Tectonics of Pakistan PDF

Patala Nammal Petroleum System Kohat Potwar Geologic Province Pakistan Regional Geologic History The total petroleum system (TPS) and assessment unit (AU) discussed herein acquired their primary structural and stratigraphic features from events associated with plate movements that occurred from la...

Patala Nammal Petroleum System Kohat Potwar Geologic Province Pakistan Regional Geologic History The total petroleum system (TPS) and assessment unit (AU) discussed herein acquired their primary structural and stratigraphic features from events associated with plate movements that occurred from latest Paleozoic to the present. From Permian through Middle Jurassic time, the Indian plate was located in the Southern Hemisphere between the African, Antarctic, and Australian plates and comprised part of southern Gondwana. The area that is now the Indus Basin, Kohat-Potwar geologic province,and northern India, was a shallow continental shelf on which carbonates, shales, and sandstones were deposited. western part of the shelf through the Late Jurassic and is represented by the interbedded shales and thick limestones of the Springwar Formation and as much as 1,400 m of the Middle and Upper Jurassic Sulaiman Limestone Group on western and northern portions of the plate. In the area that is now the Kohat-Potwar geologic province, shelf and shallow-marine stages are preserved in the rocks of the Permian Nilawahan, Permian and Triassic Zaluch, Triassic Musa Khel,and Jurassic and Cretaceous Surghar Groups. During the Late Jurassic, Madagascar, India, Australia, Antarctica, and the Seychelles began to break away from Africa, forming the Somali Rift Basin. Late Jurassic rifting also initiated separation of Australia and Antarctica from India. During Early Cretaceous time, the Indian plate drifted northward Along the eastern portion of the Indian plate the Rajmahal Trap volcanics were deposited. On the northwest margin of the plate, marine shales and limestones of the Lower Cretaceous Sembar and Goru Formations were deposited. On the eastern shelf of the Indian plate, the Bolpur and Ghatal Formations were deposited. During the latest Cretaceous, the Indian plate continued to drift northward toward the Eurasian plate and the sea floor of the Bengal Basin began to form. Northward plate movement continued during the Late Cretaceous, and a transform fault became active. Rifting between Madagascar and the Seychelles portion of the Indian plate initiated formation of the Mascarene Basin. Extensional faulting occurred as the western part of the Indian plate sheared southward relative to the main plate. Counterclockwise rotation of the Indian plate was initiated, and the Seychelles portion of the Indian plate began to break away. Latest Cretaceous was also a time of intense volcanism in western India, with the rifting, which leaving the Cambay and Kutch Grabens floored with the Deccan Trap basalts. The rifting event in the Cambay and Kutch areas might be related to the extensional faulting and shear zone. From the Late Cretaceous through middle Paleocene, trap deposits and Oblique convergence of the Indian plate with the microplates constituting the southern edge of the Eurasian plate resulted in wrench faulting and development of regional arches such as the Jacobad and Sargodha Highs in the Indus Basin. The Indian plate continued to move northward at an accelerated rate of 15--20 cm/yr Continued northward movement and counterclockwise rotation of the Indian plate slowly closed the Tethyan Sea. The Sulaiman-Kirthar fold belt began to develop as a result of this oblique collision and rotation. Regional uplift and the resulting mountain ranges rising on the Eurasian plates to the north and west created a new sediment source From Eocene through middle Miocene, a carbonate platform buildup occurred on the shelves of the Indian plate. A trench formed along the subduction zone as the Indian plate began to slip beneath the Eurasian plate. The Eurasian plate shed large volumes of sediments into the trench as subduction continued. This terrestrial sediment influx from the rapidly rising Himalayan, Sulaiman-Kirthar, SinoBurman, and Indo-Burman Ranges. In the Kohat-Potwar geologic province shallow northeast-trending anticlines and overturned folds developed above multiple detachment surfaces. Detachment surfaces as deep as the Eocambrian (late Precambrian) salts developed as a result of continued plate convergence, and associated crustal shortening of as much as 55 km occurred. The proto-Indus, Narmada, Ganges, Brahmaputra, Megna, Chindwin, and Irrawaddy Rivers developed extensive deltas as the Himalayas and other ranges continued to shed sediments at a high rate. Today, uplift of the Himalayas and subduction of the Indian plate continues, and the growth rate of the Indus, Ganges-Brahmaputra (Megna), and Irrawaddy deltas remains high. In the Kohat-Potwar area, crustal shortening also continues. Stratigraphy The Kohat-Potwar geologic province depositional record is relatively complete from Late Proterozoic to Holocene. 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 identified within the evaporite sections. 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 shales and sandstones. Cambrian Jhelum Group, Khewra Formation, which has produced oil at the Adhi, Chak Naurang, and Rajian field.The Kussak Formation glauconitic sandstones and siltstones 'Kussak has produced oil at the Missa Keswal field. The Jutana Formation consists of sandy carbonates and sandstones.The Cambrian Baghanwala Formation consists of shales and interbedded sandstones. The Permian Nilawahan Group consists of the Tobra Formation glacial tillites, siltstones, and shales.The Dandot Formation coarse-grained sandstones and shales.The Warchha Formation coarse-grained argillaceous sandstones, and shalesThe Sardhai Formation, which consists of clays and sandstone. The Zaluch Group consisting of Permian Amb Formation carbonates, Wargal Formation carbonate sequences, and the Chhidru Formation marls and coarsening upward sandstones. The Wargal has produced oil at the Dhurnal field. Triassic musakhel group the Mianwali formation consists of shales.The Tredian formation consists of sandstone.Kingriali Formation consists of dolomite. The Triassic Formations were formerly referred to collectively as the Wulgai Formation. The Jurassic strata include Datta and Shinawari Formations consisting of nearshore variegated siliciclastics The Datta has produced oil and gas from three fields, including Dhulian, in the northwest Potwar Plateau area.The Samana Suk Formation carbonates. The Cretaceous section consists of Chichali Formation shales,and Lumshiwal Formation sandstone. Eocene Makarwal Group Hangu Formation consists of impure limestone and sandstone The lockhart formation consists of semi nodular and marl limestone The patala formation consists of shales Oil production has been attributed to the Lockhart and Patala. The overlying Eocene chart group Nammal formation s shales and interbedded limestones with a transitional contact between the Patala and the Nammal. Overlying the Nammal Eocene Sakesar or Margala Hill Formation marine limestones and shales. Oil or gas production from eight fields spanning the Potwar Plateau are attributed to the Margala Hill.The shales and interbedded limestones of the Eocene Chorgali Formation, the Miocene to Pliocene Murree Formation fluvial sandstones and siltstones and the Kamlial Formation fluvial 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 fluvial sandstones and conglomerates mark the top of the stratigraphic column in the area. Production History In 1866, the first oil well on the western Indian subcontinent was drilled at Kundal. The fi rst commercial oil discovery occurred in 1914 when the Attock Oil Company completed a 214-ft well on a thrust-faulted anticline near the town of Khaur. Since the discovery, there have been more than 340 wells drilled in the fi eld. Production at Khaur is from Sakesar and Murree Formation reservoirs. Drilling from 1920 to 1935 resulted in success at Dhulian, a structural dome 17 km southeast of Khaur. Other discoveries followed with Joya Mair in 1944, Balkassar in 1946, Karsal in 1956, and Tut in 1967. The fi rst commercial gas fi eld---Adhi---was found in 1979. The Adhi gas fi eld produced from rocks ranging in age from Cambrian to Eocene. Eighteen oil and three gas fi elds were discovered in the Kohat-Potwar geologic province between 1915 and 1996, and several prospects that were unsuccessfully tested in the 1950s and 1960s are now being re examined with good results. Production in the Kohat-Potwar geologic province comes primarily from faulted anticlinal traps in rocks ranging in age from Cambrian to Miocene, and at depths of less than 20 m to greater than 1,030 m. The largest oil and gas fi eld found to date is the Dhurnal fi eld with areal closure of 13.5 km2 and vertical closure of 500 m. The initial in-place oil estimate for Dhurnal was 102 MMBO (million barrels of oil) (Jaswal and others, 1997) The plot of cumulative new-fi eld wildcat wells versus completion year for the Kohat-Potwar geologic province (fi g. 14) shows that the rate of fi eld discovery through time has increased. Composite Petroleum System Source Rock The U.S. Geological Survey (USGS) recognizes that there are several individual total petroleum systems in this area. For assessment purposes, however, they were combined into a single composite Eocambrian-Miocene TPS (the Patala-Nammal TPS, 802601) because few correlations of source to reservoir hydrocarbons were available at the time of the assessment. In addition, multiple stacked sources and reservoirs---and extensive fault systems---allow the mixing Of hydrocarbons from multiple sources making further subdivision more diffi cult. The Paleocene Patala Formation appears to be the primary source of hydrocarbons, but other potential source rocks may be contributing in different parts of the basin. 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. Most of the available information and analyses available was derived from samples collected on the Potwar Plateau and easternmost 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. 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 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 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. 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. Maturation Thermal maturities for Kohat-Potwar rocks range, from Ro 0.3 to more than 1.6 percent. A basin profi le indicates vitrinite refl ectance equivalent maturities of 0.62 to 1.0 percent for Tertiary rocks in the productive part of the Potwar Basin 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 rock. 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. 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 fore 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). 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. 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. Production from more than one of these 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. 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. 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 fi elds 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. Seals include fault truncations and interbedded shales and the thick shales and clays of the Miocene and Pliocene Siwalik Group. Assessment Unit The Kohat-Potwar Intrathrust assessment unit, is also a composite. The rocks of this AU include sandstones, shales, and coals of deltaic to fl uvial facies and carbonates and shales of shelf environments and range in age from Eocambrian to Miocene. Assessment of Undiscovered Oil and Gas Based on data current to 1996 provided by Petroconsultants International Data Corporation, the Kohat-Potwar geologic province was ranked 174th in cumulative production and reserves of oil and gas worldwide. The Kohat-Potwar geologic province due to its long history of oil production, remaining potential, and geologic and geographic relationship with the rest of the Greater Indus Basin, it was categorized as a boutique province for the USGS World Petroleum Assessment 2000 (U.S. Geological Survey World Energy Assessment Team, 2000). Known petroleum volumes are 0.3 billion barrels of oil (BBO) and 1.9 trillion cubic feet of gas (TCFG) for a total of 0.7 billion barrels of oil equivalent (BBOE) including natural gas liquids (Petroconsultants, 1996); this volume is less than 0.1 percent of world volume (excluding the United States). In the Kohat-Potwar geologic province 18 oil and 3 gas fi elds had been discovered by 1996 (Petroconsultants, 1996). Previous estimates of undiscovered oil and gas in this region include those by Kingston (1986) and Masters and others (1998). Kingston (1986) estimated the mode of undiscovered oil in the Kohat-Potwar province at 0.1 BBO and gas at 0.1 TCF. Masters and others (1998) estimated the mean for undiscovered oil in this province as 0.12 BBO and the mean for undiscovered gas as 3.0 TCF. The 2000 assessment incorporates the petroleum system concept as defi ned by Magoon and Dow (1994). The TPS used as the geologic basis of the 2000 assessment in the KohatPotwar geologic province is the Patala-Nammal. Tables 1, 2, and 3 show the estimated ranges of assessed undiscovered oil and gas volumes allocated by assessment unit and the totals for the composite Patala-Nammal TPS. Summary Source-rock quality in the Kohat-Potwar geologic province is fair to good in numerous stratigraphic intervals (fi g. 23). Burial depths were in the past, and are today, great enough for continued generation from many of these source intervals. The Paleocene and Eocene source intervals have been in the oil and gas generation window in parts of the Kohat-Potwar geologic province for as long as 15 m.y., and, in places, generation continues today. Reservoirs are of fair to very good quality and exist in close proximity to source rocks. The presence of stacked reservoirs greatly improves opportunities for discoveries. Trap development, although widespread, was relatively late but coincided with maximum burial and probably maximum hydrocarbon generation. Continued crustal shortening and uplift in some cases enhanced trap quality in others diminished or destroyed it. Complex structures and overpressuring, although creating exploration and drilling challenges, also offer additional opportunities. Further opportunities also exist in the Kohat Plateau area where few wells have been drilled, but source-rock quality or thickness may be lower. The northern folded zone also has potential, although structural complexity makes exploration diffi cult.