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Geological Society, London, Special Publications

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2012, v.366;Geological Society, London, Special Publications

Thusu and A. CozziG. M. Bhat, J. Craig, M. Hafiz, N. Hakhoo, J. W. Thurow, B. Cambrian Basins in Asia: an introduction

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Geology and hydrocarbon potential of

Neoproterozoic–Cambrian Basins in Asia: an introduction

G. M. BHAT1, J. CRAIG2, M. HAFIZ1, N. HAKHOO1, J. W. THUROW3,

B. THUSU3

& A. COZZI2

1 Institute of Energy Research and Training, University of Jammu, India

2Eni Exploration & Production Division, San Donato Milanese, Italy

3 Department of Earth Sciences, Maghreb Petroleum Research Group,

University College London, London, UK

*Corresponding author (e-mail: [email protected])

In 2005, the Maghreb Petroleum Research Group(MPRG), University College London, initiated amajor research programme focused on the relativelypoorly understood Neoproterozoic petroleum sys-tems of the world. A series of research projects wereundertaken to understand the generation and entrap-ment of hydrocarbons in this unique geological timeinterval, which is dominated by several episodes of global glaciations and post-glacial transgressions,coupled with basin development and rifting on amore local scale (Craig et al. 2009). The researchstarted with a field-based study of the Neoprotero-

zoic sequences in North Africa (Libya, Moroccoand Mauritania) and northern India (Rajasthan andJammu & Kashmir).

A series of international conferences, with fieldexcursions/workshops, were run in parallel with theresearch programmes. The first of these was held atthe Geological Society of London in November2006 and the proceedings were published in 2009in Geological Society London, Special Publication326, entitled, ‘Global Neoproterozoic PetroleumSystems: the emerging potential in North Africa’(Craig et al. 2009). The second international confer-

ence was held at the University of Jammu in 2008with a focus on the Neoproterozoic petroleum sys-tems of Asia, including India, Pakistan, Oman,China and Siberia (Bhat et al. 2008) (Fig. 1). Thiscurrent volume contains some of the papers pre-sented at the Jammu conference, in addition to newresearch on the geology and hydrocarbon potentialof the Neoproterozoic –Cambrian basins of Asia.A third and concluding conference and an associ-ated third Geological Society Special Publicationwill focus on the Neoproterozoic petroleum systemsin regions of the world not covered in the previousvolumes (mainly North and South America, westernand southern Africa and Australia) and will com-plete the project to provide a global synthesis of the Neoproterozoic petroleum systems.

Thepresent volumecontains fifteenpaperscover-ing the Neoproterozoic petroleum systems of India(Ojha, Ram, Kumar and Majid et al .), Pakistanand (Jamil & Sheikh and Siddiqui), Oman (Cozziet al .), China (Turner) and Siberia (Howardet al .). The remaining five papers concentrate onvarious aspects of Neoproterozoic geology and pal-aeobiology, including stratigraphy (Tewari) andtectonics (Mishra & Mukhopadhyay) of the NWHimalaya, salt tectonics in Oman (Smith), acritarchsinOman(Butterfield & Grotzinger)andthepalaeo-biology of the Vindhyan succession in central India

(Sharma & Shukla).This Introduction provides a synthesis of the key

conclusions in a palaeogeographic context, but fordetails the reader is referred to the relevant articlesin this volume.

Siberian plate

Howard et al . present a comprehensive review –based on field and borehole data – of the geologyand petroleum systems of the Riphean– Vendian seq-

uence in Siberia, associated with shallow-water plat-form sediments deposited on a passive margin andsubsequently uplifted during regional compression.SE Siberia contains a world-class Neoproterozoichydrocarbon province with in-place oil and gasreserves of 4 Bbbl and more than 38 tcf gas. About67% of these reserves are located in Vendian silici-clastic sediments and the remaining 33% are in theRiphean and Late Vendian – Early Cambrian carbon-ates. The high ‘helium’ content in the Vendian reser-voir lithics is derived either from uranium decay orbasement rocks. In the present case, this high heliumconcentration, and its retention, reflect long-livedefficient traps protecting early hydrocarbon charge.

The Cis-Patom Trough is an important Ripheanand Vendian regional source kitchen from which

From: Bhat, G. M., Craig, J., Thurow, J. W., Thusu, B. & Cozzi, A. (eds) 2012. Geology and Hydrocarbon Potentialof Neoproterozoic–Cambrian Basins in Asia. Geological Society, London, Special Publications, 366, 1–17.First published online September 18, 2012, http://dx.doi.org/10.1144/SP366.15# The Geological Society of London 2012. Publishing disclaimer: www.geolsoc.org.uk /pub_ethics


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3/18

long-distance hydrocarbon migration has chargedthe reservoirs in the highly prolific basins of theBaykit Anticlise, the Angara Lena Step, the Kha-tanga Saddle and the Nepa Botuoba High (Table 1).Although there is no direct evidence that the mag-matic intrusions that occur in the Cambrian–Devonian succession have affected the Neoprote-rozoic sediments, it is likely that such intrusionswould have influenced the maturation of the organicmatter in Riphean– Vendian source rocks.

Most of the hydrocarbons in the SE Siberian Plat-form are in Vendian clastic reservoirs, sealed by

interbedded siltstones and evaporites. The Ripheancarbonate sequences also have good reservoir prop-erties, largely as a result of karstification (e.g. thekarstified carbonate reservoirs of the Kamo Arch/Baykit Anticlise), which developed as a result of tectonic fracturing and subaerial exposure prior tothe deposition of the Vendian sediments. Similarkarstified carbonate gas reservoirs also occur on theflanks of the North Oman Salt Basin, where theyare sealed by Early Cambrian shales (e.g. theMakarem Field; Cozzi & Al-Siyabi 2004).

All the major hydrocarbon accumulations on theSE Siberian Platform are controlled by structuraltraps, but there are also several proven stratigraphictraps (e.g. on the Nepa-Botuoba High). Late Vendianargillaceous evaporite-rich carbonate beds and Early

Cambrian salts form excellent regional seals. Thepresence of Early Cambrian cyclic alternations of carbonate (dominantly dolomite) and salt in theSiberian Platform and in the Omani petroleum sys-tems (Huqf Supergroup) is noteworthy, althoughSiberia and Oman were located in different palaeo-latitudes during Neoproterozoic times (Fig. 2).

Indian plate

North and West India and Pakistan

During the mid to late Neoproterozoic, between 750and 550 Ma, the Earth witnessed its most extremeclimatic, atmospheric and tectonic events. Evi-dence of these extreme events has been recordedon the Indian Plate and includes the global declineof Meso-Neoproterozoic stromatolites, dismember-ment and amalgamation of landmasses, glacial dia-mictites, cap carbonates, biotic evolution and thedevelopment of Ediacaran fauna.

The Neoproterozoic and Palaeozoic successionsof the Indus Basin (Table 2) on the northwesternedge of the Indian Plate are exposed along the foot-hills of the Salt Range and in the Kirana Hills, andhave been encountered in wells drilled in thePotwar Basin, on the Punjab Platform and further

Fig. 1. Present-day configuration of landmasses and the distribution of Neoproterozoic–Cambrian Basins in Asia(China, India, Oman, Pakistan and Siberia).

G. M. BHAT ET AL.2




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Table 1. Hydrocarbon systems of Siberian platform

Super basin/basin

Tectonics

(basin type)

Sedimentation (mega

sequence)

Structural

styles

Source and

type

Migration Reservoir Seal

Siberian Plate

Siberian

Platform

(Howard

et al .)

Baykit

Anticlise

Riphean clastic/carbonate(passive margin &

molasse), Late

Vendian–Ordovician

clastic/carbonate(marine)

Platform margin

uplifted by

compressional

faults and folds

Riphean and

Vendian

shales

Long-distance

stratigraphic

controlled

migration

from platform

margin (during

Riphean–

Cambrian) and

intraplatform

depressions

(Cis-Enisey

Trough, Cis-Sayan

Syneclise and

Chunya Basin)

(during Vendian

to Silurian)

Riphean

karstified

carbonates

and Vendian

sandstones/carbonates

Vendian shales

and

carbonates,

Cambrian

salt

Cis–

Sayan–

Yenisey

Syneclise

Riphean clastic/carbonate(passive margin),

Vendian–Silurian

clastic/carbonate(marine)- Devonian–

Carboniferous clastic

(continental/marginalmarine)

Riphean rift overlain

by intracratonic

sag basin

Unproven.

Proposed

Riphean and

Vendian

shales

By analogy with other

platform basins:

Vendian to Silurian

Unknown Unknown.

Potential

Cambrian

salt

Angara Lena

Step

Riphean clastic and

carbonate (passive

margin), Vendian–

Ordovician clastic/carbonate (platform),

Jurassic clastic

(continental)

Platform Riphean and

Vendian

shales

Long-distance

stratigraphic

controlled

migration from

Riphean shales in

the Cis– Patom

Trough & ?Cis–Sayan–Yenisey

Syneclise (during

Riphean–

Cambrian) and

Vendian shales

(during Vendian–

Silurian)

Vendian

sandstones

(Parfenov

bed)

Vendian shales

and

carbonates,

Cambrian

salt


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Table 1. (Continued)

Super basin/basin

Tectonics

(basin type)

Sedimentation (mega

sequence)

Structural

styles

Source and

type


Khatanga

Saddle

Riphean clastic/carbonate(platform), Vendian–

Silurian carbonate/clastic (platform) –

Devonian–

Carboniferous clastic/carbonate (continental/marginal marine)


Vendian

shales

Long-distance

stratigraphic

controlledmigration from

Riphean shales in

the Cis– Patom

Trough & ?Cis–

Sayan–Yenisey

Syneclise (during

Riphean–

Cambrian) and

Vendian shales

(during Vendian–

Silurian)

Vendian

sandstones

(Parfenovbed), late

Riphean

karstified

carbonates

Vendian

mudstones

andanhydritic

dolomite

Nepa

Botuoba

High

Vendian–Silurian

carbonate/clastic(platform)


Vendian

shales

Long-distance

stratigraphic

controlled

migration fromRiphean shales in

the Cis– Patom

Trough & ?Cis–

Sayan–Yenisey

Syneclise (during

Riphean–

Cambrian) and

Vendian shales

(during Vendian–

Silurian)

Vendian

sandstone,

Late

Vendian–Cambrian

dolomite

Cambrian salt

Cis–Patom

Trough

Riphean clastic/carbonate(passive margin), Late

Riphean–Vendian

clastic (active margin

and foreland basin)

Silurian

compressional

deformation belt

Riphean and

Vendian

shales

Late Riphean to

Silurian

N/A N/A


6/18

south on the Thar Platform in Pakistan (Jamil &Sheikh and Siddiqui). The Neoproterozoic succes-sion is also exposed in allochthonous inliers in theHimalayan region of North India and NE Pakistan

(Fig. 3) (Hakhoo et al. 2011).In western India, the Neoproterozoic succession

crops out on the flanks of the Bikaner– Nagaur Basin(Fig.3andTable2),closetotheborderwithPakistan,and also in the Lesser Himalayan Zone of the NWHimalaya (Tewari; Mishra & Mukhopadhyay).Riphean stromatolites are important components of these Himalayan Neoproterozoic sequences (Jamil& Sheikh), particularly in the Sirban Limestonein Jammu (India) and the Hazara Formation inPakistan.

The Neoproterozoic to Cambrian succession in

western India has proven hydrocarbon potential.The Baghewala-1 well in the Bikaner– Nagaur Basinencountered heavy oil in the Sonia and GirbhakarMembers of the Neoproterozoic Jodhpur Fm. andthe Bilara (Dolomite) Fm. The Bilara Dolomite hasgood reservoir characteristics and source potential(Jamil & Sheikh). The coeval beds in Pakistan,where the succession is thicker and more deeplyburied, could generate more mature equivalents of the Baghewala-1 oil (Siddiqui). The Baghewala-1oil is geochemically similar to the heavy oil fromthe Neoproterozoic Salt Range Series in Pakistanand from the carbonate–evaporite facies of the Neo-proterozoic Huqf Group in Oman (Peters etal. 1995).The Salt Range succession is believed to be theequivalent of the Hormuz Series in Iran and the

Huqf Group in Oman (Cozzi). The Neoproterozoic–Cambrian rift basins in western India, Pakistan,southern Oman and South China were in close proxi-mity in the central portion of the Pannotia Supercon-

tinent during the Late Precambrian (Fig. 2; Peterset al. 1995).

The Marwar Supergroup in the Bikaner– NagaurBasin and the Krol Group in the Lesser Himalaya arechronostratigraphically coeval and represent mar-ginal marine successions deposited along the sameSW–NE trending continental margin (Mishra &Mukhopadhyay). The existence of proven Neopro-terozoic – Cambrian petroleum systems in SouthChina, Oman, Pakistan and western India suggeststhat this prolific petroleum province could alsoextend to the present-day Lesser Himalayan Zone,

which, at that time, was part of the same deposi-tional system (Mishra & Mukhopadhyay; Tewari).However, geochemical data and thermal modellingsuggest that the Neoproterozoic –Cambrian petro-leum system is unlikely to be present in the LesserHimalayan Zone, at least in Nahan area (HimachalHimalaya) (Mishra & Mukhopadhyay).

In north and NW Himalaya, the Neoproterozoicand younger sedimentarysuccessions have been sub- jected to multiple deformation episodes before,during and after the Himalayan Orogeny, resultingin great structural complexity. These complex fold-and-thrust belts are emerging as challenging, fron-tier hydrocarbon targets similar to the producinghydrocarbon province of the Zagros Fold Belt inIran and Iraq. The frontal part of the Himalaya in

Fig. 2. Palaeogeographic Reconstruction during the Neoproterozoic (Modified after Scotese 2009).

INTRODUCTION 5




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Table 2. Hydrocarbon systems of Indian Plate, Oman and North China Basins

Super basin/

basin

Tectonics

(basin type)

Sedimentation

(mega sequence)

Structural

styles

Source and

type


Indian Plate

Cuddapah

(Ojha;

Ram)

Chhattisgarh

(Ojha;

Ram)

NE extension of

Kurnool

Intracratonic.

Basin apparently

developed as

cratonic rift,

evolved into

deeply subsiding

one, through

episodic tectonic

pulses

Two mega

sequences

namely

Chandrapur and

Raipur, former

mainly

arenaceous,

deposited in fan

delta, deep-water

prodelta and

storm tide

dominated

prograding shelf

environment;

Raipur

megasequences,comparatively

thicker

comprising five

cycles of

sedimentation

having mixed

carbonate–

siliciclastic

succession

deposited in a

storm-dominated

shallow-water

platform

Basin structural style

is tectonically

controlled, The

north–NW

palaeoslope

together with

rapid transition

from shallow

marine to

deep-water

pelagic carbonate

sequences

indicate abrupt

deepening of the

basin, the

storm-dominatedmega sequences

indicate open

marine

circulation

conditions,

skirting NNW

margin of the

craton

Unproven; proposed

carbonates,

shale-dominated

Raipur sequences

comprising well

preserved

stromatolites and

persistent horizons

of black limestones

Unknown Unproven; proposed

to be Chanderpur

sandstones,

carbonate–

silciclastics

succession of

Raipur formation

Unknown

Bhima (Ojha;

Ram)

Further northern

extension of Kurnool

Intracratonic

Three cycles of

sedimentationequivalent to

Kurnool mega

sequences,

indicating

gradual

deepening of

marine basin

followed by

gradual

regression

Nearly horizontal

to low dips overlarge areas, some

tectonic

disturbances

around faults and

Deccan Trap

junctions

Unproven, flaggy

carbonates?

Unknown Flaggy limestones Unknown


8/18


9/18


10/18


11/18


12/18

Upper Indus

Basin (Salt

Range/

Potwar)

(Jamil &

Sheikh;

Hakhoo

et al. 2011)

The Reasi Inlier

(Sirban Fm.)

Proterozoic

carbonates

(dolomite/

limestone), chert

and organic-rich

shale; Riphean

stromatolite

biostromes;interbedded

transgressive and

regressive

sedimentation

cycles (marine

supratidal,

intertidal shallow

subtidal to inner

and carbonate

shelf

environment,

characteristic of

broad carbonate

shelf)

Platform margin

compressional

regime (folding,

thrusting and

faulting);

multiple tectonic

episodes

associated withthe drift,

rotation, collision

and under

thrusting of the

Indian Plate

Proterozoic (Palaeo-,

Meso- and

Neoproterozoic)

carbonates, shales

and algal

limestones

Unknown Dolostones with,

vuggy, inter- and

intragranular,

fracture, and

inter- and

intralayer, cavern

porosity

Chert beds/lenses,

shales and

argillites

Oman

Oman (Cozzi

et al .;

Smith;

Butterfield

&

Grotzinger)

Neoproterozoic

interior basin

part of the

Indian–

Pakistan–Oman

Plate, which

became

restricted and

evaporitic and

was later filled

by continental

clastics in the

Early Cambrian

The Neoproterozoic

Huqf Supergroup

consists of three

lithological

facies: the basal

thick glaciogenic

clastics are

overlain by

massive salt

deposits in the

centre of the salt

basin and by

shallow water

carbonates on the

flanks of it; the

later basin-fill

stage is

composed of

continental

sandstones

NE– SW oriented

evaporitic basin

formed over a

gently dipping

interior basin;

strike–slip

tectonics at Late

Neoproterozoic–

Early Cambrian

stage deformed

the flanks of the

basin

Organic-rich shales

and carbonates

deposited in the

deeper parts of the

anoxic salt basin

Self-contained

petroleum system

with the

carbonate

stringers within

the salt,

migration from

pre-salt source

rocks proven

Shallow water

carbonates and

intercalated

clastics

Neoproterozoic rock

salt


13/18

Table 2. (Continued)

Super basin/

basin

Tectonics

(basin type)

Sedimentation

(mega sequence)

Structural

styles

Source and

type


South Oman

Salt Basin

(Cozzi

et al .)

Eastern

Deformation

Front, central

SOSB and

Eastern Flank

Two mega

sequences

NE–SW strike–slip

tectonics

Dark grey, algal-rich,

laminated

dolomites

600 m of

rock salt, six

evaporitic cycles

North China

NW Tarim

Basin

(Turner)

Rift and post-rift

basin to

epicontinental

platform

Neoproterozoic

clastics,

volcanics and

carbonates;

Cambrian–

Ordovician

carbonates

Platform margin

later uplifted by

compressional

faults and folds

Cambrian shales Unknown Neoproterozoic to

Ordovician tight

carbonates

Unknown


14/18

India consists of two tectonic zones, the Lesser Hi-

malaya Zone and the Sub-Himalaya Zone. The struc-tural evolution of the frontal region in theNahan areahas been interpretedthrough sequential restorationof cross-sections, which helps to constrain trap geome-tries and trap formation relative to the timing of hydrocarbon charge (Mishra & Mukhopadhyay).Triangle zones, duplex structures and associatedback thrusts in the Sirban Limestone Fm. (Table 2)of the Reasi Inlier, near Jammu in northern India,also form potential hydrocarbon traps (Hakhooet al. 2011). In view of the very complex structuralgeometries (imbricate fans, duplex/breached dup-

lexes, stacked thrust sheets, out-of-sequence thrus-ting, reactivated ramps and so on) in the Himalayanfrontal region, rigorous structural modelling, com-bined with robust geochemical and geochronologi-cal data, are required to calibrate petroleum systemmodels and reduce exploration risk (Mishra &Mukhopadhyay).

Evidence from drilled wells confirms the pres-ence of the Neoproterozoic to Cambrian successionalong and across the eastern border of Pakistan withIndia, and it thickens towards the south. A thicknessof c. 1500 m is reported from a well (Marvi-1)drilled in the southeastern part of Pakistan. Largedomal structures that expose Middle Jurassic rocksin the cores are present in the Kirthar Fold Belt(KFB). Within the central KFB, the Neoproterozoic

to Cambrian succession may be tested at a drillable

depth and could prove to be a potential hydrocarbonplay (Siddiqui).

Central India (Vindhyan Basin)

The Indian Proterozoic basins formed part of thenorthern rim of Gondwanaland prior to its dismem-berment along six major radial fractures (Ojha). InPeninsular India, the Proterozoic basins (Bikaner–Nagaur, Vindhyan, Cuddapah, Chhattisgarh, Bastar,Bhima and Kaladgi) cover a total area of 327830 km2. They contain unmetamorphosed and rela-

tively undeformed sediments resting unconformablyon metamorphosed and deformed basement. Thesearch for hydrocarbons in these Proterozoic basinsis driven by the rapidly increasing energy demandin India, which already has highest annual rate of hydrocarbon consumption in the world. The infor-mation available from the limited Neoproterozoic –Cambrian outcrops is, however, inadequate for asses-sing the petroleum prospectivity of these basins, andmost have not been fully explored for hydrocarbons(Ram).

Indian Mesoproterozoic and Neoproterozoicsedimentary basins share similar tectonic settingsand depositional environments as their counter-parts elsewhere in the world (Oman, China, Austra-lia and so on) and contain three distinct petroleum

Fig. 3. Regional geological map of NW Himalaya with location of the Proterozoic Basins (map modified after Bhatet al. 2008 and Jamil & Sheikh.

INTRODUCTION 13




15/18

systems: the Cuddapah, Vindhyan and Indus systems(Table 2) (Ojha).

The arcuate-shaped Vindhyan Basin (Meso-Neoproterozoic) in the central part of India (Fig. 1)covers an area of c. 162 000 km2 and contains athick sequence (c. 5000 m) of shallow marine calcar-

eous, argillaceous and arenaceous rocks. The basinhas a half graben geometry (Ram et al. 1996) andis deepest towards the SW in the proximity of theSon–Narmada Fault. The Vindhyan Basin is bor-dered by the Aravalli–Delhi orogenic belt (2500–900 Ma) (Roy 1988) in the west, the Son–Narmadalineament in the SW and the Satpura Orogenic Belt(1600–850 Ma) (Verma 1991) in the SE. The Bun-delkhand Massif occupies the north-central partand is fringed by the Palaeoproterozoic Bijawar suc-cession along the eastern edge and their equivalentGwalior succession. The basin contains orthoquart-

zite–carbonate–shale successions typical of intra-cratonic to pericratonic settings (Table 2).

The Vindhyan petroleum system (Table 2) isfurther subdivided into (i) the Ganga Valley pet-roleum system, consisting of a Mesoproterozoic–Neoproterozoic system and an Ordovician–Siluriansystem; (ii) the Son Valley petroleum system; (iii)the Chambal Valley petroleum system; and (iv) theDeccan Trap Concealed Vindhyan petroleum sys-tem (Ojha). All the physical and temporal elementsof an effective petroleum system are present in theVindhyan Basin (Ojha; Kumar; Ram; Majid et al .)

but no commercial hydrocarbon accumulationshave yet been discovered (Kumar; Ram).

Southern India (Cuddapah Basin)

The crescent-shaped, convexto the west, ProterozoicCuddapah Basin (Fig. 1) occurs on the eastern Dhar-war craton (southern India) overlying an Archaeangranite-greenstone terrain (3000– 2600 Ma), closeto the western margin of the Eastern Ghats MobileBelt. This marginis widelyconsideredto be a Proter-ozoic collisional boundary (e.g. Chetty & Murthy

1994; Vijaya Kumar & Leelanandam 2008). Thebasin itselfcovers an area of 44 000 km2 and extendsfor c. 450 km along the arcuate eastern margin withan average width of 150 km. It is the second largestPurana (Proterozoic) basin in Peninsular India, afterthe great Vindhyan Basin. The arcuate north, southand western boundaries of the Cuddapah Basin aredefined by a profound unconformity. The arcuateeastern margin is marked by a prominent boundarythrust, which is parallel to the Nellore schist belt of the Eastern Ghats Mobile Belt and the East Coast(Chetty 2011).

Many theories have been proposed to account forthe complex stratigraphic and structural history of the Cuddapah Basin (e.g. Anand et al. 2003; Chetty2011), but, to date, no satisfactory model has been

established. The basin was probably initiated dur-ing a Palaeoproterozoic collisional event. An exten-sional regime, followed by major transpressionaldextral shearing events along the collisional bound-ary seem to have occurred during the Mesoprotero-zoic and resulted in the formation of imbricated

fold– thrust structures in the Nallamalai Basin(Chetty 2011). The Cuddapah Basin is divided intoseveral blocks, which are separated by basementfaults. The basin was initiated as a pericratonic riftand culminated as a foredeep with a westward-directed orogenic front and domal structures (Ram).These events had a profound influence on the hydro-carbon prospectivity of the basin by forming struc-tural traps. Moreover, the western Cuddapah Basin,in particular, developed in a thermal and tectonicregime conducive to the generation, migration andtrapping of hydrocarbons (Ojha).

The Cuddapah Basin contains a thick successionof igneous and sedimentary rocks (c. 6000 m) for-ming the Cuddapah and Kurnool supergroups. TheCuddapah Supergroup of Mesoproterozoic age con-sists of three groups: the Papaghni, Chitravathi andNallamalai groups. The rocks belonging to the Nal-lamalai Group, in the eastern part of the basin, arehighly disturbed, folded and faulted. The CuddapahSupergroup comprises a dominantly argillaceous(shales) and arenaceous (quartzites) succession withsubordinate calcareous sediments (algallimestones).The composition of gas collected from the basin

indicates the generation and migration from thermo-genic sources (Ojha). All the elements of an effec-tive petroleum system are present in the CuddapahBasin (Ojha).

Arabian plate (Oman)

The Neoproterozoic – Cambrian basins of Oman aresituated on the eastern margin of the Arabian Plate(Fig. 1), which is bounded by the Gulf of Adenspreading zone to the south, the Masirah Transform

Fault to the east and the Oman Mountains (SemailOphiolite) to the north (Looseveld et al. 1996).The southern and western margins of the ArabianPlate are formed by Oligocene–Miocene Red Seaand Gulf of Aden rift basins. The eastern and north-ern margins of the Arabian Plate are formed by theZagros Fold Belt of southwestern Iran, the fold-and-thrust belt of southwestern Turkey and theMakran Fold Belt, where subduction occurs, in theGulf of Oman (Al-Husseini 2000; Sharland et al.2001). Five phases of Arabian Plate tectonic evol-ution have been identified (Sharland et al. 2001).The Precambrian compressional phase (715 to610 Ma) was associated with the assembly of theArabian Plate and was followed by a late Precam-brian to Late Devonian extensional phase (from

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610 to 364 Ma), during which the first preservedsediments were deposited in extensional rift basins.This was followed by a Late Devonian to mid-Permian phase (from 364 to 255 Ma), a Mesozoicextensional phase (from 255 to 92 Ma) and, finally,by a late Mesozoic to present-day phase (from

92 Ma to present).The traps containing the giant and supergiant oil

and gas fields of the Arabian Plate were produced bythe reactivation of structures formed during the Pre-cambrian Amar Collision between 640 and 620 Ma.These structures form a regional, systematic and reg-ular north-, NE- and NW-trending orientation as aresult of regional compression. The Amar Collisionwas followed by the widespread extensional collapseof the Arabian Shield between c. 620 and 530 Ma,during which the north-trending anticlines remainedelevated as elongated horsts bounded by faults

(Al-Husseini 2000). During the final extensionalstage (c. 570 to 530 Ma), the regional, NW-trendingleft – lateral Najd fault system dislocated the ArabianShield by c. 250–300 km, and generated a seriesof complementary NE-trending rift basins in theArabian – Persian Gulf, Oman, Yemen, Pakistan, theRub ‘al Khali Basin and the Zagros Mountains (Pol-lastro 2002).

The major rift basins in the Arabian– PersianGulf, Oman and the Rub ‘al Khali were formedduring the Late Neoproterozoic– Early Cambrian(570 and 530 Ma). During this period, the Arabian

Plate was close to the equator (Konert et al. 2001),and thick evaporate sequences were deposited inrestricted rift basins (Pollastro 2002). In Oman,the evaporites occur in the Late Neoproterozoic–Cambrian Huqf Supergroup (Oterdoom et al.1999). The associated rift basins include the HormuzSalt Basins of the Gulf area and the Ghaba, Fahudand South Oman (Ara) salt basins of Oman. Smallercontemporaneous rift basins have recently beendocumented on seismic profiles in the central andeastern portion of the Rub ‘al Khali Basin and alongthe main Najd fault trend (Pollastro 2002). These rift

basins contain multiple, stacked petroleum systemsof Neoproterozoic to Cenozoic age. The Late Neo-proterozoic–Early Cambrian evaporite deposits of the Hormuz Series in the Zagros Fold Belt, inOman (the Ara Evaporites), in Central Iran (KermanEvaporites), in Pakistan (the Salt Range) and inIndia (Hanseran Evaporites) were part of the samegeological salt province (Fig. 2) (Smith).

The salt-bearing Late Neoproterozoic– EarlyCambrian succession of the Arabian Plate is repre-sented by the Huqf Supergroup (Abu Mahara, Nafunand Ara groups). The Huqf Supergroup is partlyexposed in two surface outcrops in Oman and hasbeen penetrated by wells (Pollastro 2002). The AraGroup (evaporite – carbonate cycle) rests uncon-formably on the Late Neoproterozoic Buah Fm.

(Nafun Group). The Shuram Fm., which underliesthe Buah Fm., contains the largest d13C (212‰)inorganic carbon negative excursion (Shuram excur-sion) in Earth history (Smith). To explain this enig-matic carbon excursion from the Ediacaran Period,‘thebalanceofevidenceatthispointsuggestsapossi-

bility of an exotic mechanism: a global diageneticevent. The most obvious scenario would involveglobal sea-level fall associated with the onset of gla-cial condition, and for the time being, the Shuramexcursion must be viewed from several perspectives’(Grotzinger et al. 2011).

The Ara Group hosts one of the best documen-ted Neoproterozoic– Cambrian petroleum systems(Table 2). The carbonate ‘stringers’ in the Ara Groupare fully enclosed in salt and are both reservoirs andsource rocks in a ‘self-charging’ petroleum system.The lateral equivalents of the Ara Evaporites in

Iran, Qatar, UAE and Saudi Arabia are the HormuzEvaporites (Smith). The Hormuz Evaporite succes-sion consists of massive salt, anhydrite, limestone,dark dolomite and some red sandstone (Kent 1970).A diverse microfossil assemblage (sphaeromorphicacritarchs, filamentous microfossils, fragmentaryvendotaenids and vaucheriacean algae) of Cryogen-ian (Abu Mahara Group; Ghadir Manqil Fm.), Edia-caran (NafunGroup; MasirahBay, Shuram andBuahformations) andEarly Cambrian(Ara Group)age hasbeen recovered from subsurface samples (Butter-field & Grotzinger) from Oman and is consistent

with the presence of organic-rich source rocks inthe Huqf Supergroup.

North China Plate (NW Tarim Basin)

During the Neoproterozoic, the Tarim Block in NWChina was connected to NW Australia and formedpart of East Gondwana before being possibly separ-ated during Late Ordovician times. Rifting duringLate Neoproterozoic (Fig. 2) resulted in the devel-opment of isolated depocentres in which shallow

marine clastic and carbonate sediments were depos-ited (Turner 2010). The Cambrian succession con-sists of an upward-shallowing carbonate succession,which eventually became emergent by the Late Cam-brian (Carroll et al. 2001).

Cambro-Ordovician marine sediments are themain hydrocarbon source rocks in the central andnorthern Tarim Basin (Table 2). The basal LowerCambrianblackphosphoriteshalerepresentsaperiodof anoxia and is similar to the Lower Cambrianphosphorite shales of Siberia, Australia and India(McKerrow et al. 1992). The widespread outcropsof the Cambrian– Ordovician megasequences reflectan uninterrupted period of carbonate sedimenta-tionin shallow depositional environments with littleevidence of tectonic influence. Subsequent phases

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of basin subsidence and sedimentation during thePalaeozoic and Cenozoic produced sufficient burialfor the maturation and migration of hydrocarbons(Turner). The Neoproterozoic to Lower Palaeozoiccarbonate-dominated succession contains poten-tial reservoirs that could be charged by Cambrian–

Ordovician source rocks (Turner). Inthe NW TarimBasin, multiple tectonic phases from the Palaeozoicto Recent have generated abundant structural traps(Lu et al. 2004). Ordovician carbonate rocks in theTazhong area contain secondary porosity and are inclose proximity to Cambrian– Ordovician sourcerocks. They form reservoirs for large oil and gasaccumulations where tectonic inversion has createdsuitable structural traps (Lu et al. 2004).

The Tarim Basin has long been recognized as aprolific region for hydrocarbon exploration, with aproven petroleum system in the Late Neoprotero-

zoic and Early Palaeozoic sequences (Table 2).

Conclusions

The papers presented in this volume provide acomprehensive overview of the geology and hydro-carbon potential of the major Neoproterozoic–Cambrian basins of Asia from Oman in the west,across the Middle East and the Indian Subcontinent,to China in the east and SE Siberia in the north.

Many of these areas (e.g. Oman, the Bikaner–Nagaur Basin in India, South China and SE Siberia)host prolific Neoproterozoic– Cambrian petroleumsystems with giant and supergiant fields. Effectivesource rocks, reservoirs and seals and appropriatehydrocarbon traps are widespread in most of theNeoproterozoic–Cambrian basins in Asia, but threekey elements seem to be critical for the developmentof effective Neoproterozoic– Cambrian petroleumsystems:

† relative tectonic stability since deposition of theNeoproterozoic– Cambrian succession;

† a relatively late phase of hydrocarbon maturationand generation;

† the presence of an effective seal (typicallyevaporites).

Theabsenceofanyoneofthesekeyelementsappearsto significantly reduce the chance of commercialvolumes of hydrocarbon being generated, migrated,trapped and preserved as conventional oil and gasfields in these Neoproterozoic– Cambrian basins.

These key elements appear to be less critical forthe development of ‘unconventional’ hydrocarbonresources. Indeed, the future hydrocarbon prospec-tivity in many of these basins in Asia may lie inthe exploration for, and production of, shale gasand shale oil directly from the ‘in situ’ thermally

mature organic-rich source rock horizons withinthe Neoproterozoic– Cambrian successions.

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