Nepal, lies between India to the South and China to the North. It extends 800 km from East to West ranges and 130 to 230 km in North- South. The country is naturally separated into four major geographical/geological zones (Figure 15 and 16) parallel to its long dimension (Frank and Fuchs, 1970; Mitchell, 1979; Stocklin, 1980; Windley, 1983).

Terai
The Terai is the Nepalese portion of the Indo-Gangetic Plain that extends from the Indian Shield in the South to the Siwalik Fold Belt to the North. The plain is a few hundred metres above sea level and usually 400 to 600 m thick. it is composed of Recent of Quaternary alluvium, boulder, gravel, silt and clay. Terai Plain is underlain by a thick, relatively flat-lying sequence of Mid to Late Tertiary molasse (Siwalik Group) which uncomformably overlies subbasins of early Tertiary to Proterozoic sediments (Surkhet, Gondwana and Vindhyan Groups) and igeneous and metamorphic rocks of the Indian Shield (Agrawal, 1977; Acharya and Ray, 1982; Raiverman et.a.l, 1983).

Siwalik Fold Belt
The Siwalik Fold Belt is from 5 to 45 km wide and rises abruptly from the Terai along the Main Frontal Thrust (MFT). It consists of a series of ridges and valleys composed of thick beds of folded and faulted Tertiary Molasse of Siwalik Group thrust to the South (Parkash et.al., 1980; Herail et. al, 1986). Gravity measurements and detailed field mapping indicate that the cores of at least some of these structures contain pre-Siwalik rocks that are considered to be hydrocarbon objectives (Friedenreich and Slind, 1986; Elber, 1989).

Lesser Himalaya
The Lesser Himalaya is a wide, stratigraphically and structurally complex zone that lies immediately north of the Siwalik Fold Belt and is separated from it by the south-verging main boundary fault (MBT). The majority of the Lesser Himal is composed of thrust sheets and nappes of metasediments and metamorphic rocks with granitic intrusions of the Midland Group. The Group is of little hydrocarbon exploration interest, although the oil gas seeps of the Dailekh (Figure 9) area occur within the Midland Group (CPIT, 1973). These seeps are interpreted to have been generated in sediments below the nappes.

Higher Himalaya
The Higher Himalaya which contains the spectacular peaks of the Great Himalayan Range, Everest, Annapurna, etc. is thrust southward over the Lesseer Himalaya along the Main Central Thrust (MCT). The zone is composed of a basal slab of metamorphic Proterozoic rocks overlain by a conformable sequence of Cambrain to Eovene Tethuyan Sediments (Bordet et al., 1981). Gas seeps occur in the upper Tethyan of northen Nepal near the village of Muktinath (Figure 16).

There are four major groups of rock units interesting for petroleum exploration in Nepal. These are the Siwalik, Surkher, Gondwana and Lakharpata (Vindhyan) Groups (Figure 15 and 16).

Siwalik Group
The surface geology of Siwalik Formation indicates thick and multi-channels of sands of reservoir quality and expected to continue into the subsurface. There is sufficient shale especially in the lower part of the section (Lower and Middle Siwalik Formations) to provide good quality seals. A few pockets of coal are present but the source rocks have not been reported from the Siwaliks. There is an effective potential sandstone reservoir. Structure and stratigraphic traps are the objectives in the Siwalik.

Surkhet Group
The Surkhet Group (Upper Cretaccous-Lower Mioccnc) outcrops in western Nepal and in the subsurface, its interpreted, lateral equivalent, the "Unnamed Formation" (Palaeogene) occurs in the Northen Indian wells close to the Nepal border. It is more than 1,000m thick in the Lumbini area of the Gandak Depression. The Surkhet Group contains potential source and seal rocks in the Swat and Melpani Formations. The reservoir rocks may be developed in the Melpani and Suntar Formations. Solid Hydrocarbons have filled the porosity of the sands of the Melpani. It is also correlated with the oil and gas producing formations of the Asam and Potwar Basins. The Surkhet Group is thus most important exploration target in Nepal.

Gondwana Group
The Gondwana Group (Upper Paleozoin to Lower Cretaceous) is expossed in various locations of west central and eastern Nepal and probably occurs beneath the Siwalik Group in the subsurface of eastern and central west Nepal. From seismic analysis it is interpreted to be 1000m thick.

The Sisne Formation could be the richest potential source rock. The lower coal bearing Sisne could be both a source and reservoir for gas and upper marine sediments may be a potential oil source rock. The Taltung Formation could be a reservoir potential. Gondawana sediments are considerd an important exploration objective in Nepal.

Lakharpata Group
The Lakharpata (Late Precambrian-Late Palaeozoic) equivalent to Vindhyan in India, is continuously exposed along the Main Boundary Thrust. It is interpreted to occur in the subsurface unconformably beneath the Siwaliks with thickness of more than 5,000m in the centre and slightly less in the west. The pre-Siwalik sediments of Lakharpata equivalent are exposed in slices within the Siwaliks of eastern Nepal. It has proven the existence of blind thrusts and blind folds in the subsurface.

A small amount of Karst has developed in the west central region. Stromatolite zones and small non-porous build-ups occur in the Group. The unconformity at the base of Siwaliks could provide a large number of attractive carbonatc as well as clastic exploration objectives.

The Sangram Formation at the base of the Lakharpata Group has good source rock potential. According to source rock basin modeling, it is in the oil and/or gas window in the subsurface beneath the Terai. The Gawar and Katuwa Formations have also source rocks potential and are considered an exploration objective.