. discovered that the molecular nitrogen d15 N values in the former USSR could be divided into two groups. One of them has d15 N values of about y10‰, the other about q14‰. The ‘‘light’’ nitrogen originates from organic matter and presumably from ammonia salts via the gaseous ammonia stage. Fig. 1 shows the features of isotopic compositions of the molecular nitrogen in the four N2-bearing basins. It appears that the molecular nitrogen d15 N
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Fig. 1. Histograms of d15 NN 2 of natural gases from the West Siberian basin ŽPrasolov et al., 1990., Yinggehai basin, Californian Great Valley ŽJenden et al., 1988a. and Mid-European basin ŽBoigk et al., 1976; Weinlich, 1991..
values differentiate among those basins and have multiple central distribution patterns different from the two group patterns suggested by Prasolov et al. Ž1990.. Figs. 2 and 3 show the plots of volume content of molecular nitrogen vs. molecular nitrogen d15 N values, and the N2rAr ratios vs. the molecular nitrogen d15 N values, respectively, which also reflect multiple central distribution patterns. In order to understand the implications of the multiple central distribution patterns of molecular nitrogen d15 N values in the petroliferous basins, it is necessary to introduce the possible origins of molecular nitrogen in subsurface. The possible nitrogen sources include: Ž1. the atmosphere, Ž2. sedimentary organic matter at the immature and the early stage of its maturation Ž R o - 0.6%., Ž3. mature Žincluding high mature. sedimentary organic matter Ž R o f 0.6– 2.0%., Ž4. post-mature sedimentary organic matters Ž R o ) 2.0%., Ž5. ammonium clay minerals in sedimentary rocks, Ž6. radiogenic nuclear reaction, Ž7. nitrogen-bearing salt in evaporites; Ž8. magmatic rocks, and Ž9. the deep crust or mantle degassing.
The molecular nitrogen from the atmosphere is characterized by the d15 N valuess 0‰ and N2rAr ratio s 38–84. This feature is considered to be unchanged in the past 3 billion years ŽSano and Pillinger, 1990.. The atmospheric N2 is not the main source for N2 in natural gas accumulations, which the evidence is that the isotopic compositions of N2 in N2-rich gas reservoirs over the world have no signs of atmospheric characteristics ŽHoering and Moore, 1958; Bokhoven and Theeuween, 1966; Maksimov et al., 1975; Prasolov et al., 1990; Krooss et al., 1995; Littke et al., 1995.. The radiogenic molecular nitrogen can be neglected because of its little contribution to gas pools, and magmatic rocks derived N2 also has little significance for it ŽKrooss et al., 1995. because nitrogen content in magmatite is the lowest in all crust rocks ŽLittke et al., 1995.. Primordial N2 from the deep crust or mantle can migrate along the active deep-setting faults and accumulate ŽKrooss et al., 1995.. The d15 N values of primordial N2 should be similar to that of N2 in the atmosphere, because atmospheric N2 is mainly from
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Fig. 2. Plot of N2 vs. d15 NN 2 of natural gases from the West Siberian basin ŽPrasolov et al., 1990., Yinggehai basin, Californian Great Valley ŽJenden et al., 1988a. and Mid-European basin ŽBoigk et al., 1976; Weinlich, 1991..
Fig. 3. Semilogarithmic plot of N2rAr ratios vs. 15 NN 2 of natural gases from the West Siberian ŽPrasolov et al., 1990., Yinggeha basin, Californian Great Valley ŽJenden et al., 1988a. and Mid-European basin ŽBoigk et al., 1976; Stahl, 1977; Weinlich, 1991..
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the upper mantle degassing during the Earth early evolution ŽZhang and Zindler, 1989.. Primordial d15 N values of N2 are supposed to vary from y2 to q1‰ by Prasolov et al. Ž1990.. The d15 N values of molecular nitrogen derived from reduction of nitrogen-bearing salt minerals in evaporites are typically equal to q4‰ ŽPrasolov et al., 1987.. Krooss et al. Ž1995. and Littke et al. Ž1995. inferred that sedimentary organic matter and ammonium clay minerals are the main sources of N2 in gas accumulations.
from the sedimentary organic matter at the immature and the early stage of its maturation Ž R o - 0.6%., with d15 N values varying from y19 to y10‰. N2 with the similar d15 N values has also been discovered in the Quaternary of the Qaidamu basin, China. Therefore, the d15 N values of N2 from sedimentary organic matter at the immature and the early stage of its maturation should vary from y19 to y10‰.
4.1. N2 from immature sedimentary organic matter
Although part of sedimentary organic nitrogen has been depleted during the immature and the early stage Ž R o - 0.6%. of its maturation, quite some of organic nitrogen still exists in sedimentary organic matter at the mature stage Ž R o f 0.6–2.0%., such as protein, amino acid, pyride and pyrrole, etc. ŽBaxby et al., 1994., which is the reason why the nitrogen content in sedimentary rocks is the highest in the crustal rocks. During the mature stage, especially in the high mature stage Ž R o f 1.0–2.0%., temperatures are high and approach those necessary to reach the activation energy Ž40 to 55 kcalrmol. to break nitrogen bonds in mature kerogen ŽKrooss et al., 1995.. Except for a small part of NH 3 generated during this stage, which is absorbed by authigenic clay minerals, such as illite and montmorillonite in sedimentary rocks ŽCooper and Evans, 1983; Oh et al., 1988., large volumes of molecular nitrogen are also generated through reactions Ž2. and Ž3. mentioned in Section 4.1. These transformations are the possible reasons of N2-rich gas pools associated with ‘‘red beds’’ and the low content of CO 2 in nature. Via the NH 3 stage, the generated molecular nitrogen is also depleted in heavy nitrogen Ž15 N., but is heavier than that from the sedimentary organic matter at the early stage of its maturation. In general, the molecules with the light isotope would decompose more readily, enriching 14 N in the gas phase relative to the remaining nitrogen-bearing organic matter ŽHoering and Moore, 1958.. The natural gases in the Pliocene Yinggehai formation of Yinggehai basin have the basic feature of an origin from the mature Žincluding high mature. sedimentary organic matter Ž R o f 0.8–2.0%.. 3 Her4 He ratios of the associated helium vary from 5.0 = 10y8 to 6.79 f 10y7 , which suggests that there is
Sedimentary organic matter at the immature and the early stage of its maturation Ž R o - 0.6%. contains proteins and amino acids in high concentration. Proteins can easily form amino acids by hydrolysis, such as COOH–CH 2 –NH 2 , and amino acids are unstable and ammoniated by microorganism. microorganism
CO 2 q H 2 q NH 3
6
COOH–CH 2 –NH 2
q Energy
Ž 1.
The NH 3 in this reaction is unstable. Part of it is absorbed by clay minerals, such as illite. Another part can transform into molecular nitrogen through the following reactions:
™ CH q 4N q 6H O, 2NH q 2Fe O ™ 6FeO q N q 3H O 8NH 3 q CO 2 3
2
4
3
2
2
2
2
Ž 2. Ž 3.
which are taken from the works of Kreular and Schuiling Ž1982. and Getz Ž1987., respectively. Via the NH 3 stage, the N2 from sedimentary organic matter at the early stage of its maturation is depleted in heavy nitrogen ŽMacko et al., 1987; Prasolov et al., 1990.. The molecular nitrogen of natural gas accumulations in the Upper Cretaceous of the West Siberian basin is one from sedimentary organic matter at the immature and the early stage of its maturation. 3 Her4 He ratios of the associated helium Ž10y8 . ŽPrasolov et al., 1990. suggest that no significant N2 from the atmosphere or the deep crust or mantle is present. The d15 N values do not indicate a contribution from nitrogen-bearing salt minerals in evaporites ŽPrasolov et al., 1990.. Thus the molecular nitrogen in the Upper Cretaceous of the West Siberian basin should be derived
4.2. N2 from mature sedimentary organic matter
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little contribution of mantle-derived helium. No trace of magmatism or nitrogen-bearing salt minerals has been discovered up to now in the basin. The molecular nitrogen in the Pliocene Yinggehai formation originated primarily from the mature Žincluding high mature. sedimentary organic matter Ž R o f 0.8–2.0%. in the Miocene Meishan formation, with an isotopic feature of d15 N values varying from y9 to y2‰. The N2 with this isotopic feature has been discovered in the Lower Cretaceous of the West Siberian basin ŽPrasolov et al., 1990. and Western Interior basin ŽHoering and Moore, 1958; Jenden et al., 1988b.. Thus, the range of d15 N values of N2 from mature organic matter should vary from y10 to y2‰. 4.3. N2 from post-mature sedimentary organic matter The geothermal energy at the mature stage of sedimentary organic matter is not high enough to release all organic nitrogen. Therefore, organic nitrogen still exists in post-mature sedimentary organic matter Ž R o ) 2.0%. in high concentration. During the post-mature stage, the geothermal energy is high enough to generate molecular nitrogen through pyrolysis ŽKrooss et al., 1995.. Thus, a large amount of molecular nitrogen can be generated during this stage. Krooss et al. Ž1995. and Littke et al. Ž1995. investigated the N2 generation from post-mature sedimentary organic matter at high temperature by laboratory pyrolysis experiments. They discovered that the peak of N2-rich gas generation was after that of methane-rich gas generation, and using the chemical dynamics method, they concluded that the essential geotemperature to generate N2-high gas ŽN2 ) 50%. from organic matter should be at least 3008C in nature. Natural gases in a reservoir in Permian Rotliegend and Triassic Buntsandstein formations of the MidEuropean basin, especially in its northern part, have typical features of the generation from post-mature sedimentary organic matter. Marine evaporates were deposited in this basin, but there are no saltpeter or other nitrogen-bearing salt minerals in significant amount ŽLittke et al., 1995.. Therefore, the hypothesis of evaporite-derived N2 ŽPrasolov et al., 1990. has been denied. Although the magmatism is obvi-
ous, its contribution to gas accumulation can be neglected. On the contrary, the magma heat should have had accelerated the N2 generation from sedimentary organic matter at high temperature ŽKrooss et al., 1995.. Most of the molecular nitrogen, in Permian Rotliegend and Triassic Buntsandstein formations, should be generated from the Upper Carboniferous post-mature source rocks. The typical feature of molecular nitrogen d15 N values from this type of generation should range from q4 to q18‰. The occurrence of N2 with this origin in a large scale may indicate that it is mainly the post-mature gas reservoired in petroliferous basins ŽKrooss et al., 1995; Littke et al., 1995.. Heavy N2 Ž d15 N f q5– q 18‰. discovered in the rift basin systems of eastern China, American Arkansas and Volga-Ural basin may be of the same origin. 4.4. N2 from ammonium clay minerals . in sedimenThe inorganic fixed nitrogen ŽNHq 4 tary rocks is primarily derived from organic matter. Some part of NH 3 from immature and mature sedimentary organic matter can be absorbed by clay minerals generating during the diagenesis of sedimentary rocks. In other words, ammonium clay min. erals can be formed by fixed nitrogen ŽNHq 4 replacing Kq in clay minerals ŽCooper and Evans, 1983; Oh et al., 1988; Baxby et al., 1994; Krooss et al., 1995; Littke et al., 1995.. The fixed nitrogen in clay minerals shows a high thermal stability and cannot release molecular nitrogen unless at a high temperature. This temperature should be higher than 10008C during laboratory pyrolysis experiment and higher than 5008C under geological conditions according to chemical dynamic calculation ŽWhelan et al., 1988.. Thus, molecular nitrogen can be generated from metamorphism of ammonium clay mineral-bearing sedimentary rocks, with the d15 N value varying from q1.0 to q4.0‰. The natural gases in the Cretaceous and Tertiary of the California Great Valley have complex origins involving mixing of multiple sources, with a narrow range of molecular nitrogen d15 N values from q0.9 to q3.5‰. Jenden et al. Ž1988a. suggested this kind of molecular nitrogen should originate from Upper Jurassic Fraciscan metasedimentary complex. There-
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fore, the isotopic composition of N2 from ammonium clay mineral may vary from q1 to q4‰. 5. Conclusions There are multiple origins of N2 in the subsurface and they often mix together in N2-bearing gas reservoirs in petroliferous basins. Therefore, widely ranging d15 N values of N2 have been observed and puzzles scientists. Based on analyzing and contrasting four typical N2-bearing basins, West Siberian basin, Yinggehai basin, Mid-European basin and Great Valley, and the study of the molecular nitrogen’s possible origins and the corresponding isotopic composition features, we can conclude on the isotope geochemistry of molecular nitrogen. Values between y19‰( d15 NN 2 ( y10‰ represents the N2 from sedimentary organic matter at the immature and the early stage of its maturation Ž R o - 0.6%.. N2 with y10‰- d15 NN 2 ( y2‰ originates from mature Žincluding high mature. sedimentary organic matter ŽŽ R f 0.6–2.0%, especially in 1.3–2.0%.. Values of y2‰ - d15 NN 2 - q1‰ may reflect the N2 from the mantle. d15 NN 2 s 0‰ and N2rAr s 38–84 characterizes N2 from atmosphere. Values of q1‰ s d15 NN 2 - q4‰ indicate N2 from metamorphism of ammonium clay mineral-bearing sedimentary rocks. d15 NN 2 s q4‰ is the isotopic composition feature of N2 from saltpeter in evaporites. Values between q4‰ - d15 NN 2 ( q18‰ imply that N2 may originate from post-mature sedimentary organic matter Ž R o ) 2.0%.. The isotopic compositions of N2 from multiple origins appear irregular. Gas pools containing N2-high ŽN2 ) 50%. may be dominantly from post-mature sedimentary organic matter. The occurrence of N2-high gas pools may indicate that this gas is mainly derived from post-mature source rocks in petroliferous basins. N2-rich gas ŽN2 ) 15%. may originate from mature, especially high mature sedimentary organic matter. Natural gases containing N2 could also be formed by metamorphism of ammonium clay mineral-bearing sedimentary rocks. Acknowledgements This work was supported by China National Scientific Fund, the China Petroleum Innovation
329
Fund and the Scientific Fund of University of Petroleum, China. [PD]
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