Gas potential of Proterozoic and Phanerozoic shales from the NWHimalaya, India: Inferences from pyrolysis

Devleena Mani a,⁎, D.J. Patil a, A.M. Dayal a, S. Kavitha a, Mateen Hafiz b, Naveen Hakhoo b, G.M. Bhat b

a CSIR-National Geophysical Research Institute, Hyderabad, Indiab Institute of Energy Research and Training (IERT) and Department of Geology, University of Jammu, Jammu & Kashmir, India

a b s t r a c ta r t i c l e i n f o

Article history:

Received 26 December 2013

Received in revised form 10 April 2014

Accepted 10 April 2014

Available online 24 April 2014

Keywords:

Himalaya

Shale

Shale gas

Organic matter

Rock Eval

Organic richness and kerogen properties of sixty-seven shales, obtained from the outcrops and underground

mines of Jammu, Kashmir and Ladakh regions of Northwest Himalaya, India, have been studied to evaluate

their gas generation potential using Rock Eval pyrolysis. Ranging in age from the Proterozoic to Tertiary, organic

matter content and characteristics of the carbonaceous and coaly shales vary widely, indicating that sedimentary

and burial history significantly affected the preservation and maturation of organic components in rocks.

The total organic carbon (TOC) content ranges from 0.01 to 1.2% in the Permian–Jurassic and Paleozoic–Tertiary

shales of Ladakh to as high as 32.5% in the Eocene shales/coaly shales from Jammu. The thermo-labile hydrocar-

bons (S1) and those from cracking of kerogen (S2), released from the pyrolysis of Eocene Subathu shales, are ob-

served to be high (from 0.1 to 2.6 and from 0.5 to 15.5 mg HC/g rock, respectively). Rock Eval thermal maturity

parameters, indicated by Tmax (temperature at highest yield of S2; N490 °C) and calculated vitrinite reflectance

(1.5 to 3.7 Ro %), suggest a post-mature, dry gas stage for the hydrocarbon generation. Based on hydrogen

index (HI) and Tmax correlations, organic matter in the Subathu shales is characterized by a late metagenetic

gas prone Type III kerogen and a fair to excellent gas potential is exhibited by these shales.

The interbedded shale units in the Proterozoic Sirban Limestone Formation, occurring as an isolated inlier in

Jammu, show TOC values from 0.1 to 1.4% with quite low S1, S2 and HI values. Thermal maturity of shales

within the stromatolitic Sirban succession indicates a post-mature and/or already spent hydrocarbon

stage. The Plio-Pleistocene carbonaceous clays, lignites and mudstones from the Lower Karewa Group of

Kashmir basin are organically rich with TOC content ranging from 5.86 to 29.4%. A thermally immature,

Type II/Type III kerogen is indicated by the HI (109 to 278 mg HC/g TOC) and Tmax (399 to 427 °C) plots

of the Karewa samples. The exposed Lower Triassic Black shales from the Permian–Triassic boundary sec-

tions in Kashmir are comparatively lean in organic matter, with TOC values ranging from 0.18 to 0.93%,

whereas the Permian–Jurassic and Paleozoic–Tertiary shales from Ladakh have a TOC content ranging

from 0.01 to 1.22%. The pyrolyzable organic carbon is b0.1% and the residual organic carbon contributes sig-

nificantly to the TOC content of these shales. The other Rock Eval parameters (S1, S2, HI) are quite low and

indicate a poor source potential. Themajority of shales from the Tethys and Trans Himalayan regions appear

to have undergone metamorphism and exhumation of organic carbon associated with the Himalayan

orogeny.

© 2013 Published by Elsevier B.V.

1. Introduction

The significant amount of natural gas trapped in organic-rich, fine

grained sedimentary shale rocks has emerged as an unconventional

energy resource in recent times globally (Boyer et al., 2006). Shale

gas, primarily methane, is generated in place from the buried organic

matter within the sediments under influence of heat and pressure.

The shale acts as a source as well as reservoir where the generated gas

remains stored within the intra- and inter-particle mineral pores and

organopores (Loucks et al., 2012; Ross and Bustin, 2009); natural frac-

tures in the shale, or adsorbed onto the organic matter (Boyer et al.,

2006). Advanced exploration technologies such as horizontal drilling

and hydraulic fracturing have allowed access to large volumes of shale

gas that were earlier uneconomical to produce (EIA, 2013; Horsfield

and Schulz, 2012; USGS, 2012). Shale gas technology has been largely

pioneered in the United States of America and the gas production

from several plays such as the Barnett, Haynesville, Fayetteville,

Woodford, and Marcellus offer an answer to the growing demand of

clean energy.

Ahead of drilling and production, shale gas plays spur a closer look at

shale rocks in terms of its organic matter distribution, richness and

International Journal of Coal Geology 128–129 (2014) 81–95

⁎ Corresponding author.

E-mail address: devleenatiwari@ngri.res.in (D. Mani).

http://dx.doi.org/10.1016/j.coal.2014.04.007

0166-5162/© 2013 Published by Elsevier B.V.

Contents lists available at ScienceDirect

International Journal of Coal Geology

j ourna l homepage: www.e lsev ie r .com/ locate / i j coa lgeo

properties that contribute largely to the gas generation potential, evalu-

ation of energy resource and delineation of target horizons (Boyer et al.,

2006). Compositional characteristics of organic matter provide useful

insights onto the variations in depositional environments that prevailed

during sedimentation (Romero and Philip, 2012). Geochemical attri-

butes of sedimentary organic matter such as the TOC content, kerogen

type and thermalmaturity are important parameters for the assessment

of gas shale potential toward hydrocarbon generation (Horsfield and

Schulz, 2012; Jarvie et al., 2007; Romero and Philip, 2012). Open system

pyrolysis of shales using Rock Eval is one of the basic organic geochem-

ical methods for characterization of the sedimentary organic matter.

The pyrolysis technique is based on the steady heating of rock samples

so that the total evolved hydrocarbons can bemonitored as a function of

temperature (Behar et al., 2001; Espitalie et al., 1987; Lafargue et al.,

1998). Released hydrocarbons and the carbon di and mono oxides gen-

erated from the organic/mineral sources aremonitored byflame ioniza-

tion (FID) and infrared (IR) detectors, respectively (Behar et al., 2001;

Lafargue et al., 1998).

For the Indian subcontinent, about 63 trillion cubic feet of recover-

able shale gas has been estimated from the sedimentary basins of Cam-

bay, Krishna–Godavari, Cauvery and Damodar Valley (DGH, 2013; Klett

et al., 2012). Among other frontier basins, those of theHimalayan region

appear prospective for the hydrocarbon resources (Bhattacharya and

Chandra, 1979; DGH, 2013). The Himalayan Foreland basin has no sig-

nificant oil and gas shows, however; owing to favorable geological con-

ditions for hydrocarbon generation and entrapment, it is considered

prospective. The Karewa and Spiti–Zanskar basins are categorized po-

tentially prospective due to their analogy with similar hydrocarbon

producing basins of the world (DGH, 2013; Jokhan Ram, 2005). The Hi-

malayan orogeny has been associatedwith the continental collision and

active tectonic prevails in the region (Molnar, 1986; Powell and

Conaghan, 1973). Plate junctions and major tectonic features have

been related to some of the largest oil and gas fields of the world

(Bullard, 1973; Nair et al., 1979). Studies have revealed that the level

of organic metamorphism in Himalayan foreland basins is favorable

for the occurrence of mainly thermo-catalytic gaseous hydrocarbons

(Bhattacharya and Chandra, 1979; Verma et al., 2006). The exploratory

knowledge, specifically for shale gas, in the tectonically active Himala-

yan region is in the knowledge building stage due to the constraints of

complex structure and tectonics of the fold and thrust belts and

logistics.

In the present work, organic geochemical characterization of shales

from Jammu, Kashmir and Ladakh regions of Northwest Himalaya has

been carried out using Rock Eval pyrolysis. Sixty-seven Proterozoic and

Phanerozoic shales were collected from the outcrops and underground

mines, which are physiographically located within the Outer, Tethys

and Trans Himalaya (Fig. 1). The sampled areas include the Cenozoic

Foothills belt of Outer Himalaya, comprising of the Eocene Subathu and

interbedded shales of Proterozoic Sirban Formations, carbonaceous

clays, mudstones and lignites of Karewa Group and the Black shales of

Permian–Triassic boundary sections of Kashmir basin along with the

Permian–Jurassic shales from the Zanskar basin of Tethyan Himalaya

and the Paleozoic–Tertiary shales from the Indus–Shyok and Karakoram

zones of Ladakh, Trans Himalaya (Figs. 1–4). Organic richness and kero-

gen properties of the varied age shales studied here provide useful in-

sights onto the abundance, distribution and thermal maturation of

Fig. 1. Geological map of Himalaya showing the different tectonic features along with the present study regions of Jammu, Kashmir and Ladakh.

Modified after Sorkhabi and Macfarlane (1999).

82 D. Mani et al. / International Journal of Coal Geology 128–129 (2014) 81–95

organic matter for the evaluation of its gas generation potential. The re-

sults obtained are interpreted in light of the sedimentary and burial his-

tory of the sampled strata located in different tectonic zones of the

Himalaya, which might have influenced the geological preservation

and maturation of sedimentary organic matter and accordingly, their

gas generation potential.

Fig. 2. Geological map of Jammu region showing the sample collection points for the Subathu and Sirban shales.

Modified after Hakhoo et al. (2011).

Fig. 3. Outline map of Kashmir basin showing the sample collection points for the Permian/Triassic shales and Karewa sediments.

Modified after Bhat (1989).

83D. Mani et al. / International Journal of Coal Geology 128–129 (2014) 81–95

2. Geologic setting and stratigraphy of studied areas

The Himalaya is the youngest and highest mountain range in the

world. The closing and subduction of the Tethyan Ocean between

India and Asia during Paleozoic, followed by collision of the two con-

tinents resulted in the rise of theHimalaya and a variety of deformation-

al structures and collision induced lithologies (Yin and Harrison,

2000). Fig. 1 illustrates the various tectonic elements of Himalaya

(Sorkhabi and Macfarlane, 1999). From south to north, there exist

six tectono-stratigraphic/tectono-geomorphic zones: the Outer

(Sub) Himalaya, the Lesser (Lower) Himalaya, the Higher (Great)

Himalaya, the Tethys (Tibetan) Himalaya, the Indus Suture Zone

and the Trans-Himalaya (Gansser, 1964). The arcuate Himalayan

belt stretches uninterruptedly for about 2500 km from west to east.

The frontier basins of Himalaya have thick sedimentary successions

and range in age from the Proterozoic to Pleistocene.

2.1. Outer Himalaya Zone

Forming the southernmost zone, the Outer Himalaya consists of

about 10 km thick Cenozoic sedimentary pile, predominantly continen-

tal molasses, ranging in age from Paleocene to Upper Pleistocene

(Thakur, 1993). Its northern boundary is limited by the Main Boundary

Thrust (MBT), which separates the Cenozoic belt from the pre-Tertiary

Lesser Himalayan strata (Fig. 1). To the south, the topographic break

against the alluvial plains of Ganga basin is expressed as a fault called

the Himalayan Foot Hill Fault (HFF) or the Main Frontal Thrust (MFT).

The Cenozoic sedimentary units of the Outer Himalaya are located in

four different tectonic settings: the Foot Hill belt, the Potwar Plateau

of Pakistan, the Ganga Basin and the Late Cenozoic Intermontane basins

(Thakur, 1993). Stratigraphically, the Tertiary Foot Hill belt in Jammu

has been divided into three principal groups: the Subathu Group, the

Murree (Dharamsala) Group, and the Siwalik Group (Karunakaran

and Ranga Rao, 1979; Table 1-1).

2.1.1. Eocene Subathu Formation, Jammu

Subathu Group and its equivalents occur north of the Siwalik Group,

as tectonically influenced outcrops extending from Potwar eastwards

through Jammu, Himachal Pradesh, Garhwal and Kumaun to their east-

ern limit in Nepal (Thakur, 1993). It consists of olive to dark green shales

interbedded with nummulitic limestone beds, carbonaceous shales and

coal seams. It is considered as source rock for the hydrocarbons in the

Himalayan Foreland basin in analogy with the adjoining petroliferous

Kohat–Potwar basin of Pakistan (DGH, 2013).

In the Jammu hills, the Subathu Group rocks occur as discontinu-

ous outcrops in three structural belts. From south to north these are:

1) Kalakot region, mainly around Kalakot, Metka, Mahogala, Riasi,

2) near Satra, south of Main Boundary Thrust and 3) near Mandi in

Himachal Pradesh (Fig. 2). In Jammu & Kashmir, the thickness varies

from about 80 to 100 m. The outcrops of Subathu Group in Kalakot,

Jammu consist of olive green shales with a thickness of about 80 m,

juxtaposed against the Sirban limestone Formation. The sequence

begins with coaly shales and coals of paralic origin becoming marine

upwards with olive green foraminiferal shales and lenticular lime-

stones (Thakur, 1993). Rich and diverse vertebrate fauna of Eocene

age, with a predominance of mammalian fossils, has been recovered

Fig. 4. Geological map of Ladakh showing the sample collection points for the Permian–Jurassic and Paleozoic–Tertiary shales.

After GSI (2013).

84 D. Mani et al. / International Journal of Coal Geology 128–129 (2014) 81–95

Table 1

Generalized stratigraphy of theCenozoics of Jammu Foot Hills-1; Karewa Beds, Kashmir-2; P/T section, Guryul Ravine-3; Tethyan Zone, Zanskar-4; Indus–Tsangpo Suture Zone, Ladakh-5.

After, Gansser (1964), Sweet (1970), and Thakur (1993) (not to scale).

85

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128–129(2014)81–95

from the Subathu sediments in the Kalakot area (Kumar and Sahni,

1985).

Significant palynological changes attributed to the variation in depo-

sitional environment from marine to fresh water is observed in the

Subathu sediments of Jammu and Dagshai sediments of the Simla

Hills, Himachal Pradesh (Sahni et al., 1983). Three main facies within

the Subathu Group have been identified, which suggest strandline con-

ditions characterized by transgressive and regressive phases, beginning

with subsidence and transgressive epicontinental sea followed by uplift

and shallowing of the basin, finally further uplift of the region to com-

plete withdrawal of the sea. These facies are the black-gray facies at

the base, overlain by the green facies and the red facies on the top, re-

spectively (Sahni et al., 1983; Thakur, 1993).

The Subathu Group is overlain by the Murree Group, and the later is

juxtaposed against the Siwalik Group along the Main Boundary Thrust

(=Reasi thrust in Jammu region) (Klootwijk et al., 1986).

2.1.2. Proterozoic Sirban Formation, Jammu

In the Jammu Sub-Himalaya, the Proterozoic Sirban Limestone

represents an allochthonous unit that crops out as detached inliers

(viz. (i) Dandili–Devigarh, (ii) Kalakot–Mahogala, (iii) Reasi and

(iv) Dhansal–Sawalkot (Lophri) inliers) toward the south of the MBT

(Fig. 2; Table 1-1; Craig et al., 2013; Hakhoo et al., 2011). The Sirban

Limestone succession consists of thickly bedded, highly jointed, hard,

and dark to light gray (silicified) dolostone, limestone characterized

by microbial mats and stromatolites interbedded with thin chert and

shale beds (up to about 10 m thick) and occasional 15–20 cm thick oo-

litic limestone and tempestite (storm deposit) beds (Craig et al., 2013).

The inliers occur in a belt about 80 km long and about 8–20 kmwide. In

the Reasi Inlier, the Sirban Formation is juxtaposed against the Tertiary

sedimentary successions of the Subathu- and the Murree Groups in the

northern part and against the Siwalik Group in the south (Klootwijk

et al., 1986).

2.1.3. Cenozoic Karewa Formation, Kashmir

The intermontane Kashmir basin, in NWHimalaya is a northwestly–

southeastly elongated depression, which is filled with about 1300 m of

weakly consolidated fluvio-lacustrine sediments of Plio-Pliestocene

age known as Karewas (Fig. 3). It has been divided into two broad

lithologic units (Table 1-2). The Lower Karewas are characterized

by mud-stones, unconsolidated sandstones, lignite layers, and con-

glomerate horizons. The Upper Karewas are laminated claystones,

sandstones and some conglomeratic layers and are devoid of lignites

(Kotila, 1990; Roy, 1975).

2.1.4. Permian–Triassic Section, Kashmir

The Permian–Triassic (P/T) boundary deposits are well exposed at

several sections in Kashmir like the Guryul Ravine, Barus, Pahalgam

etc. (Fig. 3). At Guryul Ravine section, the recognizable boundary is

the contact between the Zewan and Khunamuh Formations, which co-

incides with the regression level of Late Permian and Transgression

level of the Early Triassic (Table 1-3; Kapoor, 1996). It is marked by a

plane where maximum group of Permian biota disappeared and

new elements of Triassic nature appeared (Kapoor, 1996). The Zewan

Formation consists of carbonates, sandy shales, shales and calcareous

sandstones. The Formation is richly fossiliferous with bryozoans,

gastropods, foraminifera, conodonts etc. The succeeding Lower Triassic

Khunamuh Formation is characterized by alterations of limestones and

shales (Kapoor, 1996). Occurrence of the benthic mollusks suggests a

baythal environment. The stratigraphic sequence of Permo-Triassic

rocks is more than 300 m at Barus Spur and consists of calcareous and

carbonaceous shales, limestones, pebbly horizons and sandstones. Ex-

posures of boundary section at Pahlgam are much thicker and sandier

(Brookfield et al., 2003; Gupta and Brookfield, 1986).

2.2. Tethys Himalaya Zone

Themain belt of the Tethys Himalaya Zone lies in between the Indus

Tsangpo Suture and the Higher Himalaya Zone (Fig. 1; Thakur, 1993). In

the NW Himalaya, the well developed sequence of Tethyan Zone is ex-

posed in northern Kumaun, Malla Johar, Spiti and Zanskar regions

(Fig. 1). It comprises of over 10 km thick sequence of rocks, which are

predominantly fossiliferous and range in age from the Late Precambrian

to Cretaceous or Lower Eocene. To the north, the Tethyan sequence is

separated from the Indus Suture Zone by a south hading thrust, called

the Zanskar Thrust; and to the south, the sequence overlies the Central

Crystalline of the Higher Himalaya along a tectonic contact designated

as the Tethyan Thrust. The sedimentary sequence represents the de-

posits of the earlier Southern Tethys sea over the north facing Indian

margin (Thakur, 1993).

2.2.1. Permian–Jurassic Kulling Formation & Lilang Group, Zanskar

The Tethyan Himalaya Zone occupies 70 km wide belt in the

Zanskar mountains of Ladakh. The Zanskar sequence forms a gigantic

synclinorium called the Zanskar synclinorium showing a northwest

closure. The Late Precambrian to Lower Eocene sequence (c. 15 km

thick) here has two distinct facies in Mesozoic — the shallow shelf and

the deeper basin to slope facies. The Spongtong Klippe of the ophiolitic

rocks occurs in the core of synclinorium. It overlies theMesozoic units of

the Zanskar Tethys, thus representing their southward tectonic trans-

port from the root zone (Thakur, 1993). Stratigraphically, the Zone

has been divided into the Southern, Northern and Eastern Zanskar

(Thakur, 1993).

The Southern Zanskar includes the Late Precambrian to the Lower

Eocene sequence of the Southern part of the Zanskar Synclinorium

located between the Zanskar Valley and the mountain passes of the

Chalung La, Kangi La and Spanboth (Fig. 4). The principal strati-

graphic units are shown in Table 1-4. Overlying the volcanics of

Lower Permian, the Kulling Formation comprises of the sandstone,

shale and limestone with thickness ranging from 30 to 55 m

(Fig. 4; Table 1-4; Thakur, 1993). The Kulling Formation is succeeded

by 1000m thick sequence of dominantly carbonates described as Lilang

Group (Table 1-4; Thakur, 1993). The Northern Zanskar unit comprises

of the Zanskar Carbonates, Shilakong Formation, Lamayuru Formation,

Linghset Limestone and Kong Slate from north to south. The area of

Zanskar mountains lying east of the Zankar river and west of Nimaling

Range has been referred as Eastern Zanskar (Thakur, 1993). A tectonic

unit Tso–Morari Crystalline domal structure is an important unit in

the Eastern Zanskar.

2.3. Trans Himalaya

The Trans Himalaya is located north of the Tethys Himalaya Zone

and it includes the Indus and Shyok sutures and Karakoram Zone

(Fig. 1; Thakur, 1993). The Indus Suture Thrust (IST) separates the

largely south dipping Indus Suture rock unit from the shelf facies Tethys

Himalaya sequence and the Karakoram Thrust (KT) demarcates a

boundary between the Shyok Suture and the Karakoram Zone (Fuchs,

1977, 1984; Thakur, 1993).

2.3.1. Paleozoic–Tertiary Shyok Group, Nubra–Shyok Valley

The Ladakh region shows a well exposed cross-section of the Indus

Tsangpo Suture Zone (ITSZ). The ITSZ represents the boundary between

the Indian plate and the Karakoram–Tibet block of the Eurasian plate

(Thakur and Misra, 1984). Locally, it is referred to as the Indus Suture

Zone in Ladakh. The Indus Tsangpo Suture is divided into Indus and

Shyok sutures. The principal tectonostratigraphic units in the western

part of the Indus Suture, west of Leh and Shyok Suture Zone in the

Nubra–Shyok region have been described in Table 1-5. The Lamayuru

Formation (Lower Triassic to Middle Jurassic) appears to be an accreted

unit to the suture (Thakur, 1993). It has an average thickness of 3000 m

86 D. Mani et al. / International Journal of Coal Geology 128–129 (2014) 81–95

and consists of the shale, siltstone and graded sandstone. The Dras For-

mation constitutes the volcanics of the Late Jurassic to Early Cretaceous

age. The Cretaceous Nindam Formation consists of alternating thinly

and thickly bedded sandstone, siltstone and shale togetherwith bedded

tuffs and has an average thickness of 3000m. The Indus Formation con-

sists of thickly interbedded succession of predominantly conglomerate,

sandstone, siltstone and shale together with subordinate calcareous

shale and limestone. The entire sequence of the Indus Formation ranges

from the Lower Cretaceous to the Oligocene with shallow marine to a

near shore continental environment of the deposition (Thakur, 1993).

The main tectonostratigraphic units of the Shyok Suture Zone in the

Nubra–Shyok region are the Khalsar Formation, the Shyok Volcanics,

Saltoro Andesites, Hundri Formation, the Nubra ophioloitic malange

and the Saltoro Molasse (Table 1-5; Juyal, 2006). These rock formations

do not show a regular stratigraphic sequence but occur as silvers of

mélange (Thakur, 1993). The Karakoram metasedimentaries occur as

tectonic slices within the Shyok Suture Zone (Table 1-5; Fig. 4).

3. Field study and sample collection

A total of sixty-seven samples were collected from the underground

mines and the outcrops of Jammu, Kashmir and Ladakh regions of

Northwest Himalaya, India (Fig. 1). Samples collected from the subsur-

face mines were un-weathered and intact. When sampling the out-

crops, care was taken to collect the fresh, consolidated shales having

least signs of weathering by hammering out about ameter of outer por-

tion of the horizons. Area wise description of the sampling horizons is

mentioned below.

3.1. Jammu region

A total of twenty-nine shales/coaly shales and few coals belonging

to the Eocene Subathuwere collected from the interbedded shale ho-

rizons of underground mines in coal fields of Kalakot, Kotla and

Mahogla and outcrops at Salal, Kanthan and Kalimitti areas of Jammu

(Table 2; Fig. 2). Point samples of shales from different horizons and

the coal samples lying below the shaleswere collected.Where possible,

samples were collected at an approximate spacing of about 0.5 to 1.5m,

representing the vertical section of mines. Carbonaceous and coaly

shales were collected from the Subathu outcrops in Salal and Kanthan,

whereas the coaly shales at regular spacing of approximately 1 m

were sampled from a hillock at Kalimitti.

Three interbedded shale samples from the Proterozoic Sirban

Formation were collected from a thrust bound sliver (c. 40 m thick)

from the Tattapani area of Kalakot–Mahogala Inlier (Fig. 2).

3.2. Kashmir Region

The carbonaceous clays and lignites of the Karewa Group were

sampled from the Nichahom area of Baramulla District (Fig. 3). The

Nichahom section exposes the thickest lignites of the Hirpur Forma-

tion of Lower Karewas. The lignites occur in the Dubjan Member of

Hirpur Formation. Five carbonaceous samples were collected from

the different horizons of lignites exposed near the bottom portion

of a cliff wall for the study.

A total of sixteen Black Shales from the Permian–Triassic boundary

exposures at the Guryul Ravine section, 3 kmnorth of Barus, and Kathsu

village, Pahlgam in Kashmir basin were collected (Fig. 3). At Guryul

Ravine section, about nine shales from approximately spacing of about

0.5 to 1 m were samples, while seven point samples were collected

from the Barus and Pahlgam area.

3.3. Ladakh region

The Permian Jurassic shales from the Indus Suture Zone (ISZ) of

the Zanskar region were sampled for the study. The shales were

collected from the region between Kargil and Leh, around Lamayuru

and Saraks (Fig. 4). The Permian–Jurassic shales belong to the Undiffer-

entiated Kulling Group and Lilang Group of the Northwest Himalaya.

The Paleozoic–Tertiary shales were collected from the Shyok Suture

Zone (SSZ) of the Nubra–Shyok Valley, Northern Ladakh around the

villages of Shukur, Hunder and Diskit (Fig. 4). The shales belong to the

Shyok Group of Northwestern Himalaya. A total of fourteen samples

were collected from the exposed Formations in the ISZ and SSZ of

Ladakh–Karakoram zones.

4. Analytical procedure

4.1. Rock Eval pyrolysis

Rock Eval pyrolysis is used to estimate the petroleum potential of

rock samples by open system cracking of organic matter according to

a programmed temperature pattern. The complete process takes place

in the two ovens, pyrolysis and oxidation (combustion), respectively

of Rock Eval pyrolyzer. Pyrolysis proceeds with an initial isothermal

temperature program of 300 °C in an inert atmosphere of nitrogen.

The final temperature is set at 650 °C with a rise of 25 °C per minute.

The pyrolyzed hydrocarbons aremonitored by a flame ionization detec-

tor (FID), forming the so-called peaks S1 (thermovaporized free hydro-

carbons) and S2 (pyrolysis products from cracking of organic matter).

The method is completed by combustion of the residual rock recovered

after pyrolysis up to 850 °C, under artificial air (N2/O2). During pyrolysis

and combustion, releasedCOandCO2 aremonitored on line bymeans of

an infra-red cell. This complementary data acquisition enables determi-

nation of the organic and mineral carbon content of samples, labeled

TOC and MinC, respectively. The Tmax value is a maturity parameter

and corresponds to the temperature at whichmaximum amount of hy-

drocarbons are released from the thermal degradation of kerogen, i.e.;

the temperature atwhich S2 peak reaches itsmaximum. Among various

calculated parameters of Rock Eval, the hydrocarbon potential or hydro-

gen index, (HI) is defined by 100 × S2/TOC. The oxygen index, (OI) is

defined as 100× S3/TOC,where S3 is the CO2 released during the pyrol-

ysis. These indices help in defining kerogen types and maturation. The

experimental temperatures are set considerably higher than those

found naturally in the subsurface, so that appreciable reaction for the

generation of hydrocarbons can occur in a reasonably short time and

amount of generated hydrocarbons relative to the total potential of

the source rock can be estimated (Nuñez-Betelu and Baceta, 1994). De-

tails on Rock Eval functioning, parameters acquired, and interpretive

guidelines have been discussed by several workers (Espitalie et al.,

1987; Peters, 1986; Peters and Cassa, 1994).

4.2. Methodology

The shale sampleswere washedwithMilliQ water, air dried at room

temperature and powdered homogenously. The pyrolysis of shales was

carried out using the Rock Eval 6 pyrolyzer, Turbo version (Vinci Tech-

nologies). After obtaining a stable signal for the detectors, the instru-

ment was calibrated in standard mode using the IFP standard, 160,000

(Tmax = 416 °C; S2 = 12.43). The samples were weighed in pre-

oxidized crucibles depending upon the organic matter content

(~50–70mg of shale; and 8–15mg of coaly shale). The shale samples

were run under analysis mode using the bulk rock method and basic

cycle of Rock Eval 6 and the data was reported on dry weight basis.

5. Results

The important parameters obtained from the pyrolysis of shales

using Rock Eval 6 are given in Table 2. The Eocene shales/coaly shales

and coals collected from the underground mines of Kalakot, Kotla and

Mahogala, and outcrops of Salal, Kanthan and Kalimitti areas of

Jammu show quite high Total Organic Carbon (TOC) content ranging

87D. Mani et al. / International Journal of Coal Geology 128–129 (2014) 81–95

Table 2

Rock Eval pyrolysis results of samples from Jammu, Kashmir and Ladakh, Northwest Himalaya, India.

S no. Sample ID Type S1a S2a PIa Tmaxa S3a PC (%)a RC (%)a TOC (%)a HIa OIa VRo %a

Subathu Formation, Jammu

Bergoa Coal Mine, Kalakot

1 E-S-BK-1 Coaly shale 2.66 15.54 0.15 502 0.09 1.54 28.86 30.4 51 0 1.9

2 E-S-BK-2 Coaly shale 1.03 13.21 0.07 498 0.17 1.20 23.36 24.56 54 1 1.8

3 E-S-BK-3 Carb. shale 0.06 2.25 0.03 512 0.07 0.20 6.46 6.66 34 1 2.1

4 E-S-BK-4 Carb. shale 0.05 0.83 0.06 542 0.12 0.09 4.34 4.43 19 3 2.6

Chakkar Coal Mine, Kotla

5 E-S-CK-1 Coal 0.23 33.76 0.01 484 0.1 2.87 45.68 48.55 70 0 1.6

6 E-S-CK-2 Carb. shale 0.19 1.63 0.11 496 0.02 0.16 5.61 5.77 28 0 1.8

7 E-S-CK-3 Carb. shale 0.09 0.94 0.09 501 0.01 0.10 4.39 4.49 21 0 1.9

8 E-S-CK-4 Carb. shale 0.06 0.94 0.06 504 0 0.09 4.37 4.46 21 0 1.9

9 E-S-CK-5 Carb. shale 0.49 1.07 0.32 499 0.02 0.14 4.62 4.76 22 0 1.8

10 E-S-CK-6 Carb. shale 0.11 0.51 0.18 502 0.01 0.05 3.22 3.27 16 0 1.9

11 E-S-CK-7 Carb. shale 0.06 0.88 0.06 494 0.01 0.09 4.07 4.16 21 0 1.7

Mahogla Coal Mine, Mahogla

12 E-S-MM-1 Coal 0.34 71.62 – 488 0.48 6.09 57.44 63.53 113 1 1.6

13 E-S-MM-2 Carb. Shale 0.13 2.26 0.05 518 0.05 0.20 6.73 6.93 33 1 2.2

14 E-S-MM-3 Coal 0.25 27.35 0.01 496 0.38 2.43 51.28 53.71 51 1 1.8

15 E-S-MM-4 Carb. shale 0.05 2.51 0.02 535 0.04 0.23 10.49 10.72 23 0 2.5

16 E-S-MM-5 Carb. shale 0.21 6.57 0.03 495 0.06 0.60 13.90 14.50 45 0 1.8

17 E-S-MM-6 Carb. shale 0.33 6.50 0.05 499 0.11 0.59 10.81 11.40 57 1 1.8

18 E-S-MM-7 Carb. shale 0.05 0.63 0.07 517 0.02 0.06 2.36 2.42 26 1 2.1

19 E-S-MM-8 Carb. shale 0.12 7.72 0.01 498 0.05 0.68 13.27 13.95 55 0 1.8

20 E-S-MM-9 Carb. shale 0.15 2.50 0.06 515 0.06 0.23 7.73 7.96 31 1 2.1

Salal

21 E-S-S-1 coal 0.60 20.62 0.03 537 0.53 1.89 76.00 77.89 26 1 2.5

22 E-S-S-2 Coaly shale 0.14 3.40 0.04 587 0.99 0.43 30.34 30.77 11 3 3.4

Kanthan

23 E-S-K-1 Coaly shale 0.17 0.68 0.2 606 5.65 0.49 27.48 27.97 2 20 3.7

Kalimitti

24 E-S-KM-1 Coaly shale 0.11 3.65 0.03 602 2.87 0.65 30.90 31.55 12 9 3.7

25 E-S-KM-2 Coaly shale 0.36 4.41 0.07 573 0.26 0.52 29.41 29.93 15 1 3.2

26 E-S-KM-3 Coaly shale 0.14 3.05 0.05 570 0.14 0.31 21.15 21.46 14 1 3.1

27 E-S-KM-4 Coaly shale 0.29 3.91 0.07 600 1.05 0.56 31.90 32.46 12 3 3.6

28 E-S-KM-5 Coaly shale 0.08 1.09 0.07 598 0.54 0.21 13.04 13.25 8 4 3.6

29 E-S-KM-6 Coal 0.21 3.38 0.06 597 11.71 1.06 38.60 39.66 9 30 3.6

Sirban Formation, Jammu

Tattapani

30 P-S-TP-1 Calc. shale 0.01 0.00 0.99 322 0.08 0.00 0.05 0.05 0 160 –

31 P-S-TP-2 Carb. shale 0.20 0.06 0.76 533 0.38 0.04 0.87 0.91 7 42 –

32 P-S-TP-3 Carb. shale 0.03 0.10 0.21 527 0.43 0.03 1.33 1.36 7 32 –

Permian–Triassic Boundary, Kashmir

Guryul Ravine

33 PT-GR-1 Black shale 0.03 0.03 0.52 335 0.14 0.01 0.17 0.18 17 78 –

34 PT-GR-2 Black shale 0.03 0.04 0.39 344 0.02 0.01 0.19 0.20 20 10 –

35 PT-GR-3 Black shale 0.02 0.03 0.32 342 0.09 0.01 0.28 0.29 10 31 –

36 PT-GR-4 Black shale 0.02 0.01 0.67 336 0.04 0.01 0.23 0.24 4 17 –

37 PT-GR-5 Black shale 0.00 0.00 – – 0.08 0.00 0.19 0.19 0 42 –

38 PT-GR-6 Black shale 0.00 0.00 – 345 0.06 0.00 0.28 0.28 0 21 –

39 PT-GR-7 Black shale 0.00 0.00 – 523 0.29 0.01 0.69 0.70 0 41 –

40 PT-GR-8 Black shale 0.00 0.00 – – 0.24 0.01 0.19 0.20 0 120 –

41 PT-GR-9 Black shale 0.00 0.00 – 360 0.61 0.02 0.71 0.73 0 84 –

Barus

42 PT-BS-1 Black shale 0.00 0.00 – – 0.83 0.03 0.90 0.93 0 89 –

43 PT-BS-2 Black shale 0.00 0.00 – – 0.19 0.01 0.77 0.78 0 24 –

44 PT-BS-3 Black shale 0.00 0.00 – – 0.33 0.01 0.92 0.93 0 35 –

45 PT-BS-4 Black shale 0.26 0.00 – – 0.24 0.03 0.83 0.86 0 28 –

Kathsu, Pahlgam

46 PT-KP-1 Black shale 0.00 0.00 – – 0.34 0.01 0.42 0.43 0 79 –

47 PT-KP-2 Black shale 0.00 0.00 – – 0.24 0.01 0.39 0.40 0 60 –

48 PT-KP-3 Black shale 0.00 0.00 – – 0.29 0.01 0.30 0.31 0 94 –

Karewa sediments, Kashmir

Nichahom

49 PP-K-1 Carb. clay 0.25 9.53 0.03 424 4.23 1.05 4.81 5.86 163 72 0.5

50 PP-K-2 Shaly lignite 3.40 48.05 0.07 399 19.74 5.45 22.10 27.55 174 72 0.0

51 PP-K-3 Shaly lignite 10.25 81.69 0.11 402 18.54 8.64 20.71 29.35 278 63 0.1

52 PP-K-4 Carb. clay 0.79 21.44 0.04 419 15.42 2.67 14.13 16.80 128 92 0.4

53 PP-K-5 Shaly lignite/mudstone 1.84 28.50 0.06 407 25.81 3.91 22.26 26.17 109 99 0.2

Kulling–Lilang GRP, Ladakh

Lamayuru

54 PJ-L-1 Silty, Calc. shale 0.89 0.05 0.95 – 0 0.08 0.08 0.16 31 0 –

55 PJ-L-2 Silty, Calc. shale 0.00 0.00 – – 0.01 0.00 0.08 0.08 0 12 –

Kulling–Lilang GRP, Ladakh

88 D. Mani et al. / International Journal of Coal Geology 128–129 (2014) 81–95

from3.2 to 77.8%. Frequency distribution diagramof the TOC (%) (Fig. 5)

indicates the organic richness of the Subathu shales. The S1 values range

from 0.01 to 2.6 mg HC/g rock (milligram hydrocarbon/g of rock)

(Table 2). S2 shows an elevated value ranging from 0.51 to

71.62 mg HC/g rock. The HI ranges between 2 and 113 mg HC/g

TOC, where as the oxygen index (OI) for all studied samples is low

(b30 mg HC/g CO2). A modified van Krevelen diagram (van Krevelen,

1961) (Fig. 6) indicates that organic matter is characterized by Type III

kerogen. The Tmax of the shale samples ranges from 490 to 515 °C sug-

gesting an over mature phase for the hydrocarbons (Fig. 7). The coals

and coaly shales have TOC content N30%, and Tmax above 550 °C sug-

gesting high levels of maturity as compared to the carbonaceous shales

(Table 2; Fig. 7). The organicmatter inmajority of Subathu samples con-

sists of Type III kerogen and has generation potential for the gaseous hy-

drocarbons (Fig. 8). Rock Eval vitrinite reflectance, calculated using the

Tmax data of Subathu samples (0.018 × Tmax− 7.16; Jarvie and Lundell,

1991), ranges from 1.5 to 3.7 Ro % (Table 2). It indicates an overmature,

dry gas stage. The ratio of S1 to TOC vs depth has been used to deter-

mine the interval at which a source rock begins to expel oil. In general,

ratios between 0.1 and 0.2 have been suggested for oil generation by

Smith (1994). The S1/TOC values for the Subathu shales are in the

range of 0.01 to 0.04, except for one sample where it is 0.1. The values

well below 0.1 indicate that these shales could generate gas (Hunt,

1996). The relationship of S2 and TOC has been used to define the po-

tential of source rocks (Shalaby et al., 2012). S2 vs TOC plots show the

source rock properties of Subathu sediments to vary widely with an

overall expression of fair to excellent potential (Fig. 9).

The interbedded shale units in the Proterozoic Sirban limestone

Formation from Tattapani, Jammu show an average TOC of ~1%

(Table 2). However; all the other Rock Eval parameters such as S1,

S2, and HI are quite low for these shales. The carbonaceous clays

and lignites from the Karewa Formation, Nichahom are rich in organic

matter with the TOC content up to 29.4%. HI values are comparatively

high, ranging between 109 and 278 mg HC/g TOC. The organic matter

in majority of samples contains mixed Type II/Type III kerogen

(Fig. 10) The Tmax ranges between 399 and 27 °C suggesting an im-

mature phase for the hydrocarbon generation (Fig. 11).

The shales from the Permian–Triassic boundary in Kashmir are

lean in organic matter with TOC values of b1% (Table 2). At Guryul

Ravine section, the TOC content is between 0.1 and 0.7%; where as

those of Barus spur are between 0.7 and 0.9%. The samples from

Kathsu, Pahalgam show the TOC values between 0.3 and 0.4%. The HI

values for these shales are extremely low (Fig. 10). The Permian–Jurassic

and Paleozoic–Tertiary shales from Ladakh–Karakoram zones show

quite low TOC content of b0.5%; except for two samples where the

TOC is 1.12 and 1.22% (Table 2). The other pyrolysis parameters such

as S1, S2, HI are very low for these shales.

6. Discussion

In general, the organic content and thermal maturity of the

Proterozoic–Phanerozoic Himalayan shales observed using the Rock

Eval pyrolysis vary widely, depending upon the quality and quantity

of preserved sedimentary organic matter and geological settings. The

TOC content is quite high in the younger sediments such as the Subathu

shales of Eocene age and Plio-Pleistocene Karewas, as compared to the

Proterozoic and Paleozoic–Mesozoic shales of Kashmir and Ladakh, re-

spectively, which possess very low organic content. An overall low to

moderate HI values is observed in all the samples. Oxygen index varies

widely with quite low values in the Eocene shales as compared to the

samples fromother regions. Immature to high levels of thermalmatura-

tion characterizes the sediments, with the dominance of Type III kero-

gen in samples with significant TOC content.

Regionally, the organic matter in Subathu shales from Jammu is

characterized by organic rich; gas prone, Type III kerogen and the source

rock potential varies from fair to excellent (Figs. 5–9). The calculated

Rock Eval vitrinite reflectance varies between 1.5 and 3.7 Ro %

(Table 2), suggesting the post-mature, dry gas stage (Hunt, 1991).

Overall, the HI is low, however; the Subathu shales from Mahogla

and Kotla coal fields have comparatively higher values. Accordingly,

the thermal maturity of shales from these areas is also low as compared

to the Subathu shales from other places of Jammu. Usually, the prolific

gas–shale systems are usually characterized by high organic richness

of N3% TOC and HI values greater than 350 mg HC/g TOC (Slatt and

Table 2 (continued)

S no. Sample ID Type S1a S2a PIa Tmaxa S3a PC (%)a RC (%)a TOC (%)a HIa OIa VRo %a

Lamayuru

56 PJ-L-3 Dark gray shale 0.01 0.00 – – 0.2 0.01 1.11 1.12 0 18 –

57 PJ-L-4 Dark gray shale 0.00 0.00 – – 0.13 0.01 1.21 1.22 0 11 –

Shyok GRP, Ladakh

Shukur

58 PT-L-1 Light gray shale 0.00 0.00 1 – 0.01 0.00 0.01 0.01 0 100 –

59 PT-L-2 Calc. light gray shale 0.00 0.00 1 – 0.01 0.00 0.02 0.02 0 50 –

60 PT-L-3 Light gray shale 0.00 0.00 0.95 342 0.01 0.00 0.04 0.04 0 25 –

61 PT-L-4 Light gray shale 0.00 0.00 0.75 341 0.02 0.00 0.04 0.04 0 50 –

62 PT-L-5 Calc.,light gray shale 0.01 0.01 0.39 348 0.07 0.01 0.13 0.14 7 50 –

63 PT-L-6 Calc. light gray shale 0.01 0.01 0.52 350 0.07 0.01 0.27 0.28 4 25 –

64 PT-L-7 Calc. light gray shale 0.06 0.01 0.87 332 0.06 0.01 0.07 0.08 12 75 –

65 PT-L-8 Calc.light gray shale 0.14 0.00 1 334 0.07 0.01 0.04 0.05 0 140 –

66 PT-L-9 Calc.light gray shale 0.00 0.00 1 – 0.05 0.00 0.28 0.28 0 18 –

67 PT-L-10 Calc.light gray shale 0.00 0.00 0 – 0.06 0.00 0.06 0.06 0 100 –

S1& S2 = mgHC/g rock; Production Index, PI = S1 / (S1 + S2); Tmax = °C; S3 = mg CO2/g rock; PC = pyrolyzable organic carbon; RC = residual organic carbon;HI = mgHC/g TOC;

OI = mg CO2/g TOC; calculated vitrinite reflectance VRo % = 0.018 × Tmax − 7.16, Carb. = carbonaceous, Calc.= calcareous.a Units.

Fig. 5. Frequency distribution diagram of the TOC (%) content in Subathu shales, Jammu.

89D. Mani et al. / International Journal of Coal Geology 128–129 (2014) 81–95

Rodriguez, 2012). These values are greater than the threshold values

typical of gas generation from conventional gas-prone source rocks

and similar to the characteristics typical of Type II kerogen-rich, oil-

prone source rocks (Slatt and Rodriguez, 2012). Nevertheless, recent

studies have indicated that there might be an additional dry gas charge

for some types of organic-rich shales, subsequent to regular primary

Fig. 6. Modified van Krevelen diagram indicating the kerogen type for the Subathu shales, Jammu.

Fig. 7. HI versus Tmax plot indicating the thermal maturity of kerogen in the Subathu shales, Jammu.

90 D. Mani et al. / International Journal of Coal Geology 128–129 (2014) 81–95

and secondary decomposition reactions at geologic temperatures well

in excess of 200 °C (Ro N2.0%) (Mahlstedt and Horsfield, 2012).

The organic rich sediments of Subathu Groupmight act as source for

the late metagenetic gaseous hydrocarbons. The sapropelic and humic

type of organic matter in Subathu indicates degree of metamorphism

higher than the last stage of oil generation (Karunakaran and Ranga

Rao, 1979). Small quantities of the flaky, semi anthracite coal has been

reported in the lowest part of the sequence (Karunakaran and Ranga

Rao, 1979). Although having being subjected to low grade metamor-

phism, a high carbon ratio with respect to hydrogen of associated coal

is exhibited by the Eocene sediments (Karunakaran and Ranga Rao,

1979). Pyrolysis results show high values for the residual carbons

(Table 2). Due to tectonic deformations and subsequent thermal matu-

ration, a highly mature stage of late gas generation is expected of these

sediments, particularly when moving from base to the top of the

sequence.

Depositional environment, apart from initial organic matter

structure and precursor biota, have a significant role in generation

of such metagenetic late gas plays. Facies analysis of the outcrops

of Subathu Group shows that in the southern areas represented by

the Kalakot, Jammu and north of Kalka, Himachal Pradesh, conditions

of deposition changed from paralic to shallow marine and that marine

conditions existed from Lower to Middle Eocene (Karunakaran and

Ranga Rao, 1979). Beginning with the Upper Eocene, the rest of se-

quence was found to be continental in character (Karunakaran and

Ranga Rao, 1979). The thickness of the Subathu in these areas range

from 200 to 600 m. Sufficient thickness of organic rich strata, usually

greater than 65m, is necessary criteria in evaluation of a shale gas pros-

pect (Slatt and Rodriguez, 2012). Compared to south, in the areas to the

north of Kalakot, shallow marine conditions alternated with the non

marine conditions due to repeated transgression and regressions of

the sea (Karunakaran and Ranga Rao, 1979). The thickness here is

estimated to be of the order of 2000 m (Karunakaran and Ranga Rao,

1979). The quantitative petrological investigations suggest that these

coals are vitrinite rich, with low concentrations of inertinite and rare oc-

currences of liptinite (Singh and Singh, 1995). The Subathu sedi-

ments show features characteristic of an open sea deposit (shelf

mud, tidal flats and sand bars), partly with hyper saline and reducing

conditions with fine sediment laminae (Singh and Singh, 1995; Singh

and Srivastava, 2011; Singh et al., 2000). Late gas potential has been

associated with heterogeneous admixtures or structures in terrestri-

ally influenced, in some cases marine Type III and Type II/III coals and

shales (Mahlstedt and Horsfield, 2012).

Stratigraphic equivalents of Subathu, the Lower Dharamsala For-

mation of Himachal Pradesh, indicated gas shows during exploratory

drilling (DGH, 2013; Karunakaran and Ranga Rao, 1979). The gases of

Jwalamukhi and Nurpur wells are methane rich with low nitrogen

concentration and are dry and thermogenic in nature (C2+ b2%)

(Mittal et al., 2006). Carbon isotopic composition of methane from

these wells suggest a deep over mature source for the gases (δ13C1

~−32.0‰) (Mittal et al., 2006). In the Foot Hills of the Outer Himalaya,

the Tertiary belt has large thickness and is folded into long anticlinal

structures forming suitable traps for the entrapment of gaseous hydro-

carbons. Of this large thickness, the lower-most section comprising of

the Subathu Group has marine origin (Kurien and Rajarajan, 1979).

X-Ray diffraction studies on outcropping Subathu shales from the

Jammu area have shown the lithology to have high quartz to clay

content with almost no carbonate content (Mateen et al., 2013, per-

sonal communication). Major oxides, SiO2, Al2O3 and CaO are repre-

sentative of the main mineral phases namely quartz, clays and

carbonate, respectively. High Si/Al ratios indicate quartz-rich miner-

alogy, where as low Si/Al ratio is typically clay-rich. The fracability of

shale is an important concern for the development of plays and is

governed largely by the mineralogy (Ross and Bustin, 2009). With

high brittleness, the response of quartz rich Subathu shale to fracking

appears to be encouraging based on the preliminary lithological studies

(Mateen et al., 2013, personal communication). Estimations on expelled

and retained gas can be made, taking into account the generation po-

tential, source rock thickness and extent, along with the porosity and

permeability parameters for the Subathu shales.

The two of three interbedded shale samples collected from the

Proterozoic Sirban Limestone Formation from Tattapani, Jammu

show a TOC content of 0.91 and 1.36%. However; the HI values for

these samples is very low (b7 mg HC/g TOC). The low TOC (0.05%)

for one sample indicates the degradation of organic matter, probably

due to oxidation. The OI for the sample is quite high (160 mg CO2/g

TOC), indicating that weathering influenced the preservation of or-

ganic matter. Oxidation removes hydrogen and adds oxygen to the

kerogen, and, therefore, HI values are usually lower and OI values

higher for outcrop samples than for fresh core samples (Tissot and

Welte, 1984). The Tmax values of the shales with TOC content from

1 to 1.36% show a highly mature stage for the hydrocarbon genera-

tion. The Sirban Limestone extends westwards to the oil producing

Potwar basin in Northeast Pakistan and on basis of lithological simi-

larity it has been correlated with the outcrops to the NW in the Salt

Range and the Muzzafarabad–Punch sector (Sirban Limestone) in

Pakistan; to the NE in Dharamshala (Dharamkot Limestone); and in

Shimla (Tundapather Limestone) in Himachal Pradesh, northern India

(Bhat et al., 2009; 2012). Although the Proterozoic sediments have

been considered less suitable for oil and gas exploration; the hydrocar-

bon potential of theNeoproterozoic petroleum systems has been prom-

ising globally (Craig et al., 2013; Hakhoo et al., 2011; Jokhan Ram,

2012). In Indian subcontinent, the recent discovery of liquid hydrocar-

bons from Baghewala-1 in the Terminal Proterozoic (Vendian) and

Lower Paleozoic (Cambrian) sequences has opened up a new explora-

tion frontier for Cambrian–Infracambrian sequences (Ojha, 2012;

Jokhan Ram, 2012). The Baghewala-1 oil has been reported to be geo-

chemically similar to another heavy oil from the Infracambrian Salt

Fig. 8. HI vs TOC (%) plot indicating the gas prone source potential of Subathu shales,

Jammu.

Fig. 9. Source rock characteristics as interpreted by the relationship between the remain-

ing hydrocarbon potential (S2) and TOC (wt.%) for the Subathu shales from Jammu.

91D. Mani et al. / International Journal of Coal Geology 128–129 (2014) 81–95

Fig. 10. Modified van Krevelen diagram (HI vs OI) indicating the kerogen type of Karewa sediments from the Kashmir basin.

Fig. 11. HI versus Tmax plot for the carbonaceous sediments from the Lower Karewa, Nichahom section, Kashmir.

92 D. Mani et al. / International Journal of Coal Geology 128–129 (2014) 81–95

Range Series in the nearby Karampur-1 well in Pakistan and to oils de-

rived from carbonate–evaporite facies of the Infracambrian Huqf Group

about 2000 km to the southwest in the Eastern Flank province of south-

ern Oman (Dutta et al., 2013; Peters et al., 1995). The organic richness

and thermal maturity of the shales, as indicated by the pyrolysis data

and the presence of diverse biota dominated by various cyanobacteria,

microbial stromatolites and acritarchs (Bhat et al., 2009; Craig et al.,

2013; Raha and Sastry, 1982) in the Sirban Limestone succession

makes it a potential hydrocarbon source. The shale units between the

Sirban Limestones may possibly act as a source for gaseous hydrocar-

bons. However; the present-day high level of thermal maturity of the

shales (Tmax = 566–572 °C) also suggest a possibility that the source

may already be in a spent hydrocarbon stage.

The Plio-Pleistocene carbonaceous clays, dark mudstones and lig-

nites from Nichahom, Kashmir basin show varied range of high TOC

content (5.86–29.35%). The samples are characterized by high HI

(109–278 mg HC/g TOC) and OI (63–99 mg HC/g CO2) indicating a

lower maturity for these sediments. Modified van Krevelen diagram

(HI vs OI) for the Karewa sediments suggest that the organic matter

is characterized by mixed Type II/III kerogen (Fig. 10). An immature

stage for hydrocarbon generation is also indicated by their HI vs Tmax

plot (Fig. 11). The Karewa lignites are characterized by low percentage

of fixed carbon and high moisture content. The average of proximate

content in Karewa lignites are: Ash = 40.56%; Volatile matter =

27.04%; Moisture = 16.26%; Fixed Carbon = 16.14% (Bhat, 1989).

Due to immature level of kerogen in the present samples under consid-

eration, further geochemical and kinetic investigations on the Karewa

sediments from different regions of Kashmir are required to infer its

source potential.

The Permian–Triassic Black Shales from Kashmir have a TOC con-

tent of b1%. Majority of samples from the Guryul Ravine section are

organically lean with TOC ranging from 0.1 to 0.2%. However; two

samples (P/T-GR-7 & P/T-GR-9) have TOC content of 0.7%, indicating

characteristics of a fair source potential. These samples represent the

Lower Triassic Black Shales of Guryul Ravine P/T boundary section.

The shale samples from the Barus area have comparatively higher TOC

(0.78–0.98%); where as that of Pahlgam range from 0.31 to 0.4%. How-

ever, a commonality of the Rock Eval parameters S1, S2 being zero, a

low HI (4–20 mg HC/g TOC), and a high OI (10–120 mg HC/g CO2)

may be indicative of the weathering or metamorphic influences upon

these samples. Organic metamorphism, induced by heat (maximum

paleotemperature) and pressure, modifies the organic matter locked

in the sedimentary matrix as well as the mobile products, leading to

paraffinic oils and condensates, then to dry gas and pyrobitumens,

and finally to graphitization under severe conditions (Staplin, 1969).

The P/T sections have been reported to be metamorphosed, with abun-

dant chloritoid, indicating the sub-greenschist facies (Brookfield et al.,

2003). The TOC content obtained during pyrolysis is the sumof pyrolyz-

able and oxidative carbon present in the sample. Here, in samples

where organic content is high, the pyrolyzable carbon is b0.1%, indicat-

ing the TOC value to be derived almost entirely from the residual car-

bon. The low TOC content may also be attributed to the inherently

low organic matter deposition, as indicated by the lack of organic rich

laminae and pyrite precipitates, which generally are common in anoxic

environments (Brookfield et al., 2003; Srivastava and Singh, 1984).

With an overall low TOC, the HI, OI correlations thus, possibly indicate

altered/reworked organic matter content (Fig. 10).

The organic matter in the Permian–Jurassic and Paleozoic–Tertiary

shales from the Ladakh–Karakoram region shows low to negligible

TOC content (b0.2%). The Permian–Jurassic shales were sampled from

the Indus Suture Zone, around Saraks and Lamayuru (Fig. 4). However;

two samples (P/J-L-3 and P/J-L-4) show comparatively higher TOC con-

tent of 1.12 and 1.22%, respectively (Table 2). The Paleozoic–Tertiary

shales collected from Shyok Suture Zone around Shukur, Hunder and

Diskit areas of Nubra–Shyok Valley show quite low TOC content, rang-

ing from 0.05 to 0.33% (Fig. 4; Table 2). Other Rock Eval parameters

such as S1, S2, and HI for the Permian–Jurassic as well as Paleozoic–

Tertiary shales are low to negligible, leading to an unreliable Tmax and

a paucity in typing of kerogen and its maturation assessment toward

gas generation. Post-depositional thermal alterations leading to organic

metamorphism (Price et al., 1999) and exhumation of buried organic

carbon could possibly account for the loss of TOC content from the sed-

imentary strata. The pyrolyzable carbon for the two Permian–Jurassic

shales from IST is zero and the net TOC is contributed by the residual

carbon only (Table 2). Examining the bulk organic carbon isotope and

rank data in South Africa shales, McKirdy and Powell (1974) have

shown that post-depositional thermal alteration may account for the

anomalously heavy reduced carbon and the maturation of the kerogen

beyond a rank equivalent to 91 to 93% carbon resulting in marked en-

richment of the residual organic matter.

Contact metamorphism of organic material leads to elevated

vitrinite reflectance (%Ro), loss of TOC, increased aromatization and

changes in carbon isotope compositions (δ13C) of the residual organic

material toward the contact (Aarnes et al., 2010 and references therein).

Organic metamorphism of these shales is supported by the fact that

about 30% of the shales from Tethys and Trans Himalaya have TOC con-

tent N0.5% and reaching up to a maximum of 1.2% with high residual

carbon values. These values indicate a fair to good source rock potential.

However; the maturity assessments using the van Krevelen diagram

here reflect the present-day generation potential and not the

original source rock generation potential. Similarly, the original

TOC, (TOCO) differs from the present day TOC content (TOCP)

(TOCP/0.64 = TOCO; Jarvie and Lundell, 1991) and is generally

higher that present day observations.

Loss of hydrogen with increasing organic maturation is widely

evident (Tissot and Welte, 1984). The maturity of organic matter af-

fects the HI and hydrocarbon generation of the samples, decreasing

both. Loss of TOC is also attributed to the exhumation of buried

organic matter due to a tectonic activity. Beck et al. (1995) have pro-

vided evidence that the timing of early Himalayan thrusting and ex-

humation of bulk organic carbon (Corg) (kerogen, bitumen, and

mobile hydrocarbons including methane) from neo-Tethyan strata

above the north Indian shelf coincides with and is quantitatively

compatible with the rapid decrease of δ13Ccarb near the Paleocene–

Eocene boundary.

It has been demonstrated that oxidative weathering of organic mat-

ter in black slates is a fast process with substantial decrease of organic

matter occurring within only some decades to a century, rather than

over geological time spans (Fischer et al., 2007). The depositional

history of sediments in Himalaya has been controlled by the major tec-

tonic elements and the intensity of geological activity is reflected in the

progressive changes in the composition of organic matter in highly

sheared Himalayan shales (Brookfield and Andrews-Speed, 1984).

Occurrence of numerous slate exposures in the Himalaya, particularly

in the Tethys, Higher and Trans Himalaya (Thakur, 1993) indicate the

intense level of tectonically induced metamorphism and oxidation of

the originally deposited organic rich sedimentary strata.

The geology of Himalaya is represented by several phases of tectonic

and deformational events and the rocks in Himalayan basins have been

deposited under highly variable conditions. The Lesser, Higher and Te-

thys Himalaya belonged to the northern part of the Indian plate; and

the Trans Himalaya that includes the Indus and Shyok sutures and the

Karakoram Zone constituted the southern part of the Asian plate

(Thakur, 1993). The Lesser Himalaya sequence, dominantly of Precam-

brian to Lower Cambrian age, represents the northerly extension of an

intracratonic basin of the Peninsular India; and the Higher Himalaya

rocks have come from the mid-crustal level, whose cover of upper

crust has been largely eroded away and deposited in the Tertiary fore-

land basin (Thakur, 1993). The Tethys Himalaya sequence of fossilifer-

ous Cambrian to Cretaceous and Early Cretaceous and Early Eocene

was deposited along the north-facing Indian margin. The Indus and

Shyok sutures are the byproduct of an active margin that was located

93D. Mani et al. / International Journal of Coal Geology 128–129 (2014) 81–95

along the southernmargin of the Asian plate. Trans Himalayamagmatic

arc of Ladakh and Kohistan and a major part of the Karakoram pluton

were produced on the active margin above a subduction zone

(Thakur, 1993). The Karakoram metamorphic complex and the

Karakoram Supergroup represent the basement and the sedimentary

cover respectively of the south facingmargin of the Tibet–Karakoram

Block. The principal intracrustal thrusts, like MBT, MCT, IST, andMKT

and the Himalayan metamorphism and leucogranites were generated

as a result of collision between the two plates. Early Miocene uplift

post-Himalayan collision produced the foreland basin, south of Lesser

Himalaya in which the Tertiary Subathu sediments were deposited

(Thakur, 1993). This is reflected by the organic geochemistry of the

Subathu sediments of Outer Himalaya, which are younger and compar-

atively less mature than the Proterozoic and Paleozoic–Mesozoic suc-

cessions of Outer, Tethyan and Trans Himalaya, respectively and show

the presence of volatile hydrocarbon components (S1) and elevated

HI values.

7. Conclusion

The Rock Eval pyrolysis data of the Proterozoic and Phanerozoic

shales from Himalayan region is varied, ranging from excellent to very

good to poor. From the present pyrolysis observations, it can be inferred

that:

• Organically-rich,marine Subathu shales of the Tertiary Foot Hill belt in

theOuter Himalayan regions of Jammu could act as source for the gas-

eous hydrocarbons and have promising gas potential.

The organic matter in the Subathu shales is characterized by high

TOC content and Type III kerogen.

• The calculated Rock Eval vitrinite reflectance suggests highly mature

kerogen, derived from heterogeneous mixed marine and terrestrial

organic matter with various amounts of higher land plant material,

whichmight be crucial for thedevelopment of a high late gas potential

for the Subathu shales.

• The Lower Karewa sediments are organically rich with mixed Type

II/III kerogen. Due to lower levels of thermal maturity on samples

under consideration, further organic geochemical and kinetic in-

vestigations are required on immature samples their source poten-

tial.

• The organic matter characteristics of other Formations, namely

Proterozoic Sirban Formation of Jammu, Permian–Triassic section

of Kashmir and the Paleozoic–Mesozoic–Tertiary shales from La-

dakh–Karakoram zones, are not encouraging and/or insufficient

to describe their hydrocarbon generation potential.

• The deposition and preservation organic matter in these shales has

been influenced bymetamorphism and exhumation of organic car-

bon, inseparably linked with the evolution of the Himalaya which

involved multiple phases of tectonic deformations.

• The knowledge of the distribution, facies, and thickness of the

Subathu Group shale put together with petro-physical and litho-

logic properties and stratigraphic heterogeneity due to faults and

fractures would provide criteria for precise defining of the gas

shale horizons.

Acknowledgments

The authors acknowledge the Oil Industry Development Board (4/5/

2009-OIDB), New Delhi for providing financial aid in setting up of the

laboratory. Director, NGRI is acknowledged for permitting the publica-

tion of this work. Naveen Hakhoo gratefully acknowledges the senior

research fellowship from CSIR-India. Prof. Brian Horsfield, GFZ, is

acknowledged for the constructive scientific discussions.

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