fold and thrust belts
DESCRIPTION
Fold and thrust beltsTRANSCRIPT
From: RIES, A. C., BUTLER, R. W. H. & GRAHAM, R. H. (eds) 2007. Deformation of the Continental Crust: TheLegacy of Mike Coward. Geological Society, London, Special Publications, 272, 447–472.0305-8719/07/$15 © The Geological Society of London 2007.
This paper developed from a review of the geo-logical characteristics of fold and thrust belts thatcontain hydrocarbons. The primary data sourcefor this study is the International Explorationand Production Database marketed by IHS,which contains data on production and reservesfor gas, oil and condensate on all producing anddiscovered fields around the world, excludingonshore North America. This is the most com-plete dataset available but it must be appreciatedthat the data are of variable quality. These datahave been supplemented by information from theUS Geological Survey World Petroleum assess-ments where they are available (USGS 2000).Information on the geological characteristics ofthe fold belts in this review has been derived frompublished literature.
The purpose of the paper is to analyse statisti-cally the geological characteristics of hydrocar-bon-productive fold and thrust belts. There havebeen few previous attempts to undertake thistype of analysis (Graham et al. 1997) and mostreviews of hydrocarbon potential are limited ingeographical scope (e.g. Picha 1996; Brookfield& Hashmat 2001). The analysis presented herefocused on a few selected key parameters thatcould potentially affect hydrocarbon prospec-tivity and reserves distribution.
The review of geological characteristicsfocused on structural style observations such as
whether the fold belt is thick or thin skinned,presence of a salt detachment, the presence of asalt seal and syn-orogenic burial. The depth todetachment and the thickness of the competentbeam involved in the deformation were alsonoted but not analysed in detail because of thevariable quality of the data for these factors.Time elements, such as the age of onset of the lastdeformation phase in the fold and thrust belt andthe depositional age of the source rock, were alsorecorded and analysed. There are many otherfactors that could have been considered (e.g. pre-existing basement structures, source rock charac-teristics and age of source maturation). Theseother factors are also important but were beyondthe scope of the dataset for this paper.
In this assessment a fold belt is considered tobe any hydrocarbon province that is dominatedby compressional tectonics resulting from plateconvergence. Fold belts that have their origins asthe contractional toes to extensional systems oncontinental margins have been excluded. Thesefold belts have become important hydrocarbonprovinces as exploration drilling has moved outinto deep-water continental margins (e.g. thedeep-water Gulf of Mexico, deep-water NigerDelta and Brazil). This decision was takenbecause the fundamental tectonic drivingmechanism is different from that in a convergentmargin fold belt even though the geometric
Structural style and hydrocarbon prospectivity in fold and thrust belts:a global review
MARK COOPER
EnCana Corporation, 150 9th Avenue SW, Calgary, Alberta, Canada, T2P 2S5(e-mail: [email protected])
Abstract: A statistical analysis of reserves in fold and thrust belts, grouped by their geologi-cal attributes, indicates which of the world’s fold and thrust belts are the most prolific hydro-carbon provinces. The Zagros Fold Belt contains 49% of reserves in fold and thrust belts andhas been isolated during the analysis to avoid bias. Excluding the Zagros Fold Belt, most ofthe reserves are in thin-skinned fold and thrust belts that have no salt detachment or salt seal,are partially buried by syn- or post-orogenic sediments, are sourced by Cretaceous sourcerocks and underwent their last phase of deformation during the Tertiary. A significant obser-vation is that the six most richly endowed fold and thrust belts have no common set of geologi-cal attributes, implying that these fold belts all have different structural characteristics. Theimplication is that deformation style is a not critical factor for the hydrocarbon endowment offold and thrust belts; other elements of the petroleum system must be more significant. Otherfold and thrust belts may share the structural attributes but the resource-rich fold belts over-whelmingly dominate the total reserves in that group of fold belts. There is nothing intrinsic infold and thrust belts that differentiates them from other oil- and gas-rich provinces other thanthe prolific development of potential hydrocarbon traps. Many of the prolific, proven foldand thrust belts still have significant remaining exploration potential as a result of politicallychallenging access and remote locations.
448 M. COOPER
characteristics are obviously very similar (Rowanet al. 2004).
The IHS dataset includes reserves informa-tion for discovered fields. It takes no accountof the yet-to-find potential of the basins. Thedataset was extracted from the IHS database inMarch 2004 and was then edited to remove allfields that were not located in fold and thrustbelts.
Importance of fold and thrust belts ashydrocarbon provinces
Based on the IHS field reserves data, 14% of theworld’s discovered reserves are in fold and thrustbelts developed at convergent plate boundaries, asignificant proportion of the global reserve base.This percentage appears to be largely indepen-dent of hydrocarbon phase (Fig. 1a). The splitof oil, gas and condensate indicates that thepercentage of oil (59%) in fold and thrust beltsis very similar to the percentage of oil (54%) inall global reserves (Fig. 1b). The conclusion isthat the oil:gas:condensate ratio is roughly thesame in fold and thrust belts as it is for all global
petroleum reserves. One of the difficulties inundertaking a statistical analysis of fold belts isthat the dataset is dominated by the Zagros FoldBelt of Iran, Iraq, Syria and Turkey, whichaccounts for 49% of all the established reserves inthe fold belts around the world.
The USGS global assessment of resourcesin 2000 concluded that fold and thrust beltsamounted to 15% of the global total of undiscov-ered resources (USGS 2000). The implicationis that as known fold and thrust belt reservesconstitute 14% of global reserves, the yet-to-findis almost identically proportioned based on thetectonic setting. The conclusion is that fold andthrust belts represent an absolutely averagesample of global hydrocarbon resources, there isnothing statistically distinctive about fold andthrust belts; they are oil prone because they are avery good sample of an oil-prone world.
Hydrocarbon discoveries in fold and thrustbelts date back to the earliest days of oil explora-tion in the late 19th and early 20th centuries. Theprimary reason for these discoveries was thatearly drilling tended to focus on structurallysimple anticlines that could be mapped using thesurface geology, which mimicked the subsurface
Fig. 1. (a) Distribution of hydrocarbon type in fold and thrust belts; total volumes are indicated on the pie chartsegments. The yellow segments of the smaller pie charts indicate the proportions of global reserves in fold andthrust belts for each hydrocarbon type with the actual percentage labelled. (b) Comparison of hydrocarbon typesplit between fold and thrust belts and all global reserves.
449PROSPECTIVITY IN FOLD AND THRUST BELTS
structure at the reservoir level. This ultimatelyled to the discovery of super-giant oilfields inthe Zagros Fold Belt of Iran and Iraq, such asKirkuk, in the early decades of the 20th century.However, 80% of the giant fields were discoveredafter 1950 because of the challenges of exploringin structurally complex terrain. For example, inWyoming the first discovery was in 1900, but itwas not until the late 1970s that the first giantfield (Whitney Canyon–Carter Creek) was dis-covered (Lamerson 1982). Structural complexityis a major problem when exploring many foldand thrust belts because the surface structuralexpression is commonly decoupled from the sub-surface structural geometry at the reservoir level.Seismic imaging and ability to accurately mapcomplex subsurface structures are therefore thekeys to exploration. The reasons for geophysicalexploration and field mapping are obvious, butstructural and tectonic analysis can also improverisk assessment. Detailed structural analysisallows identification of prevalent geometricpatterns of faulting and folding. Dahlstrom(1970), Boyer & Elliott (1982) and Suppe (1983)used structural analysis to establish general rulesof structural cross-section interpretation. Theserules, when combined with field and geophysicaldata, can tightly constrain the location, shape,
and size of structural traps (e.g. Cooper et al.2004).
The Alberta foothills provide a well-documented example of how the evolution ofstructural models has affected explorationsuccess over several generations (Gallup 1975;Ower 1975; Stockmal et al. 2001). Otherexamples are the Papua New Guinea fold andthrust belt (Hobson 1986; Hill 1991; Hill et al.2004) and the Dagestan fold and thrust belt(Sobornov 1994). Revisions of structural modelparadigms still continue to yield explorationsuccess even in mature fold and thrust belts (e.g.in the Utah overthrust belt; Moulton & Pinnell2005). The common progression in the explora-tion of most fold and thrust belts is from theexploration of the simpler, near-surface struc-tures to the deeper, more complex, sub-thruststructures, which in many cases hold the largerprizes (e.g. the Alberta foothills; Gallup 1975;Ower 1975; Stockmal et al. 2001).
Fold and thrust belts included in this review
The 55 fold and thrust belts included in thisreview are shown in Figure 2 and Table 1. Thereserves data in the IHS database are organized
Fig. 2. Location map of fold and thrust belts included in this study. Each fold and thrust belt is labelled with areference number that is included in Table 1. The locations are colour coded by the predominant hydrocarbontype, and those symbols that are outlined in black indicate fold and thrust belts that Mike Coward worked onduring his career.
450 M. COOPERT
able
1. S
umm
ary
of d
ata
for
fold
and
thru
st b
elts
Fol
d be
ltC
ount
ryO
roge
nT
hin
orB
urie
dSa
ltSa
ltSa
ltO
nset
of l
ast
Com
pe-
Dep
th to
Sour
ce a
geN
o. o
fO
il +
Gas
Tot
alO
ilM
at-
thic
kse
alde
tach
-de
tach
-de
form
atio
nte
ntde
tach
-be
lt in
Con
d-(m
mre
serv
eor
urit
ysk
inne
dm
ent
men
tbe
amm
ent
Fig
. 2en
sate
boe)
(mm
Gas
inde
xag
e(k
m)
(km
)(m
m b
bl)
boe)
Alt
un S
han
FB
Chi
naH
imal
ayan
Thi
ckN
oP
arti
alN
oP
lioce
neO
ligo-
Mio
cene
4466
455
412
18O
Arc
tic
FB
Can
ada
Arc
tic
Thi
nN
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ocen
eP
alae
ocen
e2
30
3O
Ass
am F
BB
angl
ades
h, I
ndia
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imal
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4713
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550
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5033
929
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ine
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00
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e1–
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ene
286
4147
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aste
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cene
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ate
Cre
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79
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24T
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Gis
sar
FB
Uzb
ekis
tan,
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Par
tN
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art
Mio
cene
110
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ly J
uras
sic
4111
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4724
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Yes
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now
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ew Z
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/No
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tiar
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ate
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t54
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ther
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penn
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tial
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tial
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cene
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lio-P
leis
t30
105
1841
1946
G
451PROSPECTIVITY IN FOLD AND THRUST BELTS
Tab
le 1
. Con
tinu
ed
Fol
d be
ltC
ount
ryO
roge
nT
hin
orB
urie
dSa
ltSa
ltSa
ltO
nset
of l
ast
Com
p-D
epth
toSo
urce
age
No.
of
Oil
+G
asT
otal
Oil
Mat
-th
ick
seal
deta
ch-
deta
ch-
defo
rmat
ion
eten
tde
tach
-be
lt in
Con
d-(m
mre
sevr
eor
urit
ysk
inne
dm
ent
men
tbe
amm
ent
Fig
. 2en
sate
boe)
(mm
Gas
inde
xag
e(k
m)
(km
)(m
m b
bl)
boe)
Nor
ther
n R
ocki
es F
BC
anad
aR
ocki
esT
hin
No
No
No
Pal
aeoc
ene
Cam
bria
n3
235
2926
4O
Om
an F
BO
man
Zag
ros
Thi
ckY
esN
oN
oM
ioce
neL
ate
Pre
cam
b37
65
11O
15P
apua
n F
BP
apua
New
New
Bot
hN
oN
oN
oM
ioce
ne0
5Ju
rass
ic53
770
2245
3015
GG
uine
aG
uine
anP
otw
ar F
BP
akis
tan
Him
alay
anT
hin
No
No
Yes
EoC
amb
Pal
aeoc
ene
33–
5P
alae
ocen
e40
401
390
791
GP
yren
ees
Fra
nce,
Spa
inA
lpin
eT
hin
No
No
Yes
Tri
asE
ocen
e1
>3
Lat
e Ju
rass
ic26
641
1921
2562
GQ
ilian
Sha
n F
BC
hina
Him
alay
anT
hick
No
No
No
Jura
ssic
Ear
ly P
alae
oz45
022
22G
Ref
orm
a F
BM
exic
oM
exic
anT
hin
Par
tial
No
Yes
Cal
lov
Pal
aeoc
ene
1–4
5–6
Mid
dle
Cre
t9
5076
510
641
6140
6O
Roc
kies
FB
Can
ada
Roc
kies
Thi
nN
oY
esN
oP
alae
ocen
eE
arly
Car
b4
1100
936
2036
G13
San
Ber
nard
o F
BA
rgen
tina
And
ean
Thi
ckN
oN
oN
oM
ioce
ne>
3>
10E
arly
Cre
t22
4637
984
5621
O46
Saya
n-T
uva
FB
Rus
sia
Eas
t Sib
eria
nT
hick
Yes
Yes
No
Jura
ssic
>10
Lat
e P
reca
mb
4326
765
4068
07G
61Se
ram
FB
Indo
nesi
aB
anda
Arc
Thi
ckN
oN
oN
oM
ioce
neL
ate
Tri
assi
c50
211
22O
Sier
ra M
adre
Ori
enta
leM
exic
oM
exic
anT
hin
No
No
Yes
Tri
asC
ampa
nian
>5
>5
Lat
e Ju
rass
ic6
258
60G
Sout
hern
Alp
sIt
aly
Alp
ine
Thi
nY
esY
esY
esT
rias
Eoc
ene
1–4
>2
Lat
e T
rias
sic
2729
163
192
GSo
uthe
rn A
penn
ines
Ital
yA
lpin
eT
hin
Par
tial
No
Par
tial
Mio
cene
2L
ate
Tri
assi
c31
906
531
1437
OSu
laim
an F
BP
akis
tan
Him
alay
anT
hin
No
No
No
Pal
aeoc
ene
1>
5E
arly
Cre
t39
3228
1928
52G
Tai
wan
FB
Tai
wan
Tai
wan
Thi
nN
oN
oN
oP
lioce
neU
nkno
wn
4950
443
493
GT
ien
Shan
FB
Chi
naH
imal
ayan
Thi
ckY
es/P
art
No/
Yes
No
Olig
ocen
e3-
5>
10L
ate
Per
mia
n42
1121
425
7613
789
O69
Tim
or F
BE
ast T
imor
Ban
da A
rcT
hick
No
No
No
Mio
cene
Lat
e T
rias
sic
510
00
OT
rini
dad
FB
Tri
nida
d an
dC
arib
bean
Thi
nN
oN
oN
oP
lioce
ne3
3–5
Lat
e C
ret
1325
2056
730
87O
47T
obag
oU
caya
li F
BP
eru
And
ean
Thi
ckN
oN
oN
oM
ioce
ne1
>10
Lat
e T
rias
sic
1657
6412
0G
Ura
l FB
Rus
sia
Ura
lsT
hin
No
Yes
No
Per
mia
n4
4L
ate
Dev
3581
728
9437
11G
4U
tah
Wyo
min
g F
BU
SAR
ocki
esT
hin
No
Yes
No
Pal
aeoc
ene
Mid
dle
Cre
t5
600
3167
3767
GV
erac
ruz
FB
Mex
ico
Mex
ican
Thi
nP
arti
alN
oY
esL
ate
Cre
t5
5L
ate
Cre
t7
130
242
372
GY
unan
Gui
zhou
FB
Chi
naH
imal
ayan
Thi
nN
oY
esN
oT
erti
ary
>5
Lat
e T
rias
sic
460
2820
2820
G50
Zag
ros
FB
Iran
, Ira
q,Z
agro
sB
oth
Par
tial
Yes
No/
Par
tP
lioce
ne3
8M
iddl
e C
ret
3615
2170
8033
823
2508
O24
Syri
a, T
urke
y
FB
, fol
d be
lt. S
trat
igra
phic
age
s ar
e ab
brev
iate
d as
nec
essa
ry; m
m b
oe, m
illio
n ba
rrel
s oi
l equ
ival
ent;
mm
bbl
, mill
ion
barr
els.
452 M. COOPERT
able
2. K
ey fo
ld b
elt r
efer
ence
s
Fol
d be
ltR
efer
ence
sF
old
belt
Ref
eren
ces
Alt
un S
han
Fol
d B
elt
Gu
& D
i 198
9; Q
inm
in &
Cow
ard
1990
;N
ew Z
eala
ndK
nox
1982
; Pila
ar &
Wak
efie
ld 1
984;
Jin
et a
l. 20
02C
ollie
r &
Joh
nsto
n 19
90A
rcti
c F
old
Bel
tH
arri
son
& B
ally
198
8N
orth
ern
Ape
nnin
esM
atta
velli
et a
l. 19
93; Z
appa
terr
a 19
94; C
owar
d et
al.
1999
Ass
am F
old
Bel
tB
asti
a et
al.
1993
; Mal
lick
et a
l. 19
97;
Nor
ther
n R
ocki
esY
ose
et a
l. 20
01K
ent e
t al.
2002
Fol
d B
elt
Atl
as F
old
Bel
tB
eauc
ham
p et
al.
1996
, 199
9O
man
Fol
d B
elt
Gra
ntha
m e
t al.
1987
; Rob
erts
on e
t al.
1990
;M
ount
et a
l. 19
98B
alka
n F
old
Bel
tK
arag
jule
va &
Can
kov
1974
;P
apua
n F
old
Bel
tH
ill e
t al.
2004
Foo
se &
Man
heim
197
5B
angg
ai F
old
Bel
tSh
aw &
Pac
kham
199
2P
otw
ar F
old
Bel
tK
han
et a
l. 19
86; P
enno
ck e
t al.
1989
; Dol
an 1
990
Ben
i Fol
d B
elt
Illic
h et
al.
1984
; Bab
y et
al.
1995
Pyr
enee
sE
spit
alie
& D
roue
t 199
2; B
ourr
ouilh
et a
l. 19
95;
Le
Vot
et
al. 1
996
Bet
ic F
old
Bel
tB
lank
ensh
ip 1
992
Qili
an S
han
Fol
d B
elt
Che
n et
al.
1987
; Guo
& Z
hang
198
9; U
lmis
hek
1992
Bro
oks
Ran
geH
ubba
rd e
t al.
1987
; AN
WR
Ass
essm
ent
Ref
orm
a F
old
Bel
tP
eter
son
1983
; Gon
zale
z-G
arci
a &
Hol
guin
-Qui
none
s 19
91;
Tea
m U
S G
eolo
gica
l Sur
vey
1998
; Col
e et
al.
1998
Sant
iago
& B
aro
1992
Car
path
ian
Thr
ust B
elt
Rou
re e
t al.
1993
; Kre
jci e
t al.
1996
; Sla
czka
199
6R
ocki
es F
old
Bel
tB
ally
et a
l. 19
66; C
oope
r 20
00; S
tock
mal
et a
l. 20
01C
auca
sus
Fol
d B
elt
Ulm
ishe
k 19
90, 2
001;
Abr
ams
& N
arim
anov
199
7Sa
n B
erna
rdo
Fol
d B
elt
Hom
ovc
et a
l. 19
95; P
eron
i et a
l. 19
95C
haco
Fol
d B
elt
Dun
n et
al.
1995
; Mor
etti
et a
l. 19
96Sa
yan-
Tuv
a F
old
Bel
tK
onto
rovi
ch e
t al.
1990
Cub
an F
old
Bel
tB
all e
t al.
1985
; Ech
evar
ria-
Rod
rigu
ez e
t al.
1991
;Se
ram
Fol
d B
elt
Cou
rten
ey e
t al.
1988
; Syk
ora
2000
Cam
pos
et a
l. 19
96C
uyo
Fol
d B
elt
Vill
ar &
Pue
ttm
ann
1990
; Del
lape
& H
eged
us 1
995;
Sier
ra M
adre
Ori
enta
leG
onza
lez-
Gar
cia
& H
olgu
in-Q
uino
nes
1991
;U
liana
et a
l. 19
95M
arre
tt &
Ara
nda
1999
; Egu
iluz
2001
Dag
esta
n F
old
Bel
tSo
born
ov 1
994
Sout
hern
Alp
sR
oede
r 19
92; A
nelli
et a
l. 19
96D
inar
ides
Zap
pate
rra
1994
; Vel
aj e
t al.
1999
Sout
hern
Ape
nnin
esP
ieri
& M
atta
velli
198
6; B
ally
et a
l. 19
88; Z
appa
terr
a 19
94E
aste
rn A
lps
Mul
ler
et a
l. 19
88; O
rtne
r &
Sac
hsen
hofe
r 19
96;
Sula
iman
Fol
d B
elt
Raz
a et
al.
1989
; Dol
an 1
990;
Jad
oon
et a
l. 19
94Z
imm
er &
Wes
sely
199
6E
aste
rn C
ordi
lller
aK
ronm
an e
t al.
1995
; Rey
es e
t al.
2000
Tai
wan
Fol
d B
elt
Supp
e 19
80, 1
981
Gis
sar
Fol
d B
elt
Bro
okfi
eld
& H
ashm
at 1
978;
Kha
in e
t al.
1991
Tie
n Sh
an F
old
Bel
tW
ang
et a
l. 19
92; L
i et
al. 1
996;
Gao
& Y
e 19
97G
uajir
a P
rism
Rui
z et
al.
2000
Tim
or F
old
Bel
tC
harl
ton
et a
l. 19
91; S
haw
& P
ackh
am 1
992
Jura
Fol
d B
elt
Lau
bsch
er 1
962;
Mas
cle
1994
Tri
nida
d F
old
Bel
tP
ersa
d 19
85; R
ohr
1991
; Req
uejo
et a
l. 19
94K
irth
ar F
old
Bel
tD
olan
199
0; R
obin
son
et a
l. 19
99; S
chel
ling
1999
Uca
yali
Fol
d B
elt
Illic
h et
al.
1985
; Mat
halo
ne &
Mon
toya
199
5L
lano
s F
ooth
ills
Caz
ier
et a
l. 19
95; C
oope
r et
al.
1995
;U
ral F
old
Bel
tM
aste
rs &
Pet
erso
n 19
81; D
iken
shte
yn 1
986;
Dro
zd &
Pig
gott
199
6U
lmis
hek
1988
Loe
i-P
hetc
habu
n F
old
Bel
tC
oope
r et
al.
1989
; Sat
taya
rak
et a
l. 19
89U
tah
Wyo
min
g F
old
Bel
tL
amer
son
1982
; War
ner
1982
Mad
re d
e D
ios
Fol
d B
elt
Mat
halo
ne &
Mon
toya
199
5; M
oret
ti e
t al.
1996
Ver
acru
z F
old
Bel
tM
oran
-Zen
teno
199
4; J
enne
tte
et a
l. 20
03M
alar
gue–
Agr
io F
old
Bel
tM
ello
et a
l. 19
94; U
rien
& Z
ambr
ano
1994
;Y
unan
Gui
zhou
Fol
d B
elt
Che
n et
al.
1994
; Ryd
er e
t al.
1994
Man
ceda
& F
igue
roa
1995
Mat
urin
Fol
d B
elt
Tal
ukda
r et
al.
1988
; Rou
re e
t al.
1994
;Z
agro
s F
old
Bel
tB
orde
nave
& B
urw
ood
1990
; Bey
doun
et a
l. 19
92;
Par
naud
et a
l. 19
95B
erbe
rian
199
5N
E C
arib
bean
Fol
d B
elt
Spee
d et
al.
1991
; Bab
aie
et a
l. 19
92;
Wal
lace
et a
l. 20
03
453PROSPECTIVITY IN FOLD AND THRUST BELTS
by basin and sub-basin but not by fold belt. Thedataset had to be carefully reviewed and edited toextract only fields that are within fold belts andto assign each field to the appropriate fold belt.The initial dataset of nearly 22 000 fields wasreduced to just over 2900 fields. The key refer-ences for the geological attributes of each foldbelt are listed in Table 2. In several cases anattribute has been recorded as partial (Table 1),which means that it is present only over a portionof the (sub)basin area. In addition, many of thefold and thrust belts span a number of
(sub)basins that may have different attributes;in such cases the reserves in that (sub)basin havebeen appropriately attributed and the fold beltnoted as having a mixture of characteristicfactors (Table 1). Those fold and thrust belts thatMike Coward worked on during his career arehighlighted in Figure 2, which illustrates theextent of his influence in shaping the understand-ing of many of these fold and thrust belts.
The database includes 37 fold and thrust beltsthat contain giant fields (>250 million barrels ofoil equivalent mm boe); 25 of the fold and thrust
Fig. 3. Graph of the distribution of hydrocarbon reserves in fold and thrust belts, in order of decreasing totalreserves. Green, oil and condensate in mm bbl; red, gas in mm boe. The Zagros Fold Belt is excluded from thechart; the inset chart represents the fold belts vertically below at a larger scale.
Fig. 4. Location map of orogenic belt groupings for the fold and thrust belts included in this study (Table 1).
454 M. COOPER
belts have reserves of more than 1 billion barrelsof oil equivalent (bn boe) and 16 have reserves ofmore than 3 bn boe (Fig. 3 and Table 1). The foldbelts which lie in the last category include theZagros Fold Belt (one of the world’s most pro-lific hydrocarbon provinces) the Maturin Basinof East Venezuela, the Reforma Fold Belt inMexico, the Caucasus and the Tien Shan inChina (Fig. 3). The thrust and fold belts with thelargest total reserves are mostly dominated byoil, with the notable exception of the Chaco FoldBelt (Fig. 3 and Table 1).
The fold and thrust belts can be convenientlygrouped into the orogenic systems within which
they are located (Fig. 4); this allows for the analy-sis of the established reserves in the orogenicbelts. The orogenic belts are ranked by totalreserves in Figure 5; the graphs show the split ofoil, gas and condensate expressed in billions ofbarrels (bn bbl) or billions of barrels of oilequivalent for gas (bn boe). The dataset is domi-nated by the Zagros Fold Belt, which accountsfor 49% of all the established reserves in the foldbelts around the world and has four times thereserves of the next largest orogenic belt, theAndean Orogen. This strongly skews any obser-vations and conclusions drawn from the analysisof the reserves data and for this reason the
Fig. 5. Logarithmic graph of the distribution of hydrocarbon reserves in fold and thrust belts grouped byorogenic belt (Table 1), in order of decreasing total reserves. Green, oil in bn bbls; yellow, condensate in bn bbl;red, gas in bn boe.
Fig. 6. Graph of the distribution of hydrocarbon reserves in fold and thrust belts grouped by the age of onset ofthe last phase of deformation (Table 1). Green, oil and condensate in bn bbl; red, gas in bn boe. The inset chartshows the older ages of deformation at a larger scale.
455PROSPECTIVITY IN FOLD AND THRUST BELTS
Zagros Fold Belt has been isolated on subse-quent graphs. Oil is the dominant hydrocarbontype in the six orogenic belts with the largestreserves; the exception is the Andean Orogen,where gas is slightly more significant.
Analysis of fold and thrust belt reserves
Reserves by age of deformation
The reserves can be analysed by the age of theonset of the last phase of deformation in the foldand thrust belt (Fig. 6); any earlier deformationphases are ignored. This shows the Plioceneage deformation of the Zagros Fold Belt to bedominant. The Miocene and Palaeocene are thesecond and third most important times of hydro-carbon-rich fold and thrust belt development. Allother times of fold and thrust belt last phasedeformation are volumetrically insignificant bycomparison. Clearly the preservation potentialof a fold and thrust belt is enhanced when it isrelatively young, but as the age of deformationbecomes progressively greater, there is morechance of the fold and thrust belt being upliftedand eroded (e.g. the Appalachian fold and thrustbelt) or buried to uneconomic depths beneatha later passive margin (e.g. the Variscan foldand thrust belt beneath the European Atlanticmargin).
With the exception of the Urals and Sayan-Tuva fold belts, all of the 16 provinces with>3 bn boe had their last phase of deformation inthe Tertiary. The age of the last phase of defor-mation is important when considering the likeli-hood of post-charge modification of traps andpotential seal failure. It is therefore not surpris-ing that both the Ural and Sayan-Tuva fold beltshave salt seals that have helped to maintain trapintegrity over lengthy periods of geological time.The inset graph of fold and thrust belts withpre-Tertiary ages of deformation shows that theyare strongly gas dominated (Fig. 6). This is not asurprise, as the greater the age of deformation,the more likely it is that the source rock willhave entered the gas window as a result of post-orogenic burial.
Source rock age in fold and thrust belts
The age of the primary source rock in each ofthe fold and thrust belts has also been analysed.Cretaceous source rocks, which also source theZagros Fold Belt, account for nearly 75% of allfold and thrust belt reserves (Fig. 7). Even withthe Zagros Fold Belt excluded, Cretaceoussource rocks are still the volumetrically most
Fig. 7. Graphs of the distribution of hydrocarbonreserves in fold and thrust belts grouped by the age ofthe source rock (Table 1). The pie chart shows thetotal reserves for each grouping of source rock age.The other graph shows the distribution ofhydrocarbon type for each grouping of source rockage. Green, oil and condensate in bn bbl; red,gas in bn boe.
significant and have an oil:gas ratio of 70:30. Thenext most significant source rocks are the Oli-gocene (dominantly oil), the Devonian (primarilygas), the Jurassic (oil:gas ratio 50:50) and thePermian (oil dominant). Not surprisingly, thepre-Mesozoic source rocks tend to have pro-duced more gas than oil reserves, which does notnecessarily imply any correlation with the typeof source rock. It probably has more to do withthe greater likelihood of being more thermallymature as a result of greater burial since deposi-tion. The geographical distribution of sourcerock ages in fold and thrust belts are shown inFigure 8.
Reserves by deformation style
Deformation style strongly influences the distri-bution of reserves within fold and thrust belts.The parameters captured in the summary table(Table 1) that influence deformation style includewhether the fold and thrust belt is thin or thickskinned, the presence of a salt seal, the presenceof a salt detachment and whether the fold andthrust belt is buried or not.
Giant fields are often hosted by simple struc-tures in fold and thrust belts. The key factors indetermining the likelihood of their existence
456 M. COOPER
Fig. 9. Cross-sections through examples of thin- and thick-skinned fold and thrust belts to illustrate characteristicgeometries. The thin-skinned Chaco Basin section is modified from Moretti et al. (1996); the thick-skinnedQuaidam Basin section from the Altun Shan fold belt is modified from Qinmin & Coward (1990).
Fig. 8. Map showing the distribution of the age of the source rock in fold and thrust belts reviewed (Table 1).Where more than one age of source rocks is a significant contributor both colours are shown on the symbol asdiagonal stripes.
457PROSPECTIVITY IN FOLD AND THRUST BELTS
geographical distribution of thin-skinned, thick-skinned and mixed fold and thrust belts is shownin Figure 11.
Fig. 11. Map showing the distribution of deformation style in the fold and thrust belts reviewed (Table 1). Wheremore than one deformation style is present the symbol shows diagonally striped colours representing the twodeformation styles.
Fig. 10. Graph of the distribution of hydrocarbonreserves in fold and thrust belts grouped by thin- orthick-skinned deformation style (Table 1). The piechart shows the total reserves for each grouping ofdeformation style; total volumes are labeled on the piechart segments. The smaller pie charts indicate theproportions of oil and condensate to gas for eachdeformation style with the percentage of oil andcondensate labelled. Green, oil and condensate; red,gas.
include the presence of a thick competent unit(e.g. >2 km of carbonate) in the hanging wall,thus encouraging simple box folds (e.g. theZagros Fold Belt), and the depth to detachmentfor the system. These data have been captured inTable 1 but have not been analysed in detail.
Thin-skinned, thick-skinned or both thin- andthick-skinned. For the analysis of the thin- orthick-skinned style of deformation the dominantstyle of the productive structures has beenconsidered, as opposed to considering the style ofall structures in the fold and thrust belt. Thedeformation is considered to be thick skinnedif it involves a significant thickness of the crust(Coward 1983), which usually implies that thebasal detachment is within crystalline basement(Cooper 1996). Figure 9 shows type examples ofthin- and thick-skinned deformation in fold andthrust belts.
Thin-skinned deformation accounts forc. 60% of reserves in fold and thrust belts exclud-ing the Zagros Fold Belt (Fig. 10). The ZagrosFold Belt shows both thick- and thin-skinneddeformation, based on the recent work of Blancet al. (2003). The thick-skinned fold and thrustbelts have a slightly higher oil:gas ratio in com-parison with thin-skinned fold and thrust belts,but neither differs significantly from the overalloil:gas ratio in fold and thrust belts (Fig. 1). The
458 M. COOPER
Burial of fold belts. Another aspect of geometryand deformation history and style that wasexamined is whether or not the fold and thrustbelt has been buried by either syn- or post-depositional sediments, exemplified by theNorthern Apennines (Fig. 12). Normally, thrust-ing is associated with elevation and the simplestructures of the frontal zones form last and willpost-date the significant loading and hydrocar-bon generation. Burial, however, encouragesmaturation of the source after trap formation ifthe source was either immature or early matureduring the deformation.
Partially buried fold and thrust belts domi-nate the reserves distribution even when thepartially buried Zagros Fold Belt is excluded
from consideration (Fig. 13). Oil is the mostimportant hydrocarbon type in partially buriedfold and thrust belts (Fig. 3). Fold and thrustbelts, that are buried, are strongly dominated byoil, and those that are not buried are dominatedby gas (Fig. 13). The geographical distribution ofthe different classes of burial by sediment in foldand thrust belts is shown in Figure 14.
Fig. 13. Graph of the distribution of hydrocarbonreserves in fold and thrust belts grouped by burial state(Table 1). The pie chart shows the total reserves foreach grouping of burial state; total volumes are labeledon the pie chart segments. The smaller pie chartsindicate the proportions of oil and condensate to gasfor each burial state with the percentage of oil andcondensate labelled. Green, oil and condensate; red,gas.
Fig. 14. Map showing the distribution of burial state in the fold and thrust belts reviewed (Table 1). Where morethan one burial state is present the symbol shows diagonally striped colours representing the two burial states.
Fig. 12. Cross-section through the NorthernApennines, an example of a buried fold and thrustbelt; modified from Pieri (1992).
459PROSPECTIVITY IN FOLD AND THRUST BELTS
thrust belts that have salt top seals are verystrongly gas-prone, which is possibly due to theeffectiveness of the salt seal in retaining a gascharge, and the majority have a Palaeogene orearlier final phase of deformation. In fold andthrust belts with no salt seal, oil is the dominanthydrocarbon type (Fig. 15). The geographicaldistribution of salt seals in fold and thrust belts isshown in Figure 16.
Salt detachments in fold and thrust belts. The pres-ence of a salt detachment in a fold and thrust belthas a strong influence on the deformation style,tending to favour thin-skinned structures as aresult of the efficiency of the detachment(Fig. 17). The Zagros Fold Belt is problematic inthis analysis, as a recent paper (Blanc et al. 2003)suggested that the Vendian–Cambrian HormuzSalt is present only in the SE part of the ZagrosFold Belt, which is less petroliferous (Fig. 18).Fold and thrust belts with no salt detachmentdominate the reserves, excluding the Zagros FoldBelt; in fold and thrust belts with no salt detach-ment the oil:gas ratio is c. 50:50, in contrast tofold and thrust belts with a salt detachment,where the oil:gas ratio is about 80:20 (Fig. 18).This could perhaps be due to the salt detachmentinhibiting the migration of gas from secondarydeeper and more mature source rock horizonsbeneath the salt detachment into the trapslocated above the detachment. The geographical
Salt Seals in fold and thrust belts. The presenceof a salt top seal in fold and thrust belts is notsignificant in the reserves distribution, with thenotable exception of the Zagros Fold Belt, whichis oil dominant (Fig. 15). The other fold and
Fig. 16. Map showing the distribution of seal type in the fold and thrust belts reviewed (Table 1). Where morethan one seal type is present the symbol shows diagonally striped colours representing the two seal types.
Fig. 15. Graph of the distribution of hydrocarbonreserves in fold and thrust belts grouped by presence ofsalt seal (Table 1). The pie chart shows the totalreserves for each grouping of seal type; total volumesare labelled on the pie chart segments. The smaller piecharts indicate the proportions of oil and condensateto gas for each seal type with the percentage of oil andcondensate labelled. Green, oil and condensate; red,gas.
460 M. COOPER
distribution of salt detachments in fold andthrust belts is shown in Figure 19.
Reserves distribution based on deformation style.The four factors discussed above in this sectionall contribute to the deformation styles observedin fold and thrust belts. A classification of foldand thrust belts based on these four factors hasbeen developed and the fold and thrust beltsreviewed have been assigned to the appropriateclassification category. The classification uses thefactors in the following sequence and coded by asingle letter as indicated in parentheses, thick- orthin-skinned (K, thick; T, thin; B, both), burial(Y, yes; N, no; P, partial), salt seal (Y, yes; N, no;P, partial) and salt detachment (Y, yes; N, no; P,partial). The reserves distribution is shown inTable 3 and geographical distribution of the
Fig. 17. Cross-section through the Potwar Fold Belt, an example of a fold and thrust belt with a salt detachment;modified from Pennock et al. (1989).
Fig. 18. Graph of the distribution of hydrocarbonreserves in fold and thrust belts grouped by presence ofsalt detachment (Table 1). The pie chart shows thetotal reserves for each grouping of detachment type;total volumes are labelled on the pie chart segments.The smaller pie charts indicate the proportions of oiland condensate to gas for each detachment type withthe percentage of oil and condensate labelled. Green,oil and condensate; red, gas.
different categories is shown in Figure 20. TheZagros Fold Belt is dominated by oil and is boththick- and thin-skinned, partially buried, has asalt seal and no salt detachment and thus has theclassification code of BPYN.
Other categories that favour significant oilreserves are fold and thrust belts that are:
• thin-skinned (T), partially buried (P), nosalt seal (N), salt detachment (Y) (TPNY50.9 bn bbls); >99% of these reserves are in theReforma Fold Belt (Table 1);
• thin-skinned (T), buried (Y), no salt seal (N),no salt detachment (N) (TYNN 22.6 bn bbls);>99% of these reserves are in the Maturin FoldBelt (Table 1);
• thick-skinned (K), partially buried(P), nosalt seal (N), no salt detachment (N) (KPNN18.8 bn bbls); all of these reserves are in a part ofthe Caucasus Fold Belt (Tables 1 and 3);
• thick-skinned (K), buried (Y), no salt seal(N), no salt detachment (N) (KYNN 17.2 bnbbls); reserves are in parts of the Caucasus andTien Shan Fold Belts (Tables 1 and 3).
Significant gas reserves are found in thefollowing fold and thrust belt types:
• thin-skinned (T), unburied (N), no saltseal (N), no salt detachment (N) (TNNN 20.5 bnboe); >60% of these reserves are in the ChacoFold Belt (Table 1);
• thin-skinned (T), buried (Y), no salt seal (N),no salt detachment (N) (TYNN 17.4 bn boe);>99% of these reserves are in the Maturin FoldBelt (Table 1);
• thick-skinned (K), partially buried (P), nosalt seal (N), no salt detachment (N) (KPNN11.0 bn boe); all of these reserves are in a part ofthe Caucasus Fold Belt (Tables 1 and 3);
• thin-skinned (T), partially buried (P), no saltseal (N), salt detachment (Y) (TPNY 10.9 bnboe); >99% of these reserves are in the ReformaFold Belt (Table 1).
Clearly, the categories that favour large oilreserves also favour large gas reserves and interms of total reserves the top four categoriesexcluding the Zagros Fold Belt are as follows.
461PROSPECTIVITY IN FOLD AND THRUST BELTS
Fig. 19. Map showing the distribution of detachment type in the fold and thrust belts reviewed (Table 1). Wheremore than one detachment type is present the symbol shows diagonally striped colours representing the twodetachment types.
Fig. 20. Map showing the distribution of deformation style in the fold and thrust belts reviewed (Table 3). Wheremore than one deformation style is present the symbol shows only the colour of the predominant deformationstyle.
462 M. COOPER
Tab
le 3
. Cla
ssif
icat
ion
of d
efor
mat
ion
styl
e
Thi
ck o
r th
inB
uria
lSa
lt s
eal
Salt
det
achm
ent
Fol
d be
lts
Typ
eO
il &
Gas
skin
ned
code
cond
ensa
tere
cove
rabl
ere
cove
rabl
e(m
m b
oe)
(mm
bbl
)
Thi
ck &
thi
nU
nbur
ied
No
salt
sea
lN
o sa
lt d
etac
hmen
tL
lano
s (p
art)
, Mal
argu
e–A
grio
,B
NN
N37
6338
02sk
inne
dP
apua
n, N
ew Z
eala
nd (p
art)
Thi
ck &
thi
nU
nbur
ied
No
salt
sea
lSa
lt d
etac
hmen
tA
tlas
BN
NY
9410
7sk
inne
dT
hick
& t
hin
Unb
urie
dP
arti
al s
alt
Par
tial
sal
t det
achm
ent
Gis
sar
(par
t)B
NP
P24
410
72sk
inne
dse
alT
hick
& t
hin
Par
tial
ly b
urie
dSa
lt s
eal
No
salt
det
achm
ent
Zag
ros
BP
YN
1521
7080
338
skin
ned
Thi
ck &
thi
nB
urie
dN
o sa
lt s
eal
No
salt
det
achm
ent
New
Zea
land
(par
t)B
YN
N36
68sk
inne
d
Thi
ck s
kinn
edU
nbur
ied
No
salt
sea
lN
o sa
lt d
etac
hmen
tC
uyo,
E C
ordi
llera
(par
t), S
an B
erna
do,
KN
NN
7263
1606
Uca
yali,
Ser
am, T
imor
, Gis
sar
(par
t),
Loe
i Phe
tcha
bun,
Qili
an S
han
Thi
ck s
kinn
edU
nbur
ied
Par
tial
sal
t sea
lN
o sa
lt d
etac
hmen
tA
ltun
Sha
nK
NP
N66
455
4T
hick
ski
nned
Par
tial
ly b
urie
dN
o sa
lt s
eal
No
salt
det
achm
ent
Cau
casu
s (p
art)
KP
NN
1882
111
002
Thi
ck s
kinn
edP
arti
ally
bur
ied
Par
tial
sal
t sea
lN
o sa
lt d
etac
hmen
tD
ages
tan
KP
PN
3897
1386
Thi
ck s
kinn
edP
arti
ally
bur
ied
Salt
sea
lN
o sa
lt d
etac
hmen
tT
ien
Shan
(par
t)K
PY
N21
0419
95T
hick
ski
nned
Bur
ied
No
salt
sea
lN
o sa
lt d
etac
hmen
tC
auca
sus
(par
t), T
ien
Shan
(par
t), O
man
KY
NN
1724
935
56T
hick
ski
nned
Bur
ied
Salt
sea
lN
o sa
lt d
etac
hmen
tSa
yan-
Tuv
aK
YY
N26
765
40
Thi
n sk
inne
dU
nbur
ied
No
salt
sea
lN
o sa
lt d
etac
hmen
tB
alka
n, B
eni,
Cha
co, M
adre
de
Dio
s,T
NN
N59
9820
532
Lla
nos
(par
t), A
rcti
c, B
angg
ai,
NE
Car
ibbe
an, T
rini
dad,
Ass
am, K
irth
ar,
Sula
iman
, N R
ocki
es, R
ocki
es (C
an),
Tai
wan
Thi
n sk
inne
dU
nbur
ied
No
salt
sea
lSa
lt d
etac
hmen
tB
etic
, Jur
a, P
yren
ees,
Pot
war
, Sie
rra
Mad
reT
NN
Y10
4423
74O
rien
tale
Thi
n sk
inne
dU
nbur
ied
Salt
sea
lN
o sa
lt d
etac
hmen
tY
unan
Gui
zhou
, Uta
h–W
yom
ing,
Ura
lsT
NY
N25
1796
14T
hin
skin
ned
Par
tial
ly b
urie
dN
o sa
lt s
eal
Par
tial
sal
tC
arpa
thia
ns, N
Ape
nnin
es, S
Ape
nnin
esT
PN
P66
5341
58de
tach
men
tT
hin
skin
ned
Par
tial
ly b
urie
dN
o sa
lt s
eal
Salt
det
achm
ent
Ref
orm
a, V
erac
ruz
TP
NY
5089
510
883
Thi
n sk
inne
dB
urie
dN
o sa
lt s
eal
No
salt
det
achm
ent
Gua
jira,
Mat
urin
, E C
ordi
llera
(par
t)T
YN
N22
612
1744
5T
hin
skin
ned
Bur
ied
No
salt
sea
lSa
lt d
etac
hmen
tD
inar
ides
, Hel
leni
des,
Cub
anT
YN
Y97
023
8T
hin
skin
ned
Bur
ied
Salt
sea
lSa
lt d
etac
hmen
tE
Alp
s, S
Alp
sT
YY
Y35
203
463PROSPECTIVITY IN FOLD AND THRUST BELTS
• thin-skinned (T), partially buried (P), no saltseal (N), salt detachment (Y) (TPNY 61.8 bnboe); >99% of these reserves are in the ReformaFold belt (Table 1);
• thin-skinned (T), buried (Y), no salt seal (N),no salt detachment (N) (TYNN 40.1 bn boe);>99% of these reserves are in the Maturin FoldBelt (Table 1). It should be noted that this defor-mation style would also include the fold andthrust belts that develop as the contractional toesto extensional systems on continental margins.These fold belts have become important hydro-carbon provinces (e.g. the deep-water Gulf ofMexico, deep-water Niger Delta and Brazil), butwere excluded from this review as discussedabove.
• thin-skinned (T), unburied (N), no salt seal(N), no salt detachment (N) (TNNN 30.3 bnboe); 47% of these reserves are in the Chaco FoldBelt (Table 1).
• thick-skinned (K), partially buried (P), nosalt seal (N), no salt detachment (N) (KPNN29.8 bn boe); all of these reserves are in a part ofthe Caucasus Fold Belt (Tables 1 and 3).
This provides some insights into the types offold and thrust belt that should be explored ifsignificant reserves or a particular hydrocarbontype are desired. One constant factor is that,except for the Zagros Fold Belt, a salt seal doesnot appear to be necessary for significant reservesto be present. What is particularly striking is thatin most of the categories discussed above, thereis one fold and thrust belt that dominates thereserves in the category. The only exception isthe TNNN category, where, although the ChacoFold Belt accounts for a high proportion of thereserves, a number of other fold belts also con-tribute significant reserves (Table 3). The datasuggest that, based on deformation style, prolifichydrocarbons occur in fold belts that arethin-skinned with no salt detachment or salt seal.Whether or not the thin-skinned fold and thrustbelt is buried does not appear to be a criticalfactor.
A key observation from this analysis is thatthe six most richly endowed fold and thrustbelts have no common set of deformation styleattributes. The conclusion is that deformationstyle attributes are not critical factors in control-ling the hydrocarbon endowment of fold andthrust belts and that the non-structural elementsof the petroleum system are more important indetermining hydrocarbon endowment.
Gas- and oil-prone thrust belts
The characteristics of gas- or oil-prone fold andthrust belts are based on total gas and oil reserves
rather than the number of fold and thrust beltsdominated by each hydrocarbon type. Whether afold and thrust belt is thin- or thick-skinned doesnot appear to be a critical factor in determiningthe dominant hydrocarbon type; however, theother factors described above definitely are.
• Gas-prone fold and thrust belts are charac-terized by a pre-Palaeogene age of the last defor-mation phase, Palaeozoic source rocks, a saltseal, no salt detachment and are not at presentburied by syn- or post-orogenic sediment.
• Oil-prone fold and thrust belts are character-ized by a Cenozoic age of the last deformationphase, post-Jurassic source rocks; no salt seal(excepting the Zagros Fold Belt), a salt detach-ment and are currently buried or partially buriedby syn- or post-orogenic sediment.
This very clear distinction provides a poten-tial tool for the exploration of fold and thrustbelts where a particular hydrocarbon type is thegoal of the exploration programme. In theauthor’s opinion the most important factor isthe age of the last deformation.
Exploring petroleum systems in fold andthrust belts
The area of a fold and thrust belt that is mostprospective for hydrocarbons is the externalfoothills belt between the leading thrust of theinternal zone and the limit of thrusting in theforeland basin, whether emergent or buried.Commercial quantities of oil and gas have beendiscovered in almost 50 fold and thrust belts(Table 1). Predictably, fields are aligned parallelto the structural trend. Structural traps are usu-ally present throughout the belt, yet hydrocarbonreserves tend to be located in a fairly discretezone within the thrust belt. In many cases theproductive region is a band along the externalfringe of the thrust belt. This is because normallythe generation and expulsion front moves aheadof the deformation front and the normal asym-metry of the basin encourages migration into theforeland. As a result, there is a stronger possibil-ity of the frontal thrust creating a giant field(>250 mm boe) than for structures that arefurther back from the thrust front.
Toward the hinterland (internal region of theorogenic belt), the reservoir horizons tend to bebreached and flushed, older source rocks may beovermature, and younger source rocks, presentin the clastic foredeep, may be absent. Towardthe foreland (external region of the orogenic belt)of the productive trend, the reservoir horizonstend to be depositionally thinner, the source
464 M. COOPER
rocks may be immature, the young source rocksmay overlie the reservoir, and the structural trapsmay be small or absent.
Thrust belts have long been considered diffi-cult areas in which to explore for hydrocarbons.One of the reasons for this view is the difficultyof predicting subsurface structure. However, thekey risk in many fold and thrust belts is whethertrap formation predated hydrocarbon genera-tion. Many of the hydrocarbons generated endup in the foreland in stratigraphic traps, in tarbelts and within old structures buried beneaththe foreland basin sediments. This problem issummarized below.
• Thrust systems elevate rocks above theirregional elevation, thus potentially removing thesource rocks from the generating window (A inFig. 21).
• If the source is intra-thrust sheet then onlyat the trailing edges will the source be still atregional elevation and capable of generatinghydrocarbons. The available fetch will depend onthrust sheet size and will be degraded as displace-ment increases because less source volume willstill be at regional elevation (B and Bp in Fig. 21).
• If the source is in the roof sequence the sameproblems as for intra-horse sourcing applyand communication with the reservoir may bedifficult to achieve (C in Fig. 21).
• If the source is in the footwall similar prob-lems apply but in addition migration pathwayswill be limited by the availability of across-faultjuxtapositions of reservoir and source (D inFig. 21).
• The system will work if subsequent burial ofthe thrust belt, by syn-orogenic sediments, putsthe entire system, including source and traps,in the maturity window. In this case, trapspredate the generation and migration of hydro-carbons.
• The system will work if the thrusting is syn-chronous with, or shortly post-dated rapid burialby, foreland basin sediment. The loading effect ofthe thrust belt creates the accommodation spacein the foreland basin, which is then progressivelycannibalized by the prograding thrust system.In this case, the structures develop as the sourcerocks are in the maturity window; much of theearly charge may migrate into the foreland basinbut the later charge is trapped.
• The system will work if the thrusting isresponsible for pushing the source into thegeneration window (E in Fig. 21).
Despite having potential source and reservoirrocks several of the fold and thrust belts includedin this review have only modest resource endow-ments (Table 1), probably because of problemswith the timing of maturation and structuration.The really prolific fold and thrust belts, with>10 bn boe of reserves (Zagros, Reforma,Maturin, Caucasus, Chaco and Tien Shan), allhave world-class source rocks. A good rule ofthumb is that one discovery in a particularstructural zone of a fold and thrust belt mitigatesthe primary risk, timing of maturation andmigration in relation to trap formation, andnormally there will be a number of otheraccumulations.
Future potential in fold and thrust belts
To assess the future potential of fold andthrust belts, the data in the USGS 2000 WorldPetroleum Assessment were used (USGS 2000).Unfortunately, this assessment covers onlyselected hydrocarbon provinces and thus doesnot provide a complete dataset of yet-to-find(YTF) resource estimates. For those fold andthrust belts where an assessment did exist, amaturity index was calculated, which is the
Fig. 21. Cross-section based on Figure 9 annotated to illustrate different configurations of structural geometryand source rock position (see text for discussion).
465PROSPECTIVITY IN FOLD AND THRUST BELTS
YTF % of the sum of the discovered reserves andthe YTF; the higher this number is the lessmature the exploration of the fold and thrust belt(Table 1). All fold and thrust belts that have ayet-to-find resource estimate of >2 bn boe werecompiled into Table 4. Two fold and thrust beltswere added to Table 4 for which no YTF num-bers were available from the USGS but which arebelieved to offer significant additional resourcepotential. The location of the fold and thrustbelts is shown on the world map (Fig. 22). Manyof the thrust belts in Table 4 remain relativelypoorly explored for a number of reasons thatinclude remote location (Gissar, Tien Shan) andlimited or lack of access for international oilcompanies (Zagros, Reforma).
The advent of technology that allows produc-tion in ultra-deep water over the last 20 years hasdriven exploration of the fold and thrust beltsthat develop as the contractional toes of exten-sional systems on passive margins (Rowan et al.2004). Some of these systems host significantreserves; for example, the Gulf of Mexico foldbelts, the toe of the Niger Delta and deep-waterBrazil. These and other similar areas still offersignificant undiscovered resource potential.
Conclusions
This paper presents a statistical summary ofreserves in fold and thrust belts based primarily
on an IHS dataset of reserves for discoveredfields to which a number of geological attributeshave been added (Table 1). This provides apowerful summary dataset of the key geologicalcharacteristics of the fold belts included in thereview, which has been used to interrogate theIHS dataset. The analysis of the data has identi-fied which of the world’s fold and thrust belts arethe most prolific hydrocarbon provinces and theattributes of these provinces. The most prolificfold and thrust belt is the Zagros Fold Belt,which accounts for 49% of all reserves in fold andthrust belts, and the Zagros Fold Belt has, as aresult, been isolated during the analysis to avoidskewing the other conclusions.
Fold and thrust belts represent an averagesample of the world’s hydrocarbon resources andhave a virtually identical oil:gas:condensate ratioto the global resource endowment. Fold andthrust belts are oil prone (59% oil) becausethey are an almost perfect representation of anoil-prone world. Excluding the Zagros Fold Belt,most of the reserves are contained in fold andthrust belts that are thin-skinned, have no saltdetachment or salt seal, are partially buried bysyn- or post-orogenic sediments, are sourced byCretaceous source rocks and underwent the lastphase of deformation during the Tertiary. Theparticularly telling observation, however, isthat the top six most richly endowed fold andthrust belts have no common set of structural
Fig. 22. Map of fold and thrust belts that offer >2000 mm boe of yet-to-find (YTF) resources (Table 4). NA, notapplicable.
466 M. COOPER
attributes. The top six also fall into an attributeset that may also be shared with other fold andthrust belts but within which they dominate,making up >90% of the reserves within theattribute set. This implies that the resource-richfold and thrust belts all have a unique combina-tion of characteristics that is not necessarilyrepeatable in others. This also implies that struc-tural attributes are not the critical factor control-ling the distribution of hydrocarbon reserves infold and thrust belts. If the structural attributesof fold and thrust belts are not critical factors inresource endowment then what are the criticalfactors? In common with other prolific petro-leum provinces, these are the presence of a world-class source rock and the presence of a regionallyeffective reservoir–seal couplet. The Zagros FoldBelt illustrates this perfectly; the most prolificfold and thrust belt is essentially the deformedNE margin of the Arabian Basin, the world’smost prolific petroleum province, with which itshares many petroleum system elements. Thusthere is nothing intrinsic in fold and thrust beltsthat differentiates them from other oil- and gas-rich provinces other than the prolific develop-ment of potential hydrocarbon traps. The verydistinctive characteristics of oil-prone and gas-prone fold and thrust belts offers a potential toolfor targeting a particular hydrocarbon type.
The success or failure of hydrocarbonexploration in a fold and thrust belt is primarilycontrolled by the relative timing of source rockmaturation, hydrocarbon migration and trapdevelopment; unless the timing is favourablyconfigured it is unlikely that exploration will besuccessful. The analysis of the maturity indexshows that many of the prolific fold and thrustbelts still have significant remaining upside;
remote location and politically challengingaccess have limited opportunities for thispotential to be realized.
I wish to thank M. Warren and J. Squires for their valu-able comments on the evolving drafts of the manuscript,M. Allen for discussions on the Zagros Fold Belt,IHS for the use of their reserves database, and EnCanaCorporation for permission to publish this paper.C. Kluth is thanked for a constructive review, andF. Peel made a number of insightful comments andrecommendations that substantially improved the finalversion of this paper.
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