23. isotopic composition of methane carbon · pdf fileand concentrations, thereby allowing...

23. ISOTOPIC COMPOSITION OF METHANE CARBON AND THE RELATIVE CONTENT OFGASEOUS HYDROCARBONS IN THE DEPOSITS OF THE MOROCCAN BASIN OF THE

ATLANTIC OCEAN (DEEP SEA DRILLING PROJECT SITE 415 and 416)

E. M. Galimov, V. A. Chinyonov, and Ye. N. Ivanov, V. I. Vernadsky Institute of Geochemistry and AnalyticalChemistry, U.S.S.R. Academy of Sciences, Moscow, U.S.S.R.

INTRODUCTION

At Site 415, drilling was planned to penetrate 3000meters of oceanic sediments, thus affording a uniqueopportunity to study the geochemical evolution of or-ganic matter in basin conditions. Drilling such a deepborehole made especially acute the problem of prevent-ing a possible blowout of hydrocarbons. The programof geochemical investigations aboard Glomar Challeng-er therefore was devised to produce the necessary infor-mation to ensure drilling safety and prevent ocean pol-lution. One of the principal methods serving this pur-pose was the chromatographic analysis of gases.

Drilling to the contemplated depth proved impos-sible: Hole 415A penetrated 1079 meters sub-bottom,and Hole 416A achieved a total depth of 1624 meters.The gas content and composition in the sediment didnot reach parameters which could be regarded as criti-cal. However, the experience of applying chromato-graphic analysis of gases aboard ship proved it to be avery useful method of estimating hydrocarbon typesand concentrations, thereby allowing uninterrupteddrilling with minimal risk of blowout.

As a result of gas analysis aboard ship, data were ob-tained on the content of gas in the profile and its hydro-carbon composition. Gas samples in sealed vacuum con-tainers were delivered to the Carbon Geochemistry Lab-oratory (GEOCHI, Moscow) where the isotopic compo-sition of carbon in the gases was measured.

EXPERIMENTAL PROCEDURE

The gas samples were taken aboard ship by piercingthe plastic jacket of the core sampler with the needle ofa vacuum-tight syringe. The gas was transferred fromthe syringe into a pre-vacuumized test tube closed withan airtight rubber stopper. From this test tube, the gaswas analyzed chromatographically. For shore-basedanalysis, the gas was stored and transported in the sametube.

Aboard ship, gas analysis was conducted on twochromatographs using the same analytical procedureemployed on Leg 47. The CH4 and CO2 analysis wasperformed utilizing the Carl Model 8000 gas chromato-graph with a catarometer as the detector. The column (5ft long, with 78 in. o.d., packed with 8% Carbowax 1540on Anacrome ABC, 90 to 100 mesh) was kept at a cons-tant temperature of 40°C. The analysis of hydrocar-bons, from C2 to C5, was conducted on a HewlettPackard Model 5710A chromatograph with a flame-

ionization detector and a system of dual columns towork in the compensational mode of operation. Thisdual system involved one column (4 ft long, with % in.o.d., packed with Spherosyl) joined with another col-umn (12 ft long, with % in. o.d., packed with 20% OV101 on Anacrome 11/110 AC). Helium served as thecarrier gas; the flow rate was 20 cmVmin. The furnacetemperature was programmed as follows: initially at60°C for 4 min, then increased to 200°C at the rate of16°/min, then kept constant at 200°C for 16 min. Thesample previously had been drained of air and methaneby being passed through a coil cooled to -70°C.

The isotopic analysis of gas was performed on aVarian MAT-230 mass spectrometer. Prior to analysis,the gas from the vacuum container was passed throughan ascarite tube to be purified of carbon dioxide andthrough a cold trap for methane to be separated fromhigher hydrocarbons. Methane then was oxidized toCO2 in the circulation system of a reactor filled withCuO at 800° C. The CO2 obtained was freed of air andwater impurities in cold traps, and of N2O in a furnacewith elementary copper at 500° C. Analysis of the iso-topic composition of methane carbon was conductedwith an accuracy of ±0.1 per mill. The results are givenin δ13C values as related to the PDB standard.

RESULTS

Tables 1 and 2 show the results of the chromato-graphic analysis of gas samples from Holes 415A and416A. Methane concentration in the samples variesfrom immeasurably low content to 50 to 60 per cent.The content of carbon dioxide in all the samples is low,barely exceeding the background values in the labora-tory air. The hydrocarbon fraction of the gas is repre-sented almost entirely by methane. The content of totalheavy hydrocarbons in all the samples is less than 1 percent.

The hydrocarbon composition of gas in samples thatwere analyzed immediately after recovery is differentfrom the composition in the same samples after beingstored for several hours at room temperature. In the lat-ter case, as shown in Table 3, the samples are almosttwice as rich in heavy hydrocarbons. This is evidentlycaused by the different time needed for different hydro-carbons to evolve from the sediment. The hydrocarboncomposition of the gas analyzed after a certain period oftime seems to be truer to in situ conditions. According-ly, only the analyses of these gases are included in thetables presented in this report.

615

E. M. GA. JMOV, V. A. CHINYONOV, YE. N. IVANOV

TABLE 1Concentration of CH4 and CC>2 and C~-C5 Hydrocarbons in the Samples of Site 415

Core

415-3415-5

415A-5415A-6415A-7415A-8415A-9415A-10415A-11415A-12415A-13415A-14

Stratigraphy

MioceneMiocenePaleocenePaleoceneM.-U. Cret.M.-U. Cret.CenomanianCenomanianCenomanianCenomanianCenomanianAlbian

Sub-BottomDepth

(m)

137.5-147273.5-283443.0-452452.0-509.5509.5-519

519-585.5642-652709-718.5794-804880-889.5956-965.5

1032-1041.5

CH4

{%

0.30.3

27.320.2

4.716.729.857.549.634.126.059.8

CO 2

vol)

0.160.080.140.250.130.290.200.160.170.200.160.25

C 2 H 6

0.02.9

83.254.843.7

126.2239.0414.3733.6424.2588.0

1222.3

C 3 H 8

0.00.00.00.02.1

11.631.055.6

113.463.097.4

142.0

i-C4H1 0 «-C 4H 1 0

(ppm by volume)

0.00.00.00.00.02.7

11.619.260.028.253.170.4

0.00.00.00.00.02.26.88.4

22.010.919.224.3

/-C 5H 1 2

0.00.00.00.00.01.35.67.0

18.59.0

18.020.1

«-C 5H 1 2

0.00.00.00.00.00.62.81.78.44.56.77.8

Concentration of CH^ and COjTABLE 2

and Cj-C- Hydrocarbons in the Samples of Site 415

Core

416A-1416A-2416A-3416A-5416A-6416A-7416A-9416A-11416A-12416A-13416A-14416A-15416A-16416A-17416A-18416A-19416A-20416A-21416A-22416A-23416A-24416A-25416A-26416A-28416A-29416A-30416A-31416A-32416A-34416A-36416A-40416A-45416A-49416A-50416A-51416A-52416A-53

Stratigraphy

M.-U. MioceneL. MioceneU. OligoceneAlb.-1. EoceneL. AptianHauterivianHauterivianHauterivianHauterivianHauterivianValanginianValanginianValanginianValanginianValanginianValanginianValanginianValanginianValanginianValanginianValanginianValanginianValanginianValanginianValanginianValanginianValanginianValanginianValanginianValanginianValanginianValanginianBerriasianBerriasianBerriasianBerriasianTithonian-Berrias

Sub-BottomDepth

(m)

146-155.5293-307.5450-459.5754-763.5887-896.5

991.5-10931176.9-1185.41194.8-1204.31204.3-1213.81213.8-1222.11223.4-1232.91232.9-1242.51242.5-1252.01252.0-1261.51261.5-1271.0

1271-1277.51277.5-1287.0

1290-1299.51299.5-1309.11309.1-1318.61318.6-1324.71327.7-1336.71337.2-1346.71356.4-1365.91365.7-1375.21375.2-1384.61384.6-1394.11394.1-1403.61406.8-1416.41425.4-1435.01454.5-1463.91501.2-1510.61539.1-1548.61548.6-1558.01558.0-1567.41567.4-1576.81576.8-1586.2

CH4

(%

0.00.0

11.410.722.0

0.50.280.060.170.080.080.100.190.090.090.090.150.070.230.150.110.080.090.040.040.040.020.030.120.0180.020.0010.0330.0360.0520.0520.02

co2vol)

0.130.130.360.090.110.090.100.090.050.050.040.080.060.060.080.060.080.050.130.080.080.070.080.070.050.050.040.040.040.040.040.010.060.070.050.050.08

C 2 H 6

0.00.0

18.035.046.7

2.44.05.0

11.13.85.03.25.35.02.43.24.55.29.26.15.43.33.01.33.32.60.91.94.80.70.90.990.50.731.00.950.56

C 3 H 8

0.00.00.00.50.50.50.00.01.50.20.30.070.150.600.30.60.71.83.32.11.91.41.00.451.81.00.351.02.40.40.40.780.260.380.320.590.37

/ • C 4 H 1 0 rz-C4H1Q

(ppm by volume)

0.00.00.00.00.0

0.5a0.00.00.60.00.00.00.00.00.00.00.210.450.970.640.560.350.350.220.450.430.00.10.70.00.040.05

0.34a0.140.220.160.1

0.00.00.00.00.0

0.00.00.40.00.00.00.00.00.00.00.170.310.930.560.540.700.50.220.450.690.00.320.850.00.20.24

0.290.170.130.17

C5H12

0.00.00.00.00.00.40.00.00.360.00.00.00.00.00.00.00.120.31.10.880.740.700.850.520.81.110.00.41.30.00.470.290.20.30.380.340.3

Isobutane + n-butane.

616

ISOTOPIC COMPOSITION OF METHANE CARBON

TABLE 3Comparison Between Hydrocarbon Composition of Gas Samples Taken Immediately

After Recovery and After Standing at Room Temperature for Two Hours

Core

415A-13

415A-14

Section

12

11

Modeof Sampling

ImmediatelyAfter 2 hrstandingImmediatelyAfter 2 hrstanding

C H 4

(

42.6

30.462.4

59.8

C 2 H 6

595.3

627.01044.3

1222.3

C 3 H 8

69.0

94.096.6

142.0

/-C4H1 0

32.6

57.240.3

70.4

" ' C 4 H 10 ?"(

(ppm by volume)

12.2

19.214.1

24.3

: 5

11

1912

20

T

.8

.2

.3

.7

π-C 5 H 1 2

3.4

7.36.7

7.8

The values of absolute gas concentrations reflect onlyan approximation of the distribution throughout thesection. The content of gas in the sample depends notonly on its concentration in the sediment, but also onthe conditions of core and gas sampling. Consequently,relative values are more reliable for discussion. Theserelative values are reflected in the content of heavy hy-drocarbons in per cent of the total C2 to C5 in Tables 4and 5, as well as a number of individual hydrocarbonratios in Tables 6 and 7.

The results of methane carbon isotopic compositionmeasurements are given in Tables 8 and 9. The amountof higher methane homologues in all the samples wastoo small to be measured.

Much like the chemical composition, the isotopiccomposition depends to some extent on the samplingconditions (Table 10). Methane carbon from the gassample taken after holding the core at room tempera-ture for several hours is richer in the 13C isotope by ap-proximately 1 per mill.

DISCUSSION

Figures 1 and 2 give an idea of gas content variationsthroughout the drilled interval of Holes 415 A and 416A.As already mentioned, the methane content is charac-terized by relative values. With the core-sampling tech-nology presently used, a considerable portion of sedi-ment gas is lost in the course of drilling and recovery.These figures, therefore, provide no more than a quali-tative picture of gas distribution throughout the section.

Gas content in the top 200 to 300 meters of the sec-tion is only slight. This is characteristic of both Holes415A and 416A. Further downhole, gas content beginsto increase and, in Hole 415A, remains high over thewhole section. In Hole 416A, the interval with a notice-able gas content is replaced at the depth of 900 to 1000meters by a profile section extremely poor in gas.

A successive appearance of increasingly heavier gas-eous hydrocarbons is observed at both sites. In Hole415A, ethane was first detected in the interval from273.5 to 283 meters, with hydrocarbons heavier thanethane being completely absent. At a depth of 509.5 to519 meters, propane appears for the first time. Similar-ly, the first appearance of ethane has been recorded inHole 416A at a depth of 450 to 459.5 meters. Measur-able concentrations of propane were found in the nextcore sample (416A-5 at 754 to 763.5 m), and butane andpropane appear at the depth of about 1000 meters.

A picture thus emerges of consecutive actuation ofthe mechanisms whereby organic matter generates in-creasingly heavier hydrocarbons with depth. The regu-larity in the growth of the content of heavy hydrocar-bons with depth becomes especially clear in the lowerportions of Hole 416A.

The total gas content in the profile of Hole 416A, asalready has been noted for the whole turbiditic profile,is very low starting from Core 416A-7 (991.5 to 1093 m).It stays within 0.01 to 0.2 per cent down to the deepestdeposits. Despite low gas concentration, there is a clear-ly defined tendency for the content of heavy hydrocar-bons to increase with depth. From Figure 3 it is seen thatat a depth of 1250 meters (from Cores 416A-13 to 416A-19) the C2 to C5 fraction consists mainly of ethane. Withincreasing depth, the ethane fraction decreases until itconstitutes less than half of the gas samples from the de-posits lying below 1400 meters. Especially striking is theincrease of pentane content; at these depths it becomesthe main component of the C2 to C5 fraction of the gas.Since pentane essentially starts the gasoline series of li-quid hydrocarbons, its high relative concentration maybe regarded as an indication of the capability of organicmatter to generate liquid hydrocarbons at correspond-ing depths.

In this respect, one's attention is also attracted by theinversion of the ratio i-C4/n-C4 at depths of more than1300 to 1350 meters (Figure 4). In the upper portion ofthe section, isobutane predominates; in the lower por-tion, the inverse relationship is observed. It is to benoted that the ratio i-C4/n-C4 in oils is generally lessthan unity.

The picture of the distribution of carbon isotopes ingases for the sections investigated (Figure 5) has the fol-lowing noteworthy characteristic features:

1) The deposits in the upper 1000 meters of bothsites contain methane which is isotopically very light.

2) The methane isotopic composition in both siteschanges progressively with depth.

3) The gradient if isotopic composition variation isnot the same in the profiles of Sites 415 and 416.

4) At about 1300 meters in Hole 416A, a gas was en-countered whose isotopic composition is close to that ofthe gases of oil deposits.

The variation of gas isotopic composition with depthtowards the depletion of the light isotope is characteris-tic of the gases of the sedimentary crust in general (Gali-mov, 1969). We believe the most probable explanation

617

E. M. GALIMOV, V. A. CHINYONOV, YE. N. IVANOV

Concentration of C2 to CTABLE 4

Hydrocarbons on the Methane = Free Basis in the Samples of Site 415

Core

415-3415-5

415A-5415 A-6415A-7415A-8415A-9415A-10415A-11415A-12415A-13415A-14

Stratigraphy

MioceneMiocenePaleocenePaleoceneM.-U. Cret.M.-U. Cret.CenomanianCenomanianCenomanianCenomanianCenomanianAlbian

Sub-BottomDepth

(m)

137.5-147273.5-283

443-452452-509.5

509.5-519519-585.5642-652709-718.5794-804800-889.5956-965.5

1032-1041.5

C2H6

n.d.a100100100

95.087.380.581.876.778.675.182.2

C3H8

n.d.n.d.n.d.n.d.

5.08.0

10.411.011.911.712.4

9.5

/-C4H10

n.d.n.d.n.d.n.d.n.d.1.93.93.86.35.26.84.7

K-C4H10

n.d.n.d.n.d.n.d.n.d.1.52.31.72.32.02.51.6

<•-C5H1 2

n.d.n.d.n.d.n.d.n.d.0.91.91.41.91.72.31.4

«-C5Hi2

n.d.n.d.n.d.n.d.n.d.0.41.00.30.90.80.90.6

n.d. = not detected.

TABLE 5Concentration of C2 to C5 Hydrocarbons on the Methane-Free

Basis in the Samples of Site 416

TABLE 6Hydrocarbon Ratios for the Gas Samples of Site 415

Stratigraphy

Sub-BottonDepth(m) C 2 H 6 C 3 H 8 (-C4H10 «-C 4H 1 0 C 5 H 1 2

416A-1416A-2416A-3416A-5416A-6416A-7416A-9416A-11416A-12416A-13416A-14416A-15416A-16416A-17416A-18416A-19416A-20416A-21416A-22416A-23416A-24416A-25416A-26416A-28416A-29416A-30416A-31416A-32416A-34416A-36416A-40416A-45416A-49416A-50416A-S1416A-52416A-53


146-155.5298-307.5450-459.5454-763.5877-896.5

991.5-10931176.9-1185.41194.8-1204.31204.3-1213.81214.8-1222.41223.4-1232.51232.9-1242.51242.5-1252.01252.0-1261.51261.5-1271

1271-1277.51277.5-1287

1290-1299.51299.5-1309.11309.1-1318.61318.6-1327.71327.7-1336.71337.2-1346.71356.4-1365.91365.7-1375.21375.2-1384.61384.6-1394.11394.1-1403.61406.8-1416.41425.4-1435.01454.5-1463.91501.2-1510.61539.1-1548.61548.6-1558.01558.0-1567.11567.4-1576.81576.8-1586.2

n.d.10098.698.962.8n.d.n.d.79.595.094.098.097.089.089.084.078.964.659.459.359.151.252.147.948.544.6n.d.51.147.8n.d.44.842.138-539.647.843.837.3

n.d.n.d.n.d.1.41.1

13.6n.d.n.d.10.75.06.02.03.0

11.011.016.012.322.321.320.420.821.717.516.626.517.2n.d.

26.923.9n.d.

19.933.220.020.615.327.224.7

n.d.n.d.n.d.n.d.n.d.

n.d.n.d.4.3


3.75.66.26.26.15.46.18.16.67.4

n.d.2.77.0


n.d.n.d.2.9


n.d.n.d.n.d.n.d.n.d.10.5n.d.n.d.

2.6


3.03.86.05.65.9

10.88.88.16.6

11.8n.d.8.6

2.02.1

9.910.2

26.2b7.6 15.8

10.5 8.17.4 6.06.7 11.3

j"n.d. = not detected.Isobutane + M-butane

2.13.77.18.58.1

10.914.919.311.819.0n.d.10.712.8n.d.23.412.415.416.318.215.620.1.)

of this phenomenon is that at great depths there general-ly is a more transformed, organic-matter-generatedmethane which is less enriched in the light isotope. Thisphenomenon is associated with the nature of the intra-molecular distribution of carbon isotopes in biologicalsystems (Galimov, 1973).

At the early stages of organic matter transformation,usually in the top 500 meters of the sedimentary crust,methane is encountered whose isotopic composition isfrom - 80 to - 60 per mill. The gases of the brown coalstage are somewhat poor in the light isotope: the δ13C of

Core Stratigraphy

Sub-Bottom

Depth

(m) c 2 + c 3 (, +«)c 4

415-3

415-5

415A-5

415A-6

415A-7

415A-8

415A-9

415A-10

415A-11

415A-12

415A-13

415A-14

Miocene

Miocene

Paleocene

Paleocene

M.-U. Cret.

M.-U. Cret.

Cenomanian

Cenomanian

Cenomanian

Cenomanian

Cenomanian

Albian

137.5-147

273.5-283

443-452

452-509.5

509.5-519

519-585.5

642-652

709-718.5

794-804

880-889.5

956-965.5

1032-1041.5

n.d.

n.d.

3284.0

3686.0

1230.0

1156.0

980.0

1135.0

519.0

632.0

332.0

402.0

n.d.

n.d.

n.d.

n.d.

20.6

10.9

7.7

7.4

6.5

6.7

6.0

8.6

n.d.

n.d.

n.d.

n.d.

n.d.

2.4

1.7

2.0

1.4

1.6

1.3

1.5

i-C

n.d.

n.d.

n.d.

n.d.

n.d.

1.2

1.7

2.2

2.7

2.6

2.7

2.9

methane is -60 to -50 per mill. With a still higher de-gree of organic matter transformation, correspondingto the main stage of oil formation, methane is charac-terized by δ13C values in the range of —50 to -35 permill. An isotopically heavier methane (-35 to -20 permill) most frequently is associated genetically with coalsof high degrees of metamorphism. In natural condi-tions, such correlation can be disturbed by the gas-migration processes, as well as by certain peculiarities ofthe geological history of specific deposits.

All other things being equal, the gas isotopic compo-sition must depend on the extent to which the reservoirrock is capable of retaining gas. In the course of geolog-ical time, gas of different generation stages is accumu-lated successively in the rock. The isotopically lightermethane of the early generation stages is gradually di-luted with the isotopically heavier one. For one or morereasons (e.g., diffusion, filtration, break-through alongfra&ures, etc.), the previously generated gas is lost. It isobvious that the rock contains an isotopically heaviermethane than would be the case if it retained all the gasformed in the course of the evolution of organic matter.

We believe that these considerations can serve as thebasis to explain a sharper change of the gas isotopic

618


TABLE 7Individual Hydrocarbon Ratios

Core

416A-1416A-2416A-3416A-5416A-6416A-7416A-9416A-11416A-12416A-13416A-14416A-15416A-16416A-17416A-18416A-19416A-20416A-21416A-22416A-23416A-24416A-25416A-26416A-28416A-29416A-30416A-31416A-32416A-34416A-36416A-40416A-45416A-49416A-50416A-51416A-52416A-53

Stratigraphy


Sub-BottomDepth

(m)

146-155.5298-307.5450-459.5754-763.5887-896.5

991.5-10931176.9-1185.41194.8-1204.31204.3-1213.81213.8-1222.41223.4-1232.91232.9-1242.51242.5-1252.01252.0-1261.51261.5-1271.01271.0-1277.51277.5-1287.01290.0-1299.51299.5-1309.11309.1-1318.61318.6-1327.71327.7-1336.71337.2-1346.71356.4-1365.91365.7-1375.21375-2-1384.61384.6-1394.11394.1-1403.61406.8-1416.41425.4-1435.01454.5-1463.91501.2-1510.61539.1-1548.61548.6-1558.01558.6-1567.41567.4-1576.81576.8-1586.2

C l

c2+

n.d.a

n.d.6300.03100.04660.02080.0

700.0120.0122.0200.0151.0300.0348.0160.0330.0236.0263.0

86.0150.0149.0122.0122.0157.0147.059.069.0

130.081.0

120.0160.0100.0

4.0253.0195.0248.0240.0133.0

c2L

C 3

n.d.n.d.n.d.7093

4.6n.d.n.d.7.4

19.016.645.035.0

8.38.05.36.42.92.82.92.82.43.02.91.82.62.51.92.01.82.21.31.91.93.11.61.5

c

3(/+«)c4

n.d.n.d.n.d.n.d.n.d.1.0n.d.n.d.1.5n.d.n.d.n.d.n.d.n.d.n.d.n.d.1.80.71.71.71.71.31.21.02.00.9n.d.2.41.5n.d.1.72.70.80.90.82.01.4

/-C,4

«-c4

n.d.n.d.n.d.n.d.n.d.1.6n.d.n.d.1.5n.d.n.d.n.d.n.d.n.d.n.d.n.d.1.21.01.01.11.00.50.71.01.00.6n.d.0.30.8n.d.0.20.2n.d.0.51.31.30.6

TABLE 9Carbon-Isotope Composition of Methane

From Gas Samples of Site 416

n.d. = not detected.

TABLE 8Carbon Isotope Composition of Methane

From the Gas Samples of Site 415

Core

415-5415A-6415A-8415A-9415A-10415A-12415A-13415A-14

Sub-BottomDepth

(m)

273.5-283452-509.6519-585.5

642-652709-718.5880-889.5956-965.5

1032-1041.5

Stratigraphy

MiocenePaleoceneM.-U. Cret.CenomanianCenomanianCenomanianCenomanianAlbian

δ l 3 C

(%o)

-71.08-70.64-69.97-68.81-65.60a

-62.89a

-62.5?a-61.39a

Note: All samples taken immediately after recovery.aMean Value for two different sections of a core.

composition with depth throughout Hole 416A as com-pared with Hole 415A. In the Cretaceous deposits ofHole 416A, a considerable portion of earlier-generationgas probably has been lost. This is also confirmed by thevery low overall gas concentration in these deposits.

It should be said that, for all the sections of oceanicdeposits, in which up to now the isotopic compositionof methane has been studied, great enrichment in thelight carbon isotope was observed. The only exceptionswere the profiles of Site 83 (Leg 10) and Site 176 (Leg18), where at depths of less than 100 meters a gas wasencountered whose isotopic composition is about - 50

Core

416A-3416A-5416A-6416A-7416A-15416A-22+23

Sub-BottomDepth

(m)

450-459.5454-763.5887-896.5

991.5-10931232.9-1242.51299.5-1309.1

Stratigraphy

U. OligoceneAlb.-1. EoceneL. AptianHauterivianValanginianValanginian

δl3c(%o)

-82.06-73.81-71.56-61.90-57.29-49.18

Note: All samples taken after standing more than 2 hours at roomtemperature.

TABLE 10Variations of ò^C of Methane Depending on

Conditions of Sampling

Core

415-5415-11

415-13

415-14

Section

121212

SampledImmediately

After Recovery

-71.08-66.52-64.66-62.25-62.90-61.48-61.30

Sampled After Standing2 to 4 hr in the ClosedCore Barrel Section

-70.80--

-61.12-

-60.97-

per mill (Claypool et al., 1973). This gas is almost cer-tainly of migratory origin. In most cases, methane hadan isotopic composition of -80 to -60 per mill. Thedeposits of the top 1000 meters of Holes 415A and 416Acontained methane with isotopic composition from - 80to - 60 per mill. By way of comparison, in the upper-zone deposits of the continental sedimentary profiles,methane is somewhat less rich in the light isotope. It isalso characteristic that the variation of its isotopic com-position with depth manifests itself much more clearlyin continental profiles. In connection with the reasoningpresented here, such a difference in the distribution ofmethane isotopic composition over the profile of ocean-ic and of continental sedimentary deposits can be at-tributed to two causes: (1) the variation of degree oforganic-matter transformation with depth in oceanic de-posits is, on the average, smaller than in the deposits ofa continental profile; (2) oceanic deposits, on the aver-age, retain the gas generated by them better than the de-posits of a continental profile.

Both causes are plausible. The first one is probablebecause organic-matter transformation in poorly con-solidated sediments of the upper portion of the oceanicsedimentary profile actually occurs at a comparativelyslower rate (see Galimov et al., this volume). The secondcause seems probable due to the possibility of gas hy-drates being formed. A combination of low tempera-tures and high pressures in the upper zone of the oceanicsedimentary profile makes it thermodynamically admis-sible that a deep gas-hydrate zone should be formed in

619


0I-

Cores

10

Methane (vol. %)

20 30 40 50 60

Methane (vol. %)

5 10 15 20

100-

300-

-

500-

700-

-

900-

1100-

3 :

5 ;

5A :

6A

7A :

8A

9A :

10A

HA :

12A ;

13A:

14A :

Holes 415. 415A

Figure 1. Methane content in gas samples, Holes 415 and415A.

many areas of the ocean. With ocean depths of morethan 2500 meters, the thickness of the gas-hydrate zonecan reach 500 meters. The mobility of gas in the hydratestate is incomparably lower than in the free state.

Finally, the presence of methane, comparatively poorin the light isotope, at depths of about 1300 metersagrees with the data on the chemical composition of gasat these depths. The scarcity of the light isotope inmethane, as well as the appearance in C2 to C5 hydrocar-bons of heavy components in appreciable concentra-tions, indicate that these depths correspond to the up-permost boundary of the zone where the petroleum-formation process develops.

REFERENCES

Claypool, G. E., Presley, B. J., and Kaplan, J. R., 1973. Gasanalysis in sediment samples from Legs 10, 11, 13, 14, 15,18 and 19. In Creager, J. S., Scholl, D. W., et al., Initial

100-

300-

500-

700-I

o•w

oGO 900H

1100-

1300-

1500-

Cores

1A

2A ;

n Λo A

5 A ]

6A ;

7A

9A ;

113A ]

21A :23A :25A i29A :30A -32A "

45A .

49A

53A :

Hole416A

a

Figure 2. Methane content in gas samples, Hole 416A.

Reports of the Deep Sea Drilling Project, v. 19: Washing-ton (U.S. Government Printing Office), p. 879-885.

Galimov, E. M., 1969. The Isotopenzusammensetzung desKohlenstoffs in den Gasen der Erdkruste. Zeitschrift fürangewandte Geologie, B. 15, v. 2, p. 63-69.

, 1973. Isotopy ugleroda v neftegazovoy geologii:Moskva (Nedra), p. 1-384. Carbon isotopes in oil-gas geol-ogy: NASA English Translation TT F-682, Washington,June 1975.

620


Hydrocarbon Content (%)

0 20 40i i

60 80 100

1200-t

1250-

1300-

1350-

fwQ

ε 1400

o

oCO

XJ

00

1450-

1500

1550

-1

Figure 3. Percentage of the C2 to C5 hydrocarbons onmethane-free basis, Hole 416A. 1 = C2H6; 2 =C3H8; 3 = C4H10; 4 = C5H12.

n -butane Content (%)

0 η Cores0 20 40 60 80 100

100-

200-

300-

400-

500-

600-

£ 700α

I 800

o00

oo

900-

1000

1100

1200

1300

1400

1500

1600

415A

8A

9A

10A

11A

12A

13A

14A

416)

12A

2OA;22A;24A:26A29A:

32A40A

45.

50AE ; 5 1

52A: : 5 3

21A23A25A28 A30A34

_1__J I L.

i-C,

n-C,

/-C,< 1

Figure 4. Percentage ofn-butane relative to the sum ofbutane isomers, Holes 415A and 416A.

621


- 8 0 - 7 0

O-i

100-

300-

500-

700-

900-

1100-

1300-

Cores415 416/415A

5A:6A7A8A

10A

11A

12A:

13A:

14A:

- 6 0

o 415

• 416

Figure 5. Variations in carbon isotopic composition ofmethane with depth, Holes 415A and 416A.

622

23. isotopic composition of methane carbon · pdf fileand concentrations, thereby allowing...

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