Himalayan Foreland Basin
Basin Introduction :
The Himalayan Mountain Range is the most prominent feature on the physical map of India. The outer hill ranges (sub-Himalayas or Foothills) of the Himalayas extending over a distance of more than 1500 km from Jammu in the west to Arunachal Pradesh in the east separate Indo-Ganga plain in the south from the mighty Himalayan mountain ranges in the north. Geological surveys of the Foothills were conducted by geologists from Geological Survey of India (GSI): Medlicott (1864), Oldham (1883), Wadia, (1928), Auden (1934). Subsequently Oil and Gas commission (ONGC) since its inception in 1956, initiated geological surveys followed by drilling of exploratory wells in this basin.
Geographic Location of the Basin
Himalayan Foothills basin is a NW-SE aligned basin located in the NW Himalayan Foothills region. The basin borders with Pakistan to the NW and Nepal to the SE. Northern and southern limits are demarcated by terrain defining faults namely, the Main Boundary Thrust (MBT) and the Himalayan Frontal Fault (HFF). Longitude: Latitude:
Physiography and Accessibility
The hill ranges in this belt form a series of WNW-ESE trending hills with intervening valleys. These hills, mostly constituted of Tertiary rocks rise to about 1800 to 2700 m. beyond the southern slopes of HFF, the area is plain having a cover of alluvium. Beas, Sutlej, Sira, Gambhar Khad, Gamrola Khad along with their tributaries form the main drainage system of the area. The basin occupies a zone of varying width from a maximum of 80 km in Bilaspur area to a very narrow width of ~20 km in the Kumaun area (figure 2).
The area is well connected by all weather motorable roads and can be approached by railways also. The Pathankot- Jogindernagar railway line passes through the area. The nearest international airport is New Delhi, from where domestic flights are available for Chandigarh and from there the area can be approached by rail and roads.
Prospective (Category III); the basin has indicated hydrocarbon shows that are considered geologically prospective (figure 3)
The total area of the basin is about 30,300 sq. km.
Age of the Basin & Sediment-thickness
The Himalayan Foreland is an area in which sequences of the order of 10 km thickness have accumulated. The oldest of these sequences date back to the Palaeocene-Eocene time. The Himalayan Foreland is the product of tectonic processes that have been taking place in the orogenic belt in the Cenozoic Era. Deformation and sedimentation are both active processes in the Himalayan Foreland.
Different Tectonic Zones with in the Basin
Tectonically, the Himalayan foothills forms a part of the Cis-Himalayan foredeep superimposed on the northern slopes of the Indian platform. The basin consists of several tectonic blocks demarcated by deep seated faults. During Late Tertiary, the major Himalayan orogeny brought significant structural deformation and reactivated some of the deep seated faults giving rise to the thrusted blocks. This enables, the division of the area into three tectonic belts, namely,
1. The inner tectonic belt
2. The median tectonic belt
3. The outer tectonic belt
The Inner Tectonic Belt
The “Inner Tectonic Belt” is wide and structurally complex. In Himachal and eastern Jammu foothills, a number of emergent imbricate thrusts have developed, exposing various sequences of Tertiary sediments (Siwalik, Dharamshala/ Murree and Subathu). These thrusts are associated with broad synclinal structures in between. Structural highs corresponding to the related tectonic setup are Sarkaghat, Paror, Rihlu and Najot, Jwalamukhi and others.
The Median Tectonic Belt
The “Median Tectonic Belt” encompasses the area between the northwest hading Jwalamukhi and Kishanpur-Mandili thrusts and the south hading Barsar and Deragopipur thrusts. The major anticlines in this belt are Seri, Suruin-Mastgarh, Balh, Chilli, Cheri and Changartalai.
The Outer Tectonic Belt
This belt encompasses the structures of the outer foothills. The northern limit of this belt is marked by the Barsar thrust, and the southern limit by the Himalayan frontal thrust. The major structures in this belt include the Deragopipur, Bharwain, Dhionsar, Ramgirdhar and Janauri anticlines.
Adjoining the foothills area, due south are the Punjab plains. Here the tectonic deformation in Tertiary sediments is minimal which show a clear basinward thickening due north.
The Himalayan mountain system is the product of collision between the Indian and Asian Plates. The timing of collision has been variously interpreted from the KTB (Jaeger et al., 1989) to about 50 m.y. (Klootwijk et al., 1979). The initial consequence of this collision resulted in the withdrawal and closure of the Tethys Sea about 50 m.y. ago. Extensive marine sedimentation represented by the Subathu Group in the Lesser Himalayan province and in the marine sequences of Ladakh terminated in the early Middle Eocene (Bhatia, 1985). After the closure of the Tethys, deformation spread southwards across India’s erstwhile shelf. Wells (1983) has provided palaeocurrent data from the Middle Eocene Kuldana Formation to show that flow into the basin took place from the west and the north. Flow from the edge of what was India’s shelf suggests that orogenic processes on the continental margin had reversed the palaeoslope.
Continued motion of the Indian plate after the India-Asia suturing led to a staged migration of the focus of the deformation across 200-300 km of the marginal area of the Indian Shield. This changeover of the mode of collision from interactive continental/oceanic island arc systems to a purely continent-to-continent type has resulted in intracontinental shortening. Major intracontinental shear associated with the Central Crystalline Zone has been recognized.
The Himalayan Foreland consists of the Murree/Dharamsala fluvio-deltaic to fluvial sequences marking the onset of (syn)-orogenic molasses sedimentation. These deposits of latest Eocene (?) or Oligocene to Early Miocene age underlie the Siwalik Group, and derived their detritus from the northern uplifts. The initiation of the Himalayan Foreland appears to be connected to the change in the mode of collision (C-C) and the initiation of deformation in the margin of the subducting Indian plate.
The Murree/Dharamsala Group extends along the southwestern flank of the Pir Panjal through Punch and Naoshera into Jammu Hills. Below Dalhousie in Himachal Pradesh, it is reduced to narrow strip. The main belt expands in width in Himachal Pradesh below Subathu where they are referred to as Dagshai and Kasauli formations. Further to the east, the development of these sequences is negligible.
A significant point is that throughout the 11,200 sq. km occupied by the Murree deposits of Kohat and the Potwar plateau, an unconformity with the Eocene is recognized by the occurrence in the basal part of the Murree of numerous fragments and pebbles of the Alveolina limestone which forms the top of the Eocene. This indicates that the Eocene basin had already been topographically inverted, in parts, and points towards the presence of new relief in the zone bordering the southern limit of the Tethys.
In contrast to this stage of the Himalayan Foreland, the area of molasses deposition expanded considerably 20 m.y. ago. The Lower Siwalik deposits are known to occur throughout the extent of the western Himalaya as well as in the eastern Himalaya. The oldest Siwalik sequences are considered to be about 19 m.y. The thickness of the Kamlial and Chinji Formations of the Lower Siwalik Subgroup ranges from several hundred to over a thousand meters. These thick fluvial sequences would imply relatively proximal uplifts to the north. Southward migration in the focus of deformation in the margin of the shield can be inferred. In the Indus River sections, the sandstones of the Siwalik Group deposited by ancestral (Indus type) rivers over the past 18 m.y. contain detrital zircons that are only 1 to 5 m.y. older than the depositional age of the sandstones. Young zircons were inferred to be a component of Himalayan surface rocks for the past 18 m.y. This led to suggest that a series of uplifted blocks have been continually present in the Himalaya since 18 m.y. The existence of 17 m.y. old fan sediments has also been documented at site 718 drilled on the Bengal Fan by Leg 116. Further, at the bottom of hole 718E, fan sediments of Early Miocene age were still being penetrated with no evidence of having reached the base of the fan. This clearly points to the presence of a voluminous sediment source for the Bengal Fan in the Early Miocene. As a corollary, considerable relief must have existed in the Himalaya even before the Miocene.
Within the Siwalik Basin, there is yet another remarkable event seen in the form of thick accumulations of multistoried sand complexes during the Nagri time interval (10 m.y. to 8 m.y.). Kyanite is an important element of the heavy mineral assemblages of these sandstones. These sand complexes occur in widely separated area such as the Potwar plateau, Haritayalnagar and Dehra Dun.
The Upper Siwalik Subgroup spans a time interval of 5 m.y. Cannibalism of the older molasse sequences indicates interactive tectonics between basin-margin-source are fold-thrust units. Cannibalism of the older basinal materials from its northern margins implies a further southward shift in the focus of deformation. Continued northward motion and the resultant compression in this zone resulted in tectonic partitioning by growth structures (intrabasinal folding, MBT splays etc.). This is expressed in fold-thrust related basinal uplifts and tectonically controlled valley-depressions. The extremely proximal depositional sites have accumulated conglomerates which have been shown to be time transgressive (about 3 m.y. and younger), and in many cases consist of clasts predominantly derived from the hanging wall older basinal sediments.
In most areas of the Foothills belt of the western Himalaya, deformation is an ongoing process and the outermost anticlines connected to the Himalayan Frontal Fault (HFF) may be as young as 0.2 m.y. The youngest terraces and fan deposits have also been uplifted and are being cannibalized presently.
Stratigraphically the area is presented by Tertiary sediments unconformably underlain by Early Cambrian to Precambrian rocks.
The Pre-Tertiary sequence in the Himalayan foothills is mostly composed of dolomitic limestones, quartzites and shales. The overlying Tertiary sequence is thickest in the inner foothills and becomes gradually thinner towards southwest. A brief description of these sediments is given below:
The Pre-Tertiary Stratigraphy of the Simla Krol belt, north of MBT, comprises of metamorphic and sedimentary rock types of Precambrian to early Paleozoic age. The Stratigraphy has been drawn from the earlier work, modified with the sub-surface information.
In sub-surface, granitic basement has been encountered in a few wells, for e.g., Saharanpur-1, in southern part of the basin where the sedimentary thickness is very less.
Pre – Tertiary
The group consists of black, carbonaceous, garnetiferous phyllites, slates, quartzites and dolomites intercalated with biotite schists and hornblende gneisse. The exposures are known from Lesser Himalayas.
This group comprises of bluish grey slate, micaceous shale, sandstone, quartzites intercalated with microcrystalline and oolitic limestone. These rocks are exposed around Simla hills. At places in the field mapping, these have been clubbed together with Chail Group of rocks which represent slightly higher grade of metamorphism.
This is dominantly a carbonate sequence made up of dolomite, shale, stromatolitic limestone and occasional magnesite and quartzite. Rocks of Deoban Group and Bilaspur/ Bandla Limestone are considered equivalent to Shali Group. These rocks occur in tectonic windows under nappe of metamorphic rocks. At places thin Nummulitic outliers are found on these rocks. These rocks are broadly considered to be homotaxial to Simla slates. Bandla limestone forms the basement of Subathu sediments mapped in Bilaspur unit.
This group of rocks is exposed north of Main Boundary Thrust (MBT). It is composed or low grade metamorphics such as slate, phyllites and quartzites. In the field, it is divisible into Mandhali, Chandpur and Nagthat formations which are named after localities in Garhwal.
This formation consists of quartzites, shales, crystalline limestone and marble interbedded with slates, phyllites, gritty quartzites and bounder beds.
It consists of mainly quartzites, phyllites and tuffaceous slates.
This formation comprises sandstones, grits, quartzites, conglomerates, purple and green slates and phyllites. The top of the group represents a strong unconformity followed by a succession of rocks classified as Mussoorie Group.
It can be subdivided into Blaini, Infra-Krol, Krol and Tal Formations.
It mainly consists of boulder beds, limestones and shale with characteristics of a glacial till. The boulder bed consists of dark grey to greenish grey clay matrix with pebbles of dark slate, greenish quartzite, grey sandstone, green siltstone and occasionally microcrystalline limestone. Limestones are characteristically pink, dolomitic and siliceous and grade into pink and purple calcareous shale and slates. These rocks along with Infra-Krol and Krol are exposed north of MBT.
It comprises dark shale, black carbonaceous shales and quartzites.
Krol formation mainly consists of limestone, often stromatolitic, shale and sandstone. The limestone is grey to greyish white, dolomitic, cherty and at places microcrystalline. The shales are red, orange and dark gray in color. The sandstones are dirty white, poorly bedded with orange stained quartz grains.
These rocks are composed of calcareous greywacke, carbonaceous shale, micaceous shale, arkosic quartzite and grey limestone. In the subsurface, it has been identified in well Mohand-1, where it consists of shale, dolomite and silty shale. Auden, who pioneered field geological investigations in Krol series, considered these as Permo-Triassic to Cretaceous in age. This was primarily based on glacial nature of Blaini boulder bed to represent deposits of early Carboniferous glacial period, which he considered as clinching evidence. Recently, from the discovery of conodonts in these rocks in Mussoorie area, a few research workers prefer to assign a Cambrian- Ordovician age to these formations.
Following a long hiatus, the sedimentary sequence belonging to Tertiary age was deposited south of the central Himalayan axis. (In contrast, nearly a complete sedimentary sequence representing whole of Palaeozoic era was deposited and preserved north of central Himalayan axis). In Simla area, these sediments have been studied in detail and mapped by geologists of ONGC and the following description is mainly based on those findings.
This formation consists of olive green to dark green shales interbedded with thin limestone bands and coal seams. Rich assemblage of Nummulite, Asilina, Rotalia etc has been reported. The Subathu Group is assigned Upper Palaeocene to Eocene age. Subathu sequence represents coastal swamps and shallow to open marine environment deposits. In subsurface, the identification of Subathu sequence is not documented with faunal evidence.
The sediments of the Dharamshala Group unconformably overlie the Subathu group. These sediments consist of claystone, siltslones, calcareous shales and sandstones. The Dharamshala group is divisible into Lower and Upper Dharamshala. The Lower Dharamshala sediments were deposited under transitional brackish water environment and upper Dharamshala mainly represent fluvial system. The equivalent of Dharamshala group is known as Murree Group in J and K.
The Siwalik Group comprises of sandstone, siltstone and clay/claystone alternations in the lower part, whereas the upper part is represented by conglomerates, pebbly bands and sandstone lenses. These are broadly divided into Lower, Middle and Upper Siwaliks, ranging from Middle Miocene to Lower Pleistocene. The sediments were deposited as channel bars, piedmont bars, floodplain and alluvial fan deposits.
The Alluvium consists of loose sand, silt, clay, pebble, gravel, boulder and kankar. It unconformably overlies different formations in different areas.
Geological History & Depositional Environment
The basin started with a shallow marine environment which changed to estuarine and deltaic with time. By mid-Miocene, continental sedimentation marked by fluvial environment dominated the scene and this set-up has continued to the recent with minor modifications. The basin had predominantly transversal dispersion pattern controlled by southerly flowing rivers emerging from the Himalaya and during the Neogene period, a master stream along the southern margin of the basin drained into the Bay of Bengal. The fluvial sedimentation in the basin was influenced by tectonism throughout its evolution.
The Early Cambrian to Precambrian limestone forms the base for the Tertiary sediments. Sedimentation was initiated in the Paleocene time under paralic conditions at the beginning of transgressive phase in which carbonaceous shale and coal were deposited followed by shallow marine conditions with the deposition of live green shales and foraminiferal limestones of Upper Paleocene to Eocene age. The sea regressed thereafter at the end of Eocene, during which red shales, laminated mudstones, grey, blue siltstones and purple calcareous sandstones of Dharmsala were deposited under transitional brackish water environment. The closing phase of marine incursion commenced due to uplifting in Proto-Himalayan source area and as a consequence, a coastal fluviatile environment was established.
Deposition of Lower Siwalik started in Middle Miocene associated with Himalayan orogeny. The northern margin of the basin also got uplifted at this stage and this axis of deposition shifted southwards, sub-parallel to Himalayan trends.
During Upper Miocene times (middle Siwalik sub-Group), a thick succession of pebbly sandstones and claystones were deposited mainly by braided and meandering streams with a concomitant shift of the depositional axis southwards. This sequence is very thick at places with sediments supply from main provenance from the rising Himalayas.
During Plio-Pleistocene orogeny, the northern part, presently the Foothills area of the basin was further uplifted and as a result of which conglomerates of Upper Siwalik sub-Group were deposited as alluvial fans at the foot of mountains. Tectonic movements of the Himalayas later produced the present topography and it was further sculptured by Quaternary glacial and interglacial processes with the deposition of high level river terrace conglomerates.
Two surface shows of oil and several shows of gas, both in surface and in wells, have been observed in this belt. Occurrence of thick veins of asphalt in limestone in the localities of Satra, Fatehpur and Jokan (Poonch area) of J&K was reported as early as 1928 by Geological Survey of India. ONGC geologists investigated the limestone in Satra in 1964-66, and reported the presence of bituminous veins in a 90 m thick limestone band.
The other known oil show is near Chomukha village on the eastern blank of Alsed Khad in Sundernagar area ( Fig. 2). This show is observed in a sandstone bed at the base of Dharmasala sequence. The rocks are highly crushed, being adjacent to the MBT. Droplets of light crude oil in cavities and joints of thin limestone bands/lenses have also been noticed in the Alsed Khad below Chomukha village. This limestone is of Upper Paleocene to Eocene age (Subathu Formation).
Numerous gas shows have been observed in the median and inner tectonic zones in Himachal Pradesh and Jammu and Kashmir, the most famous being the gas shows in Jawalamukhi temple. The gas is mostly methane (87.8%) with small amounts of C02, N2 and O2. The other main gas shows in this belt include those in Dehar Khad River near Kotla village, Suruinsar and Mansar lake areas in J&K state and from the flank of Sarkaghat anticline in Himachal Pradesh. Surface gas shows along anticlinal axis and fault traces become ubiquitously manifest during rainy season when bubbles of gas escaping through muddles of collected rainwater become detectable.
As the peak hydrocarbon generation time is also the time of primary migration, mid-Miocene, which has been shown to be the time of kerogen maturation, is a very significant and fortuitous time in the process of generation, migration and accumulation of hydrocarbons in the Himalayan foothills. This time period also coincides with major orogenic uplift of the Himalayas that caused development of major structural elements in pre- Siwalik rocks. The folds provided structural traps and the thrusts and faults could provide pathways for secondary migration of hydrocarbons.
As such the most prospective areas for exploration will be the large structures in Paleogene belts with thick sediments and the reservoir traps which might be mapped in the lower Siwalik rocks lying adjacent to or under the Paleogene source sequences along major thrusts like the Bilaspur Thrust. Large surface anticline adjacent to belt of Paleogene rocks are also promising areas. On these criteria, the whole of Tertiary belt in the offered block is promising for hydrocarbon exploration.
The prognosticated resources of the basin are 140 MMT (OIL +OEG). The density of resources considered is 10,000 T/Km2.
Habitat of Oil & Gas
The pre-Tertiary section of the Himalayan Foothills is primarily composed of mixed clastic carbonate sequence. The sandstone and carbonate reservoir facies generally have low primary porosity. However, fractures are often observed in this section. The presence of secondary porosity in the limestone hosting the hydrocarbon may not be ruled out.
The Lower Tertiary reservoir facies in out crops also have low primary porosity and fair permeability.
The Upper Tertiary (Upper Dharmasala and Lower Siwalik) reservoir facies have better development of primary porosity (8 to 15%) and fairly permeable.
The pre-Tertiary sequences in Himalayan Foothills are mainly lean in organic carbon and are over mature. The outcrop samples from Vaishnodevi limestone in Jammu & Kashmir and Bilaspur limestone in Himachal Pradesh contain 0.01 to 0.33% of organic carbon.
The Subathu rocks are the only marine sediments in the stratigraphic sequence present in the basin. In analogy with the neighboring Kohat-Potwar basin in Pakistan, Subathu sediments are believed to be source rocks of hydrocarbon in this basin. The Subathu sediments form Jammu foothills and Himachal Pradesh are characterized by poor to marginally rich organic carbon content. However, at places Subathu is encouraging in organic carbon content.
The overlying Dharmsala/Murree and Siwalik sequences represent brackish and fluviatile sediments deposited in an oxidizing environment and have marginally rich organic carbon content (0.02 to 1.02%).
The rocks which can form good reservoirs in the basin may be limestones and sandstones present in the sedimentary sequence. The limestones in the exposures are jointed and fractured and these can act as potential reservoirs in subsurface. These are most likely in pre-Tertiary sequences and Subathu Formation. The sandstones are extensively developed throughout the sedimentary sequence with an appreciable thickness. Sandstones of Subathu, Dharmsala/Murree and Siwalik thus can be good potential reservoirs in the basin.
Traps of two main types, structural and combination are likely to be present in this basin. The structural traps include large number of exposed NW-SE trending anticlines such as Janauri, Bhimbergali, Suruin-Mastgarh, Paror, Sarkaghat, Changartalai, Bharwain etc.
Further, the chances of structural entrapment in the subthrust block are considerably high, where the deformation was continuing with the accompanying possible thermal maturation of source rocks and generation of hydrocarbons.
Alternations of arenaceous and argillaceous lithologies are observed in the entire Tertiary sequence from Lower Dharmsala onwards which could provide good reservoir and cap rocks.
Petroleum Division of Geological Survey of India started the exploration activities for exploration of hydrocarbons in the area. It was considered that the area had good possibility of locating significant exploitable reserves of petroleum. Surface oil shows at Chomukha near Sundernagar and a gas show at Jawalaji temple near Jwalamukhi, in Himachal Pradesh constituted significant hydrocarbon shows. The exploration work by detailed surveys and investigations was systematically continued by ONGC established in 1956 as a national E&P Company.
Details of Exploration input.
|Semi-detailed mapping||99,163.5 Km2|
|Special studies||1,931 Km2|
|AEROMAGNETIC SURVEYS||45,000 Km2|
|GRAVITY MAGNETIC SURVEYS||30,079 Stations|
|Reflection CDP||2578.97 GLK|
|Number of wells drilled||15|
|Number of wells drilled||8|
The area is covered by detailed, semi-detailed, special studies, reconnoitory traversing and reconnoitory mapping. 15 Exploratory wells have been dilled in different structures of the basin with the discovery of two minor gas pools in Lower Siwalik section above Jwalamukhi thrust. Through this mapping, detailed stratigraphic framework has been established along with the tectonics of the foothill belt which helped in projection of subsurface geology.
Geological surveys by ONGC were carried out in this area since its inception. Detailed and semi detailed mapping, special studies on sedimentary features like current bedding, reconnoitory traversing and investigations of oil and gas shows were carried out. Geochemical samplings were undertaken in Subathu (Eocene) and Lower Dharmsala (Oligocene) sediments for laboratory analysis. Through this mapping, detailed stratigraphic framework and tectonics of the foothill belt has been established that helped in projection of subsurface geology. The geological map is shown in fig 2.
Aeromagnetic surveys carried out in 1957, gave a broad indications of basin configuration, which formed the basis for initiation of seismic and gravity- magnetic (GM) surveys from 1957-58 field season onward. Major part of the basin is covered by GM surveys (Fig. 3 & 4). Both the maps clearly show that NW/SE Himalayan trend is the dominant lineament in the basement. However, there is a strong suggestion of NE/SW (Aravali trend) lineament in the area lying between Kalka and Dehradun. Initially, seismic surveys conducted in the Punjab plains were of refraction and conventional type which were followed by more sophisticated CDP surveys during seventies. The foothill area has been partially covered by CDP surveys.
Exploratory drilling in the basin was initiated with the drilling of well Jawalamukhi-1 (JMI-1) during 1957-58. Two minor gas pools were discovered in the Lower Siwalik sub-Group section above Jwalamukhi thrust. In the area of the map, three wells have been drilled. One well, Ramshahr-1 is located in the block under offer. Chari-1 lies on the northern side of the block and Mohand-1 towards the southeastern side of the area. A total of 15 exploratory wells have been drilled in the Foothill belt.
List of Exploratory Wells Drilled
|S.No.||Well Name||Drilled Depth(m)|
Oil & Gas Resources
The total hydrocarbon resources in the Himalayan foothills from Jammu in the west to UP Himalayas in the east has been prognosticated to be about 150 million cubic meters of oil & oil equivalent gas (O&OEG). The resources in the Tertiary exposure areas of Himachal Pradesh are estimated to be around 50 million cubic meters of O&OEG.