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Площадь: 42491.89 км²
Kohat- Potwar basin
Kohat-Potwar basin (Fig. 1c) of the Himalaya foothills records the classical features of Indo-Eurasian collision and Tethys Sea closure48,49. The receding in the Tethys Sea led to the deposition of thick marine evaporites in the Kohat Plateau, along with other sediments from the Miocene to the present50. In the southern Kohat basin, namely the Karak region, the tectonically complex nature of the Kohatfold and thrust belt is characterized by evaporite-driven deformation in the form of sinkholes. These sinkholes are typically fault-controlled because evaporite is emplaced on the ground due to thrusting. Given their highly soluble nature and dissolution activities, evaporites of the Karak region showcase one of the best exposures of karstification landforms. Such extraordinary deformation features form welldeveloped, salt-dissolution sinkholes that are unique worldwide.
Fig1. World elevation map, b the geographical location of Pakistan, and c the tectonic structure of the Kohat Potwar fold and thrust belt. Yellow and pink rectangles show the coverage of the ascending (AT-173) and descending (DT-5) tracks of the Sentinel-1 images. Regional tectonic elements are shown in black lines. PB Peshawar Basin, MBT Main Boundary Thrust, JF Jhelum Fault, HKS HazaraKashmir Syntaxis, DF Domeli Fault, DT Diljaba Thrust, SRT Salt Range Thrust, KBF Kalabagh Fault, TIR Trans Indus Ranges, KR Khisore Range, PR Pezu Ranges, BF Bahadarkhel Fault, KF Karak Fault, KF Kurram Fault (b) represents the lithological units of the study area as indicated by the red frame in (a).
Because saline water dominates the hydrological system, freshwater only exists in small pockets with aquifers varying from 3 m to 90 m depth in this region51. Rainfall is regarded as the primary source of groundwater recharging, although small dams have been constructed to recharge the aquifers and avoid groundwater depletion in the past decades. Recently, the groundwater levels have gradually decreased due to the basin’s arid climate and insufficient usage of water reservoirs. This condition not only concerns the sustainability of water resources but also leads to the formation of sinkholes and land subsidence, which cause socioeconomic damage to the local community. Nevertheless, the distribution of the developing sinkholes is still poorly known. To our knowledge, only one study concerns the sinkholes in this region using a geophysical approach, which is limited to a very small area.
Geology
Geologically, the continuous collision of the Indian and Asian plates in Cretaceous-Eocene time resulted in the western Himalayan Foreland and Fold and Thrust Belts, directly representing the active Himalayan orogeny54. Kohat and Potwar fold and thrust region situated between the Main Boundary Thrust (MBT) and the Salt Range Thrust (SRT), is a classical feature of thefold-and-thrust belt of Himalaya orogeny (Fig. 2a).
Fig. 2 Regional geological and tectonic elements of the NW Himalayan foreland fold and thrust belt Pakistan, MBT Main Boundary Thrust, PT Panjal thrust. KF Kalabagh Fault, SR Surghar Ranges, NPDZ Northern Potwar deformed Zone, and the red inset shows the study area. b Lithological map of the study area with water wells shown as light blue circles, besides which the numbers indicate their depths.
The Kohat Fold and Thrust Belt is bounded by regional fault system, i.e., Surghar Range Thrust in the south, Main Boundary Thrust (MBT) in the north, Jhelum Fault in the east, and the Kurram Fault in the west (Fig. 1c). This intermontane basin hosts a sedimentary succession of Paleocene to Pliocene age Siwalik rocks, and basin fill sediments including alluvial plains and flood plains57 (Fig. 2b). Siwalik rocks (Murree Formation, Kamlial Formation, Chinji Formation and Dhok Pathan Formation) comprises of the sandstone, clays, shale and conglomerates (Fig. 2b). Karak Fault is a major thrust in southern Kohat places Eocene evaporites (Bahadarkhel Salt and Jatta Gypsum) over the Siwalik rocks. The sinkholes are scattered in this evaporitic unit in the Karak region of the southern Kohat Basin.
Stratigraphy
The Kohat-Potwar geologic province depositional record is relatively complete from Late Proterozoic to Holocene (fig. 3).
Figure 3. Generalized stratigraphy of the Kohat-Potwar area (modifi ed from OGDC, 1996; Quadri, 1996; Kemal, 1992; Iqbal and Shah, 1980; Shah and others, 1977).
Late Proterozoic metamorphic basement rocks are overlain by oil-impregnated shales, sandstones, and interbedded carbonates and evaporates of the Late Proterozoic and Lower Cambrian Salt Range Formation. The upper part of the Salt Range consists of thick carbonates overlain by evaporites marking the top of the formation. Potential source beds and oil shows have been identifi ed within the evaporite sections (Shah and others, 1977; Iqbal and Shah, 1980). The thickness of the Salt Range Formation varies from 50 to more than 1,000 m, due partly to dissolution of these evaporates. Above the Salt Range, evaporites are as much as 150 m of marine shales and massive sandstones representing braided-stream deposits of the Lower Cambrian Jhelum Group, Khewra Formation, which has produced oil at the Adhi, Chak Naurang, and Rajian fi elds (Khan and others, 1986; Petroconsultants, 1996) (fig. 4).
Figure 4. Generalized oil and (or) gas fi eld and structure map of Kohat-Potwar area (modifi ed from Kemal, 1992; Government of Pakistan, 2001; Kazmi and Rama, 1982; Khan and others, 1986; Law and others, 1998; Petroconsultants, 1996).
As much as 180 m of the Kussak Formation glauconitic shoreface sandstones and siltstones overlie the Khewra. The Kussak has produced oil at the Missa Keswal fi eld (Petroconsultants, 1996). The overlying Jutana Formation consists primarily of sandy carbonates and nearshore sandstones. The Upper Cambrian Baghanwala Formation shales and interbedded sandstones in the Potwar Plateau and Salt Range area, and contemporaneous Khisor Formation salts in the Kohat area, mark the top of the Cambrian stratigraphic sequence. The top of this sequence also marks the beginning of a hiatus that lasted until the Permian.
The Permian Nilawahan Group consists of the Tobra Formation glacial tillites, siltstones, and shales; the Dandot Formation alluvial or glacial coarse-grained sandstones and shales; the Warchha Formation coarse-grained argillaceous sandstones, and occasional shales; and the Sardhai Formation, which is similar to the Warchha except for a greater number of fi ne-grained-sandstone intervals (Shah and others, 1977; Iqbal and Shah, 1980; Kemal, 1992). Three fi elds in the southeast portion of the Potwar Plateau have produced oil or gas from Tobra reservoirs. Overlying the Nilawahan Group is the Zaluch Group consisting of Upper Permian Amb Formation shelf carbonates, Wargal Formation shelf-carbonate sequences, and the Chhidru Formation marls and coarsening upward sandstones. The Wargal has produced oil at the Dhurnal field.
Although Mesozoic rocks are generally preserved in the Salt Range and southeast Potwar Basin, part or all of the section is missing from the Kohat Plateau and northwestern Potwar deformed zone (Jaswal and others, 1997). Depositional thinning toward the west combined with erosion accounts for the missing rocks. Deposited on the unconformable Permian strata are continental, coarse- to fi ne-grained sandstones, shales, and carbonates of the Triassic Musa Kehl Group Mianwali and Tredian Formations. Overlying the Tredian are shelf carbonates of the Triassic Kingriali Formation. The Triassic Formations were formerly referred to collectively as the Wulgai Formation (Shah and others, 1977). The Jurassic strata include the Shirinab or Datta and Shinawari Formations consisting of nearshore variegated siliciclastics that contain some nonmarine-sandstone intervals (Khan and others, 1986). The Datta has produced oil and gas from three fi elds, including Dhulian, in the northwest Potwar Plateau area. Overlying these nearshore formations are as much as 900 m of Samana Suk Formation platform carbonates. The Lower Cretaceous section consists of Chichali Formation basinal shales, Sembar and Lower Goru Formations coarsening-upward shoreline packages (late highstand), and Lumshiwal Formation sandstones (maximum basin fl ooding surface). The Upper Goru, Kawagarh, or upper Moghul Kot siliciclastics representing Late Cretaceous lowstand events are present southeast of the Salt Range and on the Kohat Plateau but are not reported within the Potwar Basin.
Cenozoic deposition began with the Paleocene-Eocene Makarwal Group. Hangu Formation siliciclastics were deposited fi rst on an erosional surface marking the top of the Cretaceous Lumshiwal Formation. There is a transitional contact between the Hangu and Pab and the overlying Lockhart Formation carbonate-shelf system. The contact between the Lockhart and the shallow-marine shales and subordinate carbonates of the Patala Formation is also transitional (Shah and others, 1977; Iqbal and Shah, 1980; Kemal, 1992). Oil production has been attributed to the Lockhart and Patala. The overlying Eocene Nammal and Panoba Formations are shallow-marine to lagoonal shales and interbedded limestones with a transitional contact between the Patala and the Nammal. Overlying the Nammal and Panoba are the lower Eocene Sakesar or Margala Hill Formation marine limestones and shales. Oil or gas production from eight fi elds spanning the Potwar Plateau are attributed to the Margala Hill. Even though Iqbal and Shah (1980) indicated that the probably contemporaneous lower Eocene Bahadur Khel Salt is present only in the Kohat Plateau area (fi g. 5), oil or gas production at three fi elds on the Potwar Plateau has been attributed (Petroconsultants, 1996) to the Bahadur Khel Salt Formation (Shah and others, 1977; Petroconsultants, 1996). The Chharat Group includes marine shales and interbedded limestones of the lower Eocene Chorgali Formation, the shales of the upper Eocene Kohat Formation, and the highstand shales and ramp carbonates of the Oligocene Kirthar Formation that were deposited at least in the Kohat and northern Potwar area. Oligocene rocks are missing from most of the basin. An erosional surface marks a change to alluvial environments represented by the Miocene to Pliocene Murree Formation fl uvial sandstones and siltstones and the Kamlial Formation fl uvial sandstones and clays of the Rawalpindi Group. The Murree Formation contains the youngest reported oil-producing reservoirs in the Kohat-Potwar geologic province. Pliocene and Pleistocene Siwalik Group fl uvial sandstones and conglomerates mark the top of the stratigraphic column in the area.
Source Rock
There are several potential source rocks in the Kohat-Potwar geologic province. These include the Late Proterozoic–Lower Cambrian Salt Range; Permian Wargal, Sardhai, and Chhidru; Paleocene Lockhart; and Eocene Patala Formations (OGDC, 1996; Quadri and Quadri, 1996). Most of the available information and analyses available was derived from samples collected on the Potwar Plateau and easternmost Kohat Plateau. The remainder of the Kohat Plateau source-rock potential is not as well known. Correlated columnar sections (Shah and others, 1977) show a generalized thinning of the Paleocene-Eocene stratigraphic sequences toward the southwest part of the Kohat Plateau. Lower Cretaceous Sembar and Lower Goru temporal equivalents—the Chichali and Lumshiwal Formations- may be the youngest mature rocks with source potential throughout most of the Kohat Plateau.
The oldest potential source rocks are in the Salt Range Formation, which consists of a clastic-dominated lower section, carbonate-dominated middle section, and an evaporitedominated upper section. Potential source-rock intervals are found primarily in the upper evaporite sequence. The Permian Sardhai and Chhidru, although sandy, have suffi ciently high total organic carbon (TOC) values to have source-rock potential (Quadri and Quadri, 1997). The shallow-marine shales of the Eocene Patala Formation, ranging in thickness from 20 to 180 m, are the probable predominant oil source in the Potwar Basin (OGDC, 1996, oral commun.). Patala TOC ranges from 0.5 percent to more than 3.5 percent, with an average of 1.4 percent, and are type-II and -III kerogens. The exception to this may be the Dhurnal fi eld (fi g. 14), where Patala samples have low TOC values, whereas TOC values in the Permian Wargal are 1.0 percent and in the Lockhart they are 1.4 percent (Jaswal and others, 1997). Oil samples from Dhurnal fi eld also do not match those known to be sourced by the Patala. Sulfur content of the oils is less than 0.65 percent, except at Joyamair where sulfur content is greater than 2 percent (Khan and others, 1986).
Maturation
Thermal maturities for Kohat-Potwar rocks range, from Ro 0.3 to more than 1.6 percent. A basin profi le (OGDC, 1996) indicates vitrinite refl ectance equivalent maturities of 0.62 to 1.0 percent for Tertiary rocks in the productive part of the Potwar Basin (fig. 5).
Figure 5. Generalized cross section showing structure through the Potwar Plateau (modifi ed from Malik and others, 1988) and generalized maturity profi le (modifi ed from OGDC, 1996).
Fluid-inclusion data, with vitrinite-refl ectance data used for calibration, shows calculated and measured Ro samples between 0.6 and 1.1 percent for Cretaceous, 0.5 to 0.9 percent for Jurassic, and 0.65 to 0.95 percent for Permian rocks (Tobin and Claxton, 2000). North of the main boundary thrust fault, maturities are higher. In the northern and probably central basin, Cretaceous rocks are in the 1.0 to 1.6 percent Ro range. Dry gas generation begins near 1.3 percent Ro.
Generation and Migration
Generation of hydrocarbons most likely began in Late Cretaceous time for Cambrian through Lower Cretaceous source rocks and again from Pliocene time to the present for younger source rocks (OGDC, 1996). Burial-history plots by Law and others (1998) (fig. 6) start at about 30 Ma and therefore show only a late or second period of generation beginning 20 to 15 Ma and continuing to the present. Two distinct overpressuring regimes were reported by Law and others (1998).
Figure 6. Generalized burial-history plots for the ODGC Dakhni 1 well (left) and the Gulf Oil Fim Kassar well (right) (modifi ed from Law and others, 1998)
A Neogene overpressuring regime was attributed to tectonic compression and undercompaction, and a pre-Neogene overpressuring regime is attributed to combined hydrocarbon generation and tectonic compression The burial-history plots of Law and others (1998) also indicate that maximum burial was reached approximately 2 million years ago. Even though there were probably two distinct periods of generation from two different groups of source rocks, suffi cient source-to-reservoir correlation data were not available to clearly defi ne separate petroleum systems. In many oil and gas fi elds, there are stacked source and reservoir rocks possibly resulting in mixing of oils. Migration is primarily over short distances updip and vertically into adjacent reservoirs and through faults and fractures associated with plate collision and thrusting.
Reservoir Rocks
Reservoir rocks include Miocene alluvial sandstones, Paleogene shelf carbonates, Jurassic and Permian continental sandstones, and Cambrian alluvial and shoreface sandstones (Shah and others, 1977; Iqbal and Shah, 1980). On the Potwar Plateau, oil or gas has been produced from the following formations: Cambrian Kherwa, Kussak, and Jutana; Permian Tobra, Amb, and Wargal; Jurassic Datta; Cretaceous Lumshiwal; Paleocene Khairabad, Lockhart, Patala, and Nammal; Eocene Bhadrar, Chorgali, and Margala Hill Limestone; and Miocene Murree (Khan and others, 1986; Petroconsultants, 1996) (fig. 4). Production from more than one of these reservoirs (in one case five reservoirs) was reported at 12 of the 22 fi elds in the database (Petroconsultants, 1996). More than 60 percent of the producing reservoirs (by fi eld) are of Cenozoic age, with the majority of those being Eocene carbonates.
Sandstone porosities range from less than 5 percent to 30 percent and average 12 percent to 16 percent. Permeability ranges from less than 1 millidarcy (mD) to greater than 300 mD, with the average ranging from 4 to 17 mD (Khan and others, 1986). At the Dhurnal fi eld and probably elsewhere in the basin, hydrocarbons in the carbonate reservoirs are primarily from tectonically induced fracture porosity on strike with structural trends (Jaswal and others, 1997). Approximately 60 percent of the identifi ed producing reservoirs are carbonates.
Because oil and gas production volumes are reported by fi eld rather than reservoir in the database used (Petroconsultants, 1996) and 12 of 21 fields reported production from more than one reservoir, no attempt was made to assign volumes of oil and gas to reservoirs, ages, or lithologies.
Traps and Seals
Most of the fi elds discovered in the Kohat-Potwar geologic province to date are either overturned faulted anticlines, popup structures, or fault-block traps. In this area, anticlinal features strike generally east-northeast to west-southwest and are approximately parallel to the plate-collision zone. Many of these folded structures are amplifi ed, or they are only present above a detachment zone in Eocambrian salts. The latest trap-forming thrust events began at approximately 5 and 2 Ma (Jaswal and others, 1997). Seals include fault truncations and interbedded shales and the thick shales and clays of the Miocene and Pliocene Siwalik Group.
Data source: Patala-Nammal Composite Total Petroleum System, Kohat-Potwar Geologic Province, Pakistan. C.J. Wandrey, B.E. Law, and Haider Ali Shah, 2004
Rapid karstification process with evaporite-driven sinkholes in Southern Kohat Basin, NW Pakistan. Said Mukhtar Ahmad, Teng Wang, Saad Khan, Muhammad Waqar Azeem, Lv Fu. 2025
Следующий Бассейн: Palmyride Foldbelt