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
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