CHAPTER-IV

CARBON ISOTOPE GEOCHEMISTRY

Introduction

Isotopic properties of thermogenic gas

Isotopic properties of bacterial methane

Isotopic properties of secondary gas types

Stable carbon isotopic compositions

Results and discussion

Conclusions

______4.1 Introd

Carbon is

organic so

influence

determina

characteri

of metha

molecular

identificat

classes of

organic m

heavier g

generation

amounts

methanog

through t

1983b; Ga

Primary t

source roc

burial hi

commerci

C________duction

sotope studie

ource mater

of migrati

ation of the

ization of lig

ane, ethane

r (C1, C2, C

tion of hydr

f processes:

matter and i

gas molecul

n process oc

of other h

genesis in h

the last deca

alimov, 198

type hydroc

ck (Whiticar

story (temp

ial quantitie

CARBON_________

es (13C/12C)

rials. These

ion, recogn

e maturity o

ght hydrocar

and propan

C3 and C4) d

rocarbons.

one proces

is associate

les and liqu

ccurs throug

hydrocarbon

hydrocarbon

ades (Stahl,

8; Faber et a

carbon gases

r, 1994) are

perature and

es of oil for

N ISOTO_________

have contri

e studies are

ition of ba

of producin

rbon gases.

ne (δ13C1, δ

distribution

Sources are

ss occurs du

d with the

uid hydroca

h bacterial m

n molecules

n reservoirs

1977; Bern

al., 1992).

s, which are

mainly depe

d time) an

rm from sap

OPE GEO________

ibuted widel

also useful

acterial degr

ng organic s

The carbon

δ13C2, and δ

patterns are

e traditional

uring the th

generation

arbons (ther

methanogene

s (biogenic

have been

nard et al.,

e generated

endent on th

nd geotherm

propelic (Ty

CHAPT

OCHEMI________

ly in identif

in understa

radation, qu

source mate

n isotopic com

δ13C3) and

e employed

lly subdivide

hermal degra

of large am

rmogenic);

esis with ins

). The o

n extensively

1977; Scho

directly by

e type of the

mal gradien

ype I and II

107

TER-IV

ISTRY ______

fication of

anding the

uantitative

erials and

mposition

particular

in source

ed in two

adation of

mounts of

the other

significant

origins of

y studied

oell, 1980,

a simple

e kerogen,

t. Most

kerogen)

108

organ

sourc

Type

their

C2+ h

therm

therm

with

Fig. 4

nic matter, w

ce (Fig. 4.1)

e III kerogen

maturation h

hydrocarbon

mogenic met

mal maturity

less than 0

4.1 Relativeorganic

whereas gas

). Primary h

n are differen

history, sapr

s (Hunt, 199

thane relativ

levels. The

0.5% C2+ ga

e proportionsmatter (after

s forms from

hydrocarbon

nt from sapro

ropelic kero

95). Humic

e to C2+ com

e biogenic g

ases (Stahl,

s of natural r Hunt, 1995

m humic (Ty

n gas genera

opelic Type

gens genera

c kerogens r

mponents, yi

gases are alm

1974; Whi

gas generate5).

ypes III and

ation profile

I and II kero

ate a signific

results in hig

ielding most

most exclusi

iticar et al.,

ed from diff

d IV keroge

es from hum

ogen. Durin

cant volume

gh amounts

tly CO2 at lo

ively methan

, 1986). T

ferent types

en)

mic

ng

of

of

ow

ne,

The

of

occurrenc

to 110 oC

addition

detected i

1994). Th

thermal c

Hunt, 199

thermogen

150oC, w

They are

of oil (Tis

in limited

thermogen

150oC to

150oC, w

thermogen

4.2 Isotop

Thermoge

thermal c

1995; wh

isotopic p

organic m

generation

ce of bacteria

) and depth

to surface

in oil reserv

hermogenic

racking of c

95; Whitica

nic gases ar

with peak ge

formed from

ssot and We

d amount m

nic gas is fo

over 200oC

wet thermog

nic methane

pic properti

enic gas m

racking of c

hiticar, 1999)

properties, w

matter (Tiss

n of thermog

al methane u

(>1 kbar) ha

environmen

voirs as a co

gas may be

crude oil wit

ar, 1999). T

re main prod

eneration 12

m sapropelic

elte, 1978; H

may also be d

ormed by all

(Stahl, 197

genic gases

e becomes do

ies of therm

may be form

crude oil wit

). Thermog

which are a

sot et al., 1

genic gases c

under extrem

as also been

ts, methane

ommon comp

formed by t

th increasing

Thermogenic

ducts of the

20oC) (Hunt

(Type I and

Hunt, 1995).

derived from

kerogen typ

77; Schoell,

s decreases

ominant gas

mogenic gas

med by ther

th increasing

genic gas ha

affected by

1974). Bec

causes isotop

me condition

documented

e formed by

ponent of oi

thermal degr

g maturity (S

c gas may

mature stag

t, 1995) (Fi

d II) kerogen

Moreover,

m humic Ty

pes during ov

1983a). At

to very l

type (Fig.4.

rmal degrad

g maturity (S

as a wide ran

type, amoun

cause of kin

pic fractiona

ns of tempera

d (Whiticar,

y microbes

il field gas (

radation of k

Schoell, 198

be wet or d

ge (between

g. 4.1 and

n and therma

wet thermo

ype III kerog

ver mature s

temperature

low values

2).

dation of ke

Schoell, 198

nge of mole

nt and matu

netic isotop

ation leading

109

ature (-1.2

1992). In

has been

(Whiticar,

kerogen or

80, 1983b;

dry. Wet

70oC and

Fig. 4.2).

l cracking

ogenic gas

gen. Dry

stage from

es beyond

and dry

erogen or

83b; Hunt,

ecular and

uration of

pe effects,

g to a gas

110

Fig.4

deple

therm

deple

with

enrich

gradu

mater

very

4.2 Diagramincreasin

eted in 13C

mal maturati

eted with res

increasing m

hed in 13C

ually change

rial. Immat

dry (<5%

m showing ng temperatu

and leaves

on, later for

spect to the

maturation o

C. Thermo

e via the eff

ture thermog

C2+) (Rice,

generation ure (after Ric

the residue

rmed gas w

residual ker

f the source,

ogenic gas

fects of ther

genic gas is

1989; Hun

of natural ce and Clayp

e enriched

will be enric

rogen (Clayt

, the most m

has geoche

rmal maturat

typically d

nt, 1995). W

gas and itpool, 1981).

in 13C. The

hed in 13C,

ton, 1991).

mature gas w

emical prop

tion history

depleted in 1

With increa

ts types wi

erefore durin

although st

Consequent

ill be the mo

perties, whi

of the sour

3C and dry

sing maturit

ith

ng

till

tly

ost

ich

rce

to

ty,

mature t

(5%<C2+<

significan

Whiticar,

30‰ to -

are varyin

terrestrial

from mari

Fuex (19

thermogen

methane.

40‰ to

(>5%) (F

oil source

thermogen

matter is

ranging fr

may be de

crude oil

associated

been sug

Methane

40‰ (PD

associated

thermogenic

<15%)(Rice

ntly enriched

1994). The

60‰ (PDB)

ng from -15

l environmen

ine environm

977) and S

nic gas base

Thermogen

-58‰ (PDB

Fuex, 1977).

e rock or fro

nic gas from

characterist

from -25‰ t

erived from

at over matu

d with crud

gested as a

of thermoge

DB) indicat

d thermogen

c gas is e

et al., 19

d in 13C and

isotopic pro

) (Hunt, 199

‰ to -40‰

nts (humic s

ments (sapro

tahl (1977)

ed on C2+ c

nic gas assoc

B) range an

The wet th

om crude oi

m either ter

tically dry (

to -40‰ (PD

over mature

urity (Fuex,

e oil forme

approximatel

enic gas ass

ting higher

nic gas (Scho

enriched in

989). Ove

dry (<5% C

operties for w

95). Dry the

‰ PDB (Hun

source) is en

opelic source

) proposed

omposition

ciated with o

nd significan

hermogenic g

l itself (Stah

rrestrial hum

(<5% C2+) a

DB) (Stahl,

e oil source r

1977). The

d from crac

ly -50‰ to

sociated with

source ma

oell, 1983b).

n 13C and

er mature

C2+) (Stahl, 1

wet thermog

ermogenic g

nt, 1995). T

nriched in 13

e) (Stahl, 197

an empiric

and carbon

oil generatio

nt quantities

gas may be d

hl, 1975; Fu

mic or marin

and has met

1977). The

rocks or from

first methan

cking reactio

-60‰ (PD

h condensat

aturities rel

the gas

thermogeni

1977; Schoe

genic gas ran

gas isotopic p

Thermogenic

3C relative t

75; Fuex, 19

cal different

isotope pro

on has δ13C v

s of C2+ co

derived from

uex, 1977).

ne sapropeli

thane enrich

e dry thermo

m thermal cr

ne or thermo

ons in petro

DB) (Schoell

tes is enrich

ative to th

111

is wetter

c gas is

ell, 1983b;

nge from -

properties

gas from

to the gas

977).

tiation of

operties of

values in -

omponents

m a mature

Deep dry

ic organic

hed in 13C

ogenic gas

racking of

ogenic gas

oleum has

l, 1983b).

hed than -

hat of oil

112

4.3 Is

Roug

origin

Bacte

reduc

aceta

occur

to 11

additi

detec

1994)

Bacte

Bacte

0.5%

bacte

crack

bacte

1983b

these

proce

envir

sedim

bacte

comm

sotopic prop

ghly 20% o

nated from

erial methan

ction of CO

ate fermenta

rrence of bac

0 oC) and de

ion to surfa

cted in oil re

).

erial gas may

erial gas is d

% higher C2+

erially forme

king reaction

erially gener

b; Hunt, 19

ranges bec

esses such

ronments w

ments with a

erial methan

mercial quan

perties of B

of the worl

microbial s

ne is formed

2 and in fre

ation (Whiti

cterial metha

epth (<1 kba

ace environ

eservoirs as

y be recogn

defined as ex

component

ed methane

ns by carbon

rated methan

95; Whitica

cause of var

as migrati

ith anomalo

already enric

ne although

ntities of ba

acterial Me

ldwide natu

sources (Ric

d in shallow

eshwater env

icar and Fa

ane under ex

ar) has also b

nments, meth

a common

ized by uniq

xclusively m

s (Whiticar,

from “therm

n isotopic pro

ne vary fro

ar, 1999). A

riable sourc

ion or mic

ous 13C val

ched 13C va

these parti

acterial gas

ethane

ural gas res

ce and Clay

w marine en

vironments b

aber, 1986;

xtreme cond

been docume

hane forme

component

que molecul

methane depl

1994). It i

mogenic dry

operties. Th

m -60‰ to

Anomalous v

ces of 13C a

crobial alte

lues of sou

alues) may l

icular cond

(Nissenbaum

ervoirs are

ypool, 1981;

nvironments

by near surf

; Whiticar,

itions of tem

ented (Whiti

d by micro

of oil field

ar and isotop

leted in 13C,

is possible t

y gas” forme

he carbon is

o -110‰ (PD

values may

and because

eration. S

urces (inters

lead enrichm

itions do n

m et al., 19

estimated

; Rice, 1992

by microb

face microb

1999). T

mperature (-1

icar, 1992).

obes has be

gas (Whitica

pic propertie

with less th

to differentia

ed by therm

otopic ratio

DB) (Schoe

occur outsi

of seconda

Some specif

stitial CO2

ment of 13C

not exist wi

972). Bacter

as

2).

ial

ial

The

1.2

In

een

ar,

es.

han

ate

mal

of

ell,

ide

ary

fic

in

in

ith

ial

methane

ranging f

(PDB) usi

4.4 Isotop

The emp

discussed

determini

generative

original m

as bio d

1990).

A mixtur

phenomen

indicating

gas helps

members

Chung et

bacterial

and C1/(C

relationsh

been wide

1989). F

observed

(PDB) rel

is delineate

from 103 to

ing Bernard

pic properti

phasis on t

d above.

ing the cha

e) processes

molecular an

egradation

e of more t

non. This m

g a mixed si

to recogniz

of the mix

al., 1988; W

and thermog

C2+C3) mole

hip between

ely used to d

For cogeneti

that general

lative to met

ed with a fi

105 and in

plot (Bernar

ies of second

the unaltere

In many c

aracteristics

s may alter t

nd isotopic p

or migratio

than one gas

may be a b

ituation. M

ze that the ga

xture and th

Whiticar, 19

genic gas m

ecular concen

δ13C of me

define the m

c methane e

ly ethane is

thane (Silver

ield of valu

n δ13C prope

rd, 1978).

dary gas typ

ed gases (th

cases geoch

of a prim

their initial g

properties of

on fractionat

s type in a

bacterial gas

Molecular and

as is a mixtu

heir relative

94; Prinzho

may be recog

ntration of t

ethane and δ

mixed gas pr

ethane pairs

enriched in

rman, 1971;

ues in C1/(C

erties vary

pes

hermogenic

hemical ana

ary gas, so

geochemical

f natural gas

tion (Thom

natural gas

within a th

d isotopic p

ure and to de

e contributio

fer and Huc

gnized by us

the gas on th

δ13C of etha

roperties (Sc

in thermog

13C between

Deines, 198

C2+C3) conc

from -60‰

and bioge

alyses may

ome second

l characteris

may suffer

mpson and K

sample is a

hermogenic

properties of

etermine po

ons (Schoel

c, 1995). M

sing δ13C of

he Bernard

ane data pair

choell, 1983

genic gas, it

n 5‰ (PDB)

80).

113

centrations

to -90‰

enic) was

y help in

dary (post

stics. The

alteration

Kennicutt,

a common

gas pool,

f a natural

ssible end

ll, 1983a;

Mixtures of

f methane

plot. The

r has also

a; Berner,

has been

and 10‰

114

4.5 S

Carbo

98.89

proce

isotop

prefe

terres

result

organ

comp

precu

hydro

organ

1995)

In or

comp

Chrom

comb

and 4

notati

Wher

intern

table carbo

on has two

9% and 1.1

ess, these rel

pic composi

rred in phot

strial organi

ts in the di

nic matter re

pounds retai

ursors, varia

ocarbons of

nic and inorg

).

rder to obt

pounds, the

matograph (

busted and co

46) of CO2

ion.

re R represen

nationally a

on isotopic c

stable isot

11% respec

lative ratios

itions for ce

osynthesis o

ic matter. C

issolved ino

elative to ter

in the carb

ations in iso

different or

ganic reservo

tain the sta

component

(GC). The or

onverted int

measured.

δ13C = 100

nts the ratio

accepted sta

composition

topes, 12C a

tively. Du

may be tran

ertain carbon

on land leadi

Conversely

organic carb

rrestrial orga

bon isotopic

otopic comp

rigin and tra

oirs (Murph

able carbon

ts of the s

rganic carbo

o CO2 and th

Results ar

00 X (Rsample/

13C/12C . A

andard Pee

ns

and 13C in

uring chemi

nsformed sli

n pools. In

ing to a gene

the lower v

on being en

anic materia

c compositio

positions ma

ace the mov

y and Abraj

isotopic co

sample mix

on of the sam

he three maj

re reported

/Rstandard – 1)

All reported a

Dee Belem

the global

ical, physic

ightly, leadin

one such ex

eral depletio

vapour press

nriched in 1

als. Provide

ons of their

ay be used

vement of ca

ano, 1994; D

omposition

xtureis separ

mple is then

jor isotopic m

in conventi

)‰

analyses are

mnite (PDB

abundance

cal, biologic

ng to differe

xample, 12C

on of 13C in

sure of 13CO

13C in mari

ed that organ

r biosynthet

to distingui

arbon throu

Dowling et a

of individu

rated by G

quantitative

masses (44,

onal delta (

referred to t

B) a calciu

of

cal

ent

is

all

O2

ine

nic

tic

ish

gh

al.,

ual

Gas

ely

45

(δ)

the

um

carbonate

in South C

4.6 Resul

Thirty sev

methane

classified

locations

and GNR

Figure 4.3

The meth

PDB. It

occurred

mode of

misinterpr

stronger t

would be

significan

(δ13C2) va

values hav

The mean

30.5‰, -2

(C1) is alw

ethane (C

Such an

e fossil of Be

Carolina whi

lts and discu

ven samples

(δ13C1), eth

into two zo

west and ea

R-50. Selecte

3.

hane carbon i

might be n

the residual

f formation

reted (Colem

thermal sign

observed in

nt variability

alues have a

ve a range o

n carbon iso

21.7‰ and -

ways isotop

C2), propane

orderly dist

elemnite Am

ich is assign

ussions

s were analy

hane (δ13C2)

ones i.e., A (

ast respective

ed soil samp

isotope (δ13C

noted, howe

l gas will be

n and/or th

man, et al.,

ature (i.e. a

n an unaltere

y in their sta

a range from

of -19.4‰ to

otopic compo

-22.3‰ resp

ically lighte

(C3), butane

tribution wa

mericana for

ned a δ13C va

yzed for the

) and prop

20 samples)

ely, of the st

le locations

C1) composi

ver, that if

e isotopicall

hermal mat

1981). Su

more elevate

ed gas. The

able carbon i

m -14.6 ‰ t

-26.6‰ (PD

osition of m

ectively.It h

est among ga

e (C4) and p

as explained

rm Cretaceou

alue of 0‰.

e carbon iso

ane (δ13C3)

) and B (17 s

tations form

(Table 4.1)

itions range

f partial mic

ly enriched

turity of t

uch enrichm

ed level of th

wet gas com

isotope com

to -25.60‰.

DB).

methane, etha

has long been

aseous hydro

pentane (C5)

d from theor

us Pee Dee

otopic comp

. The samp

samples) bas

med by the lin

have been m

from -23.8 t

crobial oxid

(i.e. heavier

the source

ment would

hermal matu

mponents als

mpositions. T

The propan

ane and prop

n known tha

ocarbons fol

if the gas i

retical calcu

115

formation

osition of

ples were

sed on the

ne GNR-5

marked in

to -37.0‰

dation has

r) and the

may be

suggest a

urity) than

so display

The ethane

ne (δ13C3)

pane are -

at methane

llowed by

s pristine.

ulation by

116

Wapl

McC

and p

nearly

The m

(δ13C

genet

gases

provi

and m

two g

been

bioge

hydro

les and Tor

arty and Fel

propane sho

yfollowed in

molecular ra

C1) are used

tic classifica

s from vent

ides a plot r

methane bas

gases results

proposed th

enic in ori

ocarbon mig

rheim (1978

lbeck (1986)

ow δ13C1<δ1

n the present

atio of C1/(C

in order to

ation diagram

ts, seeps an

relating varia

sed on the h

in a linear c

hat some iso

gin but ar

gration (Prinz

8) as well a

). Generally

3C2<δ13C3 tr

t study.

C2+C3) and

delineate it

m combining

nd sediment

ations in the

hypothesis th

change of th

otopically li

re the resu

zhofer and P

as laboratory

y, carbon iso

rend in natu

carbon isoto

ts origin. B

g molecular

ts in variou

e carbon iso

hat mixing

heir isotopic

ghter metha

ult of isoto

Pernaton, 199

y heating ex

otopes of me

ural gas and

ope properti

ernard (197

and isotopic

us areas. Sc

topic proper

of various p

properties.

ane accumul

opic fraction

97).

xperiments b

ethane, etha

d this trend

es of metha

78) proposed

c properties

choell (1983

rties of etha

proportions

It has recent

lations are n

nation durin

by

ane

is

ane

d a

of

3b)

ane

of

tly

not

ng

Fig.4.3 LLocation of soil sample selectedd for Carbon isootope analyses.

1

17

118

A cla

bioge

Micro

greate

conce

a qui

proce

with

ethan

B of

the va

δ13C

from

consi

below

to -3

One

bacte

of th

ratio

highl

Gang

assic Bernard

enic activity

obial degra

er than 100

entrations of

ite isotopica

esses produc

C1/(C2+C3)

ne and highe

the Shri Ga

alues for the

value of me

Zone A and

iderable frac

w 50. The i

5.8‰ (PDB

sample (GN

erial oxidatio

e biogenic r

C1/(C2+C3)

ly diagnostic

ga Nagar are

d plot has b

y to that

dation prod

00 and cont

f ethane, pro

ally deplete

ce a wide s

ratios rangin

r hydrocarbo

anga Nagar a

e ratio (C1/C

ethane. The

d fifteen soil

ctions of eth

sotopic valu

B) and Zone

NR 99) is s

on of methan

range. Thu

and δ13C o

c of the the

a.

een used to

from therm

duces hydro

ains almost

opane, butan

ed 13C ratio

spectrum of

ng from 0 to

ons. The lig

area are sho

2+C3) are pr

adsorbed alk

samples fro

hane and pro

ue of methan

B is rangin

showing en

ne. None of

s the two im

of methane

ermal origin

differentiate

mogenic pro

ocarbon gas

exclusively

ne and pentan

o of less th

low molecu

o 50 with sig

ght hydrocar

own on the B

rovided in th

kane gases f

om Zone B c

opane and th

ne in Zone A

ng from -26

nriched 13C

f the isotopic

mportant par

and the tren

of hydroca

e the gases p

ocesses (Be

s with C1/(

y methane w

ne (C2+). Me

han -60‰.

ular weight

gnificant con

rbon gases fr

Bernard plot

he Table 4.1

from eightee

composed of

heir C1/(C2+

A isranging

6.4 ‰ to -37

which migh

c values is a

rameters, th

nd between

arbon gases

produced fro

ernard, 1978

C2+C3) rati

with very lo

ethane usual

Thermogen

hydrocarbo

ntribution fro

rom Zone A

t (Fig.4.4) a

along with t

en soil sampl

f methane wi

C3) values a

from -23.8

7.0 ‰ (PDB

ht be due t

a characteris

he gas wetne

these two a

from the Sh

om

8).

ios

ow

lly

nic

ons

om

&

nd

the

les

ith

are

‰

B).

the

tic

ess

are

hri

Fig. 4.4 L

Z

Z

Log C1/(C2+gaseous hyarea, Bikan

one B Sam

Zone A Sa

+C3) versus ydrocarbons ner-Nagaur b

mples

amples

δ13C1 (Bernfrom Zone basin (after B

nard plot) fA & B of tBernard et a

for the adsorthe Shri Ganal., 1976).

119

rbed light nga Nagar

120

Abram

classi

small

in 13C

conce

isotop

bacte

conce

range

The a

range

(Fig.

GNR

meth

greate

samp

upwa

samp

conce

30‰

remai

betwe

betwe

influe

ms (1996, 2

ify the gas i

l concentrati

C1 (>-45‰)

entration of

pe ratios (<

erially deriv

entrations of

e (-25 to -55‰

adsorbed ga

e of Type-A

4.5). There

R 77, GNR

ane concent

er than -40‰

ples (Type A

ard and reac

ples(GNR 62

entration ran

. These sam

ining four

een the Type

een that of

ence of bac

2007) used

into one of

ions of meth

and is bacter

methane (u

<-55‰) and

ved gases, a

f methane a

‰), indicati

ases from Z

and Type-I

e are eleven

55, GNR 6

tration rangi

‰, which c

A) could be c

ching the n

2, GNR 69,

nging from

mples (Type

samples (G

e A and Typ

the unaltere

cterial altera

the methan

three catego

hane (usually

rially altered

usually >100

is bacterial

and Type A

and carbon i

ng the prese

Zone A of th

category wi

samples (G

7, GNR 95

ing between

can be class

considered as

ear surface

GNR 60) a

104 to 163

I) could be

GNR 87, GN

pe I categorie

ed hydrocarb

ation on th

ne isotope a

ories: Type

y <200 ppb)

d; Type II is

00ppb) with

l sourced g

A is sedim

isotope ratio

ence of therm

he Shri Gan

ith no sampl

GNR 56, GN

, GNR 83,

n 225 to 110

sified under

s unaltered t

environmen

are of Type

ppb, and δ1

considered a

NR 99, GN

es and repre

bon microse

em. As the

and composi

e I is sedime

) and isotopi

sediment ga

depleted m

gas or mixed

ment gases

os within the

mogenic seep

nga Nagar a

les typical of

NR 59, GNR

GNR 93, G

04 ppb, and

Type A ca

thermogenic

nt. Anothe

I category,

13C1 values

as bacteriall

NR 64, and

esent the isot

eepage and t

ere are no

itional data

ent gases wi

ically enrich

ases with lar

methane carb

d with in si

with elevat

e thermogen

page.

area cover t

f Type-II zo

R 78, GNR 6

GNR 75) wi

d δ13C1 valu

ategory. The

gas migrati

r set of thr

with metha

greater than

ly altered. T

GNR 29)

topic signatu

those with t

indications

to

ith

hed

rge

on

itu

ted

nic

the

one

66,

ith

ues

ese

ng

ree

ane

n -

The

lie

ure

the

of

biogenic

Abrams (

The adsor

of Type-A

4.5). The

GNR 42,

category

280-1582

(C2+) and

considere

near surfa

methane

sample (T

samples (

and Type

unaltered

alteration

the Shri G

gases classif

1996) in the

rbed gases fr

A and Type-

ere are ten

GNR 11, G

, which are

ppb) along

d δ13C1 value

ed as unalter

ace environm

concentratio

Type I) could

(GNR 44, G

I categories

hydrocarbo

on them. A

Ganga Nagar

fied by Bern

Zone A of t

rom Zone B

-I category w

samples (GN

GNR 28, GN

characterize

g with the p

es greater th

red thermog

ment.One sa

on 178 ppb,

d be conside

GNR 07, GN

s and repres

n microseep

As there are n

r samples.

nad et al. (1

the Shri Gan

of the Shri G

with no samp

NR 31, GNR

NR 37, and

ed by large c

presence of

han -40‰. T

genic gas mi

ample (GNR

, and δ13C1

ered as bacte

NR 03, and G

ent the isoto

page and tho

no samples i

976) or Typ

nga Nagar sa

Ganga Naga

ples typical

R 35, GNR 1

GNR 43) a

concentration

ethane plus

These sample

igrating upw

R 52) is of T

values grea

erially altered

GNR49) lie

opic signatur

ose with the

in Type II ca

pe II gases d

amples.

ar area cover

of Type-II z

19, GNR 01,

are typical o

n of C1 (ran

higher hyd

es (Type A)

ward and rea

Type I categ

ater than -3

d. The rema

between th

re between t

influence of

ategory the Z

121

defined by

r the range

zone (Fig.

, GNR 21,

of Type A

ging from

drocarbons

) could be

aching the

gory, with

0‰. This

aining four

he Type A

that of the

f bacterial

Zone B of

122

Fig.44.5 Isotopic c

soil sampNagaur B

classificationples from Z

Basin (after A

n of adsorbeZone A & BAbrams, 199

ed methane eB of the Sh6).

Zone

Zon

extracted frohri Ganga N

e B Sampl

ne A Samp

om the surfaNagar, Bikan

les

ples

ace ner

Genetic c

Schoell (

(terrestria

origin an

used in Fi

and petro

gas forma

dry or de

matter an

associated

petroleum

Md (deep

TT (h) in

matter and

Nine sam

δ13C valu

the hydro

samples (

characteri

GNR 29)

source. T

hydrocarb

Ganga Na

characterizat

(1983a, 198

al or marine

nd occurrenc

ig. 4.6 indica

oleum respec

ation which

eep dry gas

nd (h) symbo

d gases (T)

m. The comp

p migration)

ndicate the

d humic org

mples from Z

ues for both m

ocarbon gase

(GNR 59, G

istic of the s

represent t

he enriched

bons in the s

agar, represe

tion of the

83b) by me

e source), w

ce of these

ate the gas o

ctively. The

follows the

es and (m)

olizes the hu

), are origi

positional sh

and Ms (sh

non-associa

anic matter r

Zone A of t

methane and

es based on

GNR 93, GN

apropelic or

he contribut

isotope valu

samples GN

ents a possib

hydrocarbon

ans of inco

which serves

gases. The

origination fr

term TT is

e principal st

is the mari

umic source

inate predom

ifts due to m

hallow migra

ated gases

respectively

the Shri Gan

d ethane, hav

the Schoell

NR 66and G

rganic matter

tions from

ues and the

NR 69 and G

bility of mix

n gases has

orporating t

s to further

non associ

rom cracking

the second

tage of oil f

ine or sapro

e. The δ13C

minantly fr

migration are

ation), respe

from saprop

y.

nga Nagar a

ve been used

l’s diagram

GNR 75) hav

r where as tw

non-associa

presence of

GNR 99 of

xing of meth

been carrie

their specifi

the underst

ated/associa

g reactions i

stage of the

formation re

opelic sourc

C1–δ13C2 sign

rom the cra

e indicated b

ectively. TT

pelicliptiniti

area, which

d to genetica

(Fig. 4.6).

ve δ13C1-δ13

wo samples

ated gases fr

f ethane and

f Zone A o

hane generate

123

ed out by

ic genesis

anding of

ated terms

n kerogen

ermogenic

esulting in

ce organic

natures of

acking of

by arrows,

T (m) and

c organic

show the

ally typify

The four

3C2 values

(GNR 95,

romhumic

d propane

of the Shri

ed at high

124

tempe

(Man

Five

classi

Three

chara

samp

sapro

The

comp

gases

comp

from

eratures wit

ni et al., 2011

samples fro

ify the hyd

e samples (

acteristic of

ples (GNR 4

opelic source

isotopic da

ponents are

s is a comm

ponents are n

the Shri Gan

Zone

th C2+ hydr

1).

om Zone B o

drocarbon ga

GNR 44, G

the non-ass

42 and GN

e.

ata of samp

generated f

mon phenom

not necessar

nga Nagar a

A Sample

rocarbons du

of the Shri

ases based

GNR 07 and

sociated gas

NR 11) repre

ples from Z

from saprop

menon (Whit

ily co geneti

area.

es

uring migra

Ganga Naga

on the sch

d GNR 37)

ses from hu

esent the no

Zone A&

pelic and hu

ticar, 1994),

ic, and is ob

ation toward

ar area have

hoell’s diagr

have δ13C

umic source,

on-associate

B indicate

umic source

which indi

bserved in th

ds the surfa

e been used

ram (Fig.4.6

1-δ13C2 valu

, whereas tw

ed gases fro

that the g

es. Mixing

icates that g

e alkane gas

ace

to

6).

ues

wo

om

gas

of

gas

ses

Fig. 4.6 P

The stage

and over-

organic m

gas sourc

type and

produced

maturity

qualitativ

alteration

gas can b

versus eth

Plot showingisotope varfrom Zone (after Scho

es of maturity

-mature are

matter in term

e rock at the

maturity o

light hydr

and the δ13

e maturity p

s etc., an es

be determine

hane and of

Zone B g carbon isoriations in et

A & B of tell, 1983a).

y ranging fr

e defined ac

ms of vitrinit

e time of ge

of source i

rocarbon ga

3C concentr

parameter. D

stimate of so

ed by compa

f ethane vers

Samples otope variatithane extractthe Shri Gan

om immatur

ccording to

te reflectanc

eneration and

nfluences th

ases. Base

ration in m

Despite the c

ource rock t

aring the sta

ses propane

ons in methted from thenga Nagar, B

re (bacterial/

Dow (1997

e (Ro) reflec

d migration.

he isotopic

ed on the

methane, δ13C

constraints of

type and ma

able isotope

(Berner and

hane related e surface soiBikaner Nag

/diagenesis)

7). The m

cts the matu

. It is know

compositio

relationship

C can be u

f mixing and

aturity of the

signature of

d Faber, 199

125

to carbon il samples gaur basin

to mature

maturity of

urity of the

wn that the

on of the

between

used as a

d bacterial

ermogenic

f methane

93; James,

126

1983

the F

Gang

relati

The δ

Shri

relati

ethan

that o

whos

four

mixed

The δ

7, GN

sugge

predo

a mix

The p

carbo

extrac

been

their

on m

Naga

; Stahl and

Fig. 4.7 the

ga Nagar ar

onship of Be

δ13C values

Ganga Na

onships of B

ne of the sam

of thermoge

se maturity r

samples (G

d source (Be

δ13C values o

NR 37, GN

esting a tr

ominantly te

xed source.

published m

on isotopic

cted from se

migrated fro

δ13C values

maturity scale

ar area, the so

Carey, 1975

δ13C values

rea are plot

erner and Fa

of adsorbed

agar are pl

Berner and

mples GNR

enic gas der

ranges betwe

GNR 93, GN

erner and Fa

of adsorbed

NR 11 and G

rendsimilar

errestrial sou

model of Bern

composition

ediments are

om deeper,

provide a g

e applied to

ource materi

5; Whiticar a

s of adsorbe

tted togethe

aber (1996).

methane an

lotted toget

Faber (1996

67, GNR 9

rived from p

een 0.5 and 2

NR 75, GNR

aber, 1996).

methane and

GNR 42) sa

to that o

urce rock wit

ner and Fabe

n and matur

e immature, t

more matur

general indic

the surface

ial might be

and Faber, 1

ed methane

er with the

nd ethane fro

ther with

6). The δ13

5, GNR 29

predominant

2.0% vitrinit

R 66, and G

d ethane from

amples of S

of thermog

th one samp

er (1996) is

rity of kerog

the alkane g

e part of the

cation for the

sediment sa

capable of g

1986; Whitic

and ethane

semi-empi

om the Zone

semi empir

3C values of

display a tr

tly terrestria

te reflectanc

GNR 59) rep

m Zone B (G

Shri Ganga

genic gas

ple (GNR 42

dependent o

gen. Howe

ases are pres

e sedimentar

e level of ma

amples of th

generating o

car, 1994).

from the Sh

rical maturi

e A samples

rical maturi

f methane a

rend similar

al source ro

e with anoth

presenting t

GNR 44, GN

Nagar areal

derived fro

2) representi

on the origin

ever the gas

sumed to ha

ry column a

aturity. Bas

he Shri Gan

il and gas.

In

hri

ity

of

ity

and

to

ock

her

the

NR

lso

om

ng

nal

ses

ave

nd

sed

nga

Fig. 4.7 PPlot of carbmaturity resurface soiBikaner-Na

Z

Z

bon isotope elationships fl samples fragaur Basin,

one A Sam

one B Sam

ratios of mfor adsorbedrom Zone A (after Berne

mples

mples

methane versud alkane gasA & B of ther and Faber

us ethane ases extractedhe Shri Ganr, 1996).

127

along with d from the ga Nagar,

128

4.7 C

The

hydro

area

burie

There

bioge

the en

from

in all

therm

in co

sourc

gener

Conclusions

hydrocarbon

ocarbons ad

indicate a

d, terrestria

e is absolut

enic sources

nriched valu

mixing and

l of the sam

mogenic gase

mbination w

ce organic m

rating oil and

n compositi

dsorbed in n

clear signat

al source ro

tely no evid

. However

ues, suggesti

bacterial ox

mples provid

es. Using th

with vitrinite

matter of ads

d gas.

ions and st

near surface

ture of ther

ck with litt

dence of hy

, the altered

ing that thes

xidation. Th

des compelli

he δ13C valu

e reflectance

sorbed alkan

able carbon

sediments

mogenic ga

tle contribut

ydrocarbon g

d isotope va

e gases have

he strong pre

ing evidence

ues as a qual

e and level o

ne gases is i

n isotope va

of the Shri

as generated

tion from m

gases genera

lues have sh

e experience

esence of C2

e of upward

litative matu

of organic m

indicated to

alues of lig

Ganga Nag

d from deep

marine sourc

ated from t

hifted towar

e contributio

2+ alkane gas

d migration

urity paramet

maturation, t

be capable

ght

gar

ply

ce.

the

rds

ons

ses

of

ter

the

of