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Sir my  question is  about Potwar Plateau that what kind of subsurface Structures are present specially along the vicinity of Dhulian, Meyal and Khuar area. Is there is any Salt tectonic is involved in the area.

Dear Mohib,

Thanks for sending me your question.

First I would like to give you an overall tectnic picture of the Potwar Basin.

Potwar Basin which is located in Potwar Plateau and is the part of  lesser Himalayas. Himalayan collision system represents an active collision orogen between the Indian and Eurasian subcontinents. The collision was active since about 55 Ma and involves continuous uplifting, erosion and deposition of sediments. This area is composed of deformed meta-sedimentary and sedimentary rocks originally deposited on the northern Indian continental margin and in the Indo-Gangetic foreland basin. This zone is south of the high crystalline Himalayas, which contain, from north to south, meta-sedimentary and igneous rocks of the northern Asian continental margins; meta-volcanic, igneous and meta-sedimentary rocks of the Kohistan Island arc terrain; and igneous and high grade metamorphic rocks of the intensely deformed northern margin of the Indian Plate. Thrust faults have been traditionally assigned for the fault contacts between these zones.

Sedimentary strata in the foreland are detached and translated along the Salt Range Thrust over the Indo-Gangetic foreland. The topographically rugged Salt Range and Potwar Plateau is the northernmost feature of Indus Basin, bounded by the Main Boundary Thrust and the Kalla Chitta Range in the north and the Salt Range in the south. Precambrian Salt and possibly Eocene evaporites and argillites are likely the main decollments that produced large scale thrusts and telescoping that had transported the sedimentary units substantial distances from the point of their origin in the Potwar and Kohat basins.

If you have map of Potwar and Kohat Basin you will notice that there are two main faults bounding the Potwar Plataue,  east to west Main Bounding Fault to the north and east to west running Salt Range thrust belt to the south and all other major folds are following the same trend. This illustrates about the compressional regime or stresses that would have active from north to south and resulting compressional features as we seen now are characterized by east-west striking folds. Soan Syncline,  roughly  trending east-west (true trend is NE-SW) is dividing Potwar Plateu into two parts, souther and northern parts. The northern part is known as North Potwar Deformed Zone (NPDZ)where Khaur, Meyal and Dhulian oilfields are located. The structures in NPDZ, being closer to the northern front from where compression was initiated, are more complex. Their complexity is due to two main reasons; thick accumulation of formations of Kuldana, Muree-Kamlial and Siwaliks which are consisting of thick pile of claystone, shale and sandstone/siltsone  which were prone to be deformed more easily than their competent and more resistant counterpart rocks i.e, limestone and dolomite. Secondly, the competent and deeply buried rock surfaces such as carbonates of Eocene and Paleocene have been folded  due to Himalayan compressional and reached to the point of failure and developed faults. These faults  have also provided additional ramp  along which softer and plastic sediments of Neogene (Muree-Kamlial and Siwaliks) have propagated upward and formed much complex surface and subsurface structures.

Take an example of Khaur field; east-west running  very acutely folded anticline is located east of Dhulian, about 100 km from Islamabad. The southern and northern dips of the Khaur anticline's flanks often measure dip from 70 to  nearly vertical. This is the first oil discovery in Pakistani soil in 1915.  Oil was found in Murree Formations continental highly porous and over pressured sands which are at places only 100 feet deep. Interestingly, it was the only field where oil was found from Murree formation sands of Miocene age whereas all the other Potwar fields have been  producing hydrocarbon from carbonates of Paleogene. It is clearly visible that oil at Khaur was initially accumulated at the Eocene carbonates but post accumulation fracturing and faulting due to continued compression by Himalayan orogeny or later any other minor deformation had developed large plumes that lead oil to the overlying sands of Murree Formation where oil was easily re-deposited. Khaur anticline was selected for first  exploration well due to reasons of the nature of the anticline and presence of the surface oil seepage near the Khaur village. Khaur was significantly displaced structures where Murree Formation was found in the core of the anticline which is about 4000 ft deep in the nearest Dhulian anticline. It can give you a measure of a magnitude of the massive uplift and later strong erosion of the Khaur anticline core area.

On the contrary, Dhulian is a fantastic symmetrical anticline, striking east-west with moderate dips along its flanks from 30-35 degrees. On the surface, it appears to be simple fold but seismic imaging help to determine the sub surface structural complexity and found out that both southern and northern flanks are bounded by thrust faults with varying degree of uplift.

Meyal Field is located in Attock district, near Pindi Gheb, 110 kilometers southwest of Islamabad in an active foreland and thrust belt in the Central Potwar Plateau of the Upper Indus Basin. In 1916, soon after the discovery of Khaur Field, the first well was drilled which was terminated in the Molasse sequence without reaching the Eocene.

Meyal-Kharpa surface structure is an east-west trending narrow steep faulted anticline with two major thrusts cutting the structure longitudinally. The subsurface structure does not lie directly underneath the surface structure. Previous exploratory attempts were mainly based on the surface geological mapping. Attock Oil Company (AOC) and Burmah Oil Company (BOC) drilled six unsuccessful wells during 1916 to 1948 in Meyal. Seismic data acquisition during 1965-1966 had helped identify the subsurface structure configuration that led to drilling of the Meyal-1 discovery well. At the Eocene level the subsurface structure is defined as east-west trending anticline bounded by thrust faults in the north and south. Most of the wells were drilled in the central part of the Eocene structure.

In all three structures Salt Range Formation of Precambrian age was found in Dhulian field (in well Dhulian 43 which is the deepest well so far in the field) consisting of massive salt and gypsum and carbonates. Meyal field's deepest drilling formation was Permian and no well yet drilled to Salt Range Formation's salt sequences.

I think salt has also supplemented in the initiation of the substantial deformation in the entire Potwar Basin along with major tectonic compression due to Himalayan Orogeney.


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Syed Tariq Hasany


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