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The Himalayan orogeny has shaped the sedimentary basins of the region, where continuous deformation formed both ‘conventional’ and ‘unconventional’ petroleum systems at multiple stratigraphic levels ranging in the age from Precambrian to Neogene. Himalaya is considered to be prospective for hydrocarbon exploration because of its suitable tectono-sedimentary environment, oil/gas shows, and the presence of commercial oil and gas discoveries in broadly similar structural settings in the eastern and western regions. Although detailed surface geological mapping, the acquisition of geological data and the drilling of wells has considerably improved the understanding of the geological and structural setting and the hydrocarbon potential of the NW Himalaya, commercial discoveries have remained largely elusive.
In the NW Himalaya the Precambrian-Cambrian sequences that are of primary interest include the Salt Range Formation (Potwar Basin), and also some sequences in the Lesser-and-Sub-Himalaya, such as the Proterozoic Sirban Limestone Formation; in the Kashmir and Bhadarwah-Chamba basins further to the northeast, and in the Garhwal Group and the Krol belt in the southeast.
The Palaeozoic sedimentary rocks exposed within the Lesser Himalaya and the Tethyan Himalaya (represented by the Kashmir, Zanskar-Spiti, Kinnaur-Uttarakhand and Kumaon basins) have been subjected to low grade metamorphism, and presently have no significant hydrocarbon generation potential. The Cambrian Khewra and the Permian Tobra formations form hydrocarbon bearing reservoirs in the East Potwar. The Palaeozoic stratigraphy of the Zanskar Tethyan Himalaya in northern India is rather similar to that of the Peshawar Basin in Pakistan. The thick argillaceous successions are the best potential hydrocarbon source rock horizons within the Palaeozoic.
The Mesozoic and Early Eocene successions of the Tethyan Himalaya were deposited in the shallow southern margin of the Tethys Ocean. In the western Himalaya, the Tethyan Himalayan succession is exposed in Kashmir, Zanskar, Chamba and Spiti basins. The Mesozoic successions include thick sequences of organic material rich argillaceous sediments. The Triassic and Jurassic strata are generally poorly developed or absent in the eastern Potwar Basin, while they get thicker towards the west Potwar and Kohat basins. The sandstones of Jurassic age are proven reservoirs, and potential source rocks are present. The Mesozoic succession of the Kashmir Basin is represented by the formations of the Triassic age. Some of the shales contain organic matter (OM) and could represent viable hydrocarbon source rocks, while some of the limestones, dolomites and sandstones have sufficient reservoir characteristics. The OM content of the argillaceous sediments within the Mesozoic-Tertiary succession of the Zanskar-Spiti Basin (Ladakh Himalaya) is appropriate for hydrocarbon generation.
The Cenozoic foreland basin of the Himalayan orogen was deformed by a southward migrating thrust system during the Late Miocene-Quaternary. The Sub-Himalaya Zone contains a sequence of Cenozoic sedimentary rocks divided into the Subathu and Dharamsala (=Murree) formations, and Siwalik Group. Hydrocarbon source rocks are present in the Subathu and Dharamsala formations; while the Lower Siwalik, Kasauli and Dagshai formations contain potential sandstone reservoirs. The Eocene Subathu Formation is a key exploration target in the NW Himalaya with both potential hydrocarbon source and reservoir rocks sealed by a thick clay sequence. The coeval shales within the Patala and Nammal formations are considered to be the main source rocks in the Potwar Basin, whereas, the fractured carbonates of Palaeocene and Early Eocene age are the main reservoirs. The Miocene Murree Formation is the youngest oil-producing horizon in the Potwar Basin. Palaeocene Hangu Sandstone and Lockhart Limestone are the main reservoirs in the Kohat Basin. The stratigraphy of Kohat-Potwar Basin extends into Margalla, Kalachitta and Samana Ranges. In these ranges the Jurassic-Eocene strata is exposed, so sub-thrust sheets could have hydrocarbon potential.
In the NW Himalaya, the surface gas seeps are characterised by a high nitrogen content, and are either thermogenic or biogenic in origin, while the gases encountered in the wells are typically methane rich (dry) with low nitrogen concentrations, indicating thermogenic origin. There appears to be a strong linear correlation between the relative concentration of methane and nitrogen in the Himalayan fore-deep gas shows. There are numerous references to biogenic gas seeps in the Plio-Pleistocene sediments and lignite fields in the Kashmir Valley, and also in the shallow Plio-Pleistocene sediments in the Peshawar Basin.
The evolution and establishment of the key petroleum system elements, the generation, expulsion, migration and accumulation (entrapment) of hydrocarbons at multiple stratigraphic levels in NW Himalaya has been controlled by the regional tectonic events. These events are associated with the source rock burial and maturation history, coupled with hydrocarbon generation, ‘peak oil’ and subsequent migration occurring concomitantly with the peak activity along the major regional thrusts. The complex and variable structural geometries have allowed a variety of traps beneath sections where source rocks have adequate burial depth, and where traps have not been breached. In NW Himalaya, the key to understand the direct relationship between tectonics and the evolution of petroleum systems are the accurate estimates for the timing of the related tectonics and that of the hydrocarbon generation, accumulation and critical moment. Here, the exploration has been hampered by the structural complexity, difficult terrain, drilling complications and poor seismic data quality. Timing of the trap formation vs. hydrocarbon charge, trap integrity, seal presence and capacity, and reservoir quality are the key geological risks that have to be addressed.
•Conventional and unconventional petroleum systems of the North-West Himalaya (India and Pakistan).•Tectonic and stratigraphic context of the sedimentary basins.•History of hydrocarbon exploration and production.•Precambrian-Cambrian, Palaeozoic, Mesozoic and Tertiary petroleum systems and hydrocarbon potential.•Oil and gas seeps, and Plio-pleistocene to Recent biogenic gas plays.