EXPLORATION Anatomy of success in oil and gas exploration in Pakistan, 1915-94

Viqar-un-Nisa Quadri
S.M.G.J. Quadri
Exploration consultants
Riyadh

Pakistan, flanked by Iran, Afghanistan, China, and India, is the size of Texas and Louisiana combined.

The Indus and Baluchistan basins cover 80% of Pakistan's total area (Fig. 1 [58154 bytes]). The country also has 230,000 sq km of marine Exclusive Economic Zone.

The law regarding E&P activity was promulgated in 1986, replacing the previous Petroleum (Production) Rules of 1949.

As a result of the new Petroleum Policy implemented in March 1994 and streamlining of the bid review and award process, acreage leased including reconnaissance during 1994 was 355,541 sq km onshore and 120,640 sq km offshore, with the number of operating groups also a record high of 46.

The major players are Oil & Gas Development Corp. (OGDC), Union Texas Petroleum (UTP), Pakistan Petroleum Ltd. (PPL), Pakistan Oilfields Ltd. (POL), Occidental (Oxy), Lasmo, OMV, Tullow Oil, and British Gas.

Sedimentary basins

Although complex and disturbed as a result of collision tectonics,1 Pakistan's geology is as fascinating as the surface geomorphology, from the complex compressional thrusted to the relatively simple extensional rifted, salt related to transform fault associated, the reefs, too, all impressive traps for petroleum, at times almost textbook examples.

However, domestic oil production at yearend 1994 was about 53,251 b/d of oil and 1.7 bcfd of gas.

Oil and gas have been found in the Potwar/Upper Indus basin and Lower Indus basin, and mainly gas with one gas/condensate discovery in the Sulaiman/Middle Indus basin. This article attempts to present brief case history outlines of typical, significant oil and gas discoveries of Pakistan 1915-94 (Fig. 2 [104358 bytes] and Table 1 [71210 bytes]) with respect to the two main productive basins, their source and reservoir sequences, in order to determine the anatomy of success in exploration in Pakistan.

U. Indus (Potwar) basin

The Potwar basin is bounded to the south by the Salt Range escarpment, where rocks of Precambrian to Tertiary are exposed.

In the north it is bounded by the Main Boundary thrust, with Paleogene and Mesozoic rocks exposed in the Kala Chitta and Margala Hills. In the east is the Jhelum transform fault, and to the west lies the Kalabagh transform fault.

Infra-Cambrian evaporites of unknown true thickness were deposited in Potwar in restricted and hypersaline environments caused by carbonate banks developed over the east bank of the Jhelum re-entrant, north of present Kala Chitta and Margala ranges, along Precambrian basement lineaments, which isolated the Potwar basin from Kashmir and Hazara, where open marine conditions prevailed.

Depositional environments of Pakistan basins are shown in Fig. 3 [81655 bytes].

Sadkal field

Sadkal field, discovered in June 1992, is in the Northern Potwar deformed zone on OGDC's Basal E.L. concession in Punjab's Attock District.

The structure is developed on the subthrust sheet and on the same geological trend as Bhal Syedan field and is bounded by faults on the northern and southern sides, with the northwestern flank also affected by a fault closer to the crest. At top of Eocene, the Kohat limestone level on 1,500 milliseconds two way time closed contour, areal extent is 4.8 sq km.

Sadkal 1 reached TDs of 3,346 m in the Middle Eocene Kuldana formation (hole A) and 4,200 m in Early Eocene Margala Hill limestone formation (hole B).

An open hole drillstem test at 3,941-4,060 ft in Paleocene Lockhart and Eocene Chorgali yielded water with traces of oil. Condensate surfaced on a second DST at 3,661-70 ft.

The Sadkal 2 well was completed as an oil producer in March 1993 with original reserves of 738,000 bbl of oil. Production in 1994 averaged 2,819 b/d of oil and 28.9 MMcfd of gas.

Missa Keswal field

Missa Keswal, formerly Qazian, oilfield was discovered in August 1991. It is in East Potwar on OGDC's Gujar Khan E.L. concession in the Rawalpindi district of Punjab.

The structure is a northeast-southwest trending surface expression of 19 by 3.2 km, eroded down to Chinjis (Mio-Pliocene). The discovery well has been drilled 2 km northeast of Qazian well X1 drilled in 1979-80 by Gulf Oil on the same structure.

Missa Keswal 1 reached TD 2,202 m in infra-Cambrian Salt Range on July 16, 1991. It is an oil producer from Eocene Sakesar limestone, Lockhart, Cambrian Jutana dolomite, and Cambrian Bhaganwala and Khewra sandstone.

Original reserves were 34.73 million bbl of oil. Production in 1994 was 4,930 b/d of oil and 8.094 MMcfd of gas.

Gas and condensate flowed to surface on a drillstem test of Cambrian Kussak at 2,013-29 m. Extrapolated reservoir pressure was 4,204 psia.

Fimkassar oilfield

Fimkassar, formerly Parhal, field is in southern Potwar on OGDC's Fimkassar concession in Punjab's Chakwal District.

The structure is located in southeastern Potwar and has a 23 by 13 km northeast-southwest trending surface expression with vertical closure 1,800-1,900 m. At Eocene level areal extent is 45 sq km and vertical closure is 58 m, with an additional 230 m due to vertical fault relief. The areal closure is comprised of 7.5 sq km crestal and 37.5 sq km fault related closure.

Gulf Oil declared Fimkassar 1X an oil discovery in June 1981. TDs were 2,730 m (hole A), 2,253 m (sidetrack hole A), 3,081 m (hole B), and 2,360 m (sidetrack hole B). The well is an oil producer from Chorgali limestone at 2,891-3,081 m. Original reserves were 30 million bbl, and 1994 production was 2,562 b/d of oil.

Dhurnal field

Dhurnal oilfield was discovered in March 1984 in the triangle zone in the foreland belt of North Potwar deformed zone in Oxy's Dhurnal M.L. concession in the Attock district of Punjab.

At the surface the field is an east-west trending sinuous syncline. At Eocene, Paleocene, and Permian levels, the structure has four way dip closure. Strike lines show it as a west-southwest/east-northeast bold anticline with well defined axial plunge. Dip lines show anticlinal turnover with both flanks terminated by two opposing thrust faults at downdip position.

A reverse fault encountered in the discovery well repeated 99 m of Sakesar formation, enhancing pay thickness and fracture intensity.

At Eocene level on 2,100 ms TWT closed contour, areal extent is 2,185 ha and vertical closure 230 ms, with additional fault closure. Miocene Murree formation claystones provide vertical and lateral seals to Eocene, Paleocene, and Permian reservoirs.

The Dhurnal 1 discovery well reached TD 4,099 m (sidetrack 4,003 m) in Sakesar on Mar. 5, 1984. The Chorgali-Sakesar formation in the sidetrack hole is a very permeable and highly productive reservoir with low skin damage.

The Dhurnal 2 appraisal well tested 3,500 b/d of oil from Permian Wargal limestone and 5,500 b/d from Paleocene Patala-Lockhart.

The field produced 2,993 b/d of oil and 9.564 MMcfd of gas in 1994. Original reserves were 50.94 million bbl.

Adhi field

Adhi oilfield, discovered in February 1978, is in East Potwar in PPL's Adhi M.L. concession area in Punjab's Rawalpindi district.

At the surface it is a northeast-southwest trending, very narrow steep anticline 64 by 12.8 km with 609.6 m vertical closure, eroded down to Chinjis.

Wells were drilled in December 1946, November 1957, October 1965, and April 1967 before oil the discovery at well No. 5.

Adhi 1 and 2 blew out due to high pressure water sands in Neogene Siwaliks at 640 m and 1,147 m, respectively. Adhi 3 was abandoned at 3,275 m as it was apparently off subsurface structure. Adhi 4 was abandoned at 2,042 m for mechanical reasons due to alternating lost circulation and high pressure water flows. Mud weighing 24 lb/gal was used to kill this well.

Adhi 5, drilled to 2,812 m, tested 50 gravity oil from Permian Tobra sandstone, 44 gravity oil from Khewra sandstone, and 27 gravity oil from Sakesar.

With original reserves of 116 bcf of gas and 10.221 million bbl of condensate, Adhi in 1994 produced 16.817 MMcfd of gas, 2,449 b/d of condensate, and 62 metric tons/day of LPG.

Lower Indus basin

The Lower Indus basin is bounded by the Indian shield to the east and marginal zone of Indo-Pak plate to the west. It is comprised of the Thar platform, Karachi trough, Kirthar foredeep and Kirthar fold belt, and Indus offshore extension.

The Badin trough located over the southeastern flank trends northeast-southwest, deviating slightly in the south to a more north-south orientation .

Bari field

Bari oilfield, discovered in 1992, is in the Badin trough on UTP's Laghari P/L concession area in Hyderabad district, Sind. This is a "B" sand stratigraphic trap, downdip to the "C" sand production in Laghari field.

Subsurface mapping shows approximately 325 ha productive, while seismic indicates about 93 ha.

The Bari 2 well has been drilled between Bari 1, in which the B sands of the Upper Sand unit of Cretaceous Lower Goru came in wet, and Laghari oilfield wells in which the B sands are absent, on the basis of gross/net B sands isopach maps. It is the first stratigraphic hydrocarbon accumulation discovered on the Badin block.

Laghari field is a three way fault closure situated on the northeastern edge of a large northwest-southeast trending horst block.

Bari 2, TD 923 m in Lower Goru, encountered oil in all 4.9 m of net (6.1 m gross) B sand.

Original reserves are 2.531 million bbl of oil; 1994 production was 2,275 b/d of oil and 537 Mcfd of gas.

Kadanwari field

Kadanwari gas field, discovered in August 1989, is on the Thar platform in the Lasmo group's Tajjal exploration license area in Khairpur district of Sind.

The structure is a small culmination on a major north-northwest/south-southeast trending nose east of the Khairpur high. It appears to have been inverted during Late Cretaceous and then tilted during Oligocene inversion of the Khairpur high.

There is a fault bounded structural closure at the two Lower Goru horizons, one near the top of the shallowest Early Cretaceous sands encountered in Bobi 1, the other deeper (equivalent to main hydrocarbon bearing Middle Sand Unit in Bobi 1) with vertical relief at both levels 30 m, although shape changes markedly with depth.

The well tested a valid structure at Sui Main limestone level, but it is uncertain whether the well tested reversal within the upper part of the Sembar formation.

The Kadanwari discovery well, TD 3,994 m in Sembar, flowed gas on drillstem tests at rates of 11.2-28.6 MMcfd on various size chokes with 133-174 b/d of water. Original reserves are 728 bcf of gas. The well is temporarily suspended.

Pasakhi field

Pasakhi field, discovered in July 1989, is in the Badin trough on OGDC's Sanghar P.L. concession in Hyderabad district, Sind.

At the Cretaceous level it is a horst block bounded by two normal faults dipping to the west and east, respectively. On top of Lower Goru sand reservoirs, areal extent is 1.3 sq km with vertical closure 53 m.

Original reserves were 9.37 million bbl, and 1994 production was 2,320 b/d from Lower Goru.

Tando Alam field

Tando Alam field, opened in May 1984, is in the Badin trough of the Thar platform on OGDC's Tando Alam M.L. area in Hyderabad district, Sind.

At Cretaceous level it is a north-south trending anticline bounded by two faults, with four way closure at Middle and Lower Goru level. Minimum areal extent is 5.5 by 2 km, and vertical amplitude is 90 ms TWT (about 120 m).

Original reserves were 20.16 million bbl, and 1994 production was 919 b/d of oil from Lower Goru.

Khaskeli field

Khaskeli field, discovered in June 1981, is in the Badin trough on UTP's Khaskeli M.L. concession area in Hyderabad district of Sind.

The structure is a tilted fault block dipping to the west and bounded by a down-to-the-east fault that divides in three branches in the center and two in the south of 30.5 to 61 m throw. The structure is separated into five blocks, and the oil field covers more than 17.8 sq km.

The discovery, by the UTP group, was the first oil found in the Lower Indus basin.

Original reserves were 8.196 million bbl, and 1994 production was 684 b/d of oil and 331 Mcfd of gas.

Producing reservoirs are Lower Goru Upper Sand unit, A1 and A2 reservoirs. Average porosity is 25%. Permeability exceeds 1 darcy.

Sui field

Sui, a giant gas field discovered in November 1952, is in the Marri-Bugti transverse uplift in Sibi district of Baluchistan, on PPL's Sui M.L. concession.

At surface it is a 52 by 12 mile, gentle anticline eroded down to Lower Siwaliks. It was drilled and discovered on surface evidence.

The discovery well, TD 3,066 m, found Sui Main limestone 600 m thick with a 226 m gas column. Porosity is 6.7-28.4%, permeability 35 md. Gas is 90% methane, 1.2% ethane, 4.5% carbon dioxide, and 3.5% nitrogen.

Original reserves were 8.624 tcf of gas, and 1994 production was 712.267 MMcfd.

Anatomy of discoveries

1. There is a clear correlation between the modest number of discoveries and an equally modest exploratory effort ( Table 2 [10454 bytes]).

2. Re-entry based on state of the art seismic and drilling techniques has yielded positive results, e.g. Pindori (ex-Chak Beli Khan), Missa Keswal (ex-Qazian), Bhangali (ex-Tanwin Bains), Fimkassar, Adhi, etc. discoveries in the Upper Indus (Potwar) basin.

3. In the Upper Indus (Potwar) basin, structures in the east appear as a result of complex tectonics involving both salt and basement roles. Structures in the west are largely detached due to salt flow. All these subsurface structures have surface expression.

Structures in the north do not have surface expression and seem to have resulted by a combination of thrust faulting to the north and folding common in the south, modified by collapse due to evacuation of underlying infra-Cambrian salt from structural lows. The powerful upward force of salt added with unreleased compression has created south dipping thrust faults.2

4. In the Lower Indus basin, the structures in the Badin trough and Thar platform are basement related, with petroleum traps located in intra-Cretaceous extension faults and associated folds, the latter being products of cover rock shearing above crustal tear zones. The structures towards the folded edge of the Kirthar foredeep in the west. and Marri-Bugti transverse uplift towards the north are convergence/ wrench related.3

5. The oil and gas fields discovered to date are structural traps, except two. The Jurassic of Dhulian field in the Upper Indus basin is structural-stratigraphic, while Bari oil field in the Lower Indus basin is a classic stratigraphic trap of Sand "B" of the Lower Goru Upper Sand unit.

6. Based on weight percent organic carbon, estimated visual carbonization scale and kerogen type, the following formations are potential source rocks: Patala, Sembar, Late Permian Chhidru, Early Permian Sardhai, Cambrian Khisor, and Salt Range.

In north central Potwar, the Patala, Chhidru, Sardhai, and Salt Range formations are the more likely source rocks, as Sembar and Khisor are absent in this area. The proven source rock in Potwar is Patala shale and packstone, with depositional environment outer shelf. In the Lower Indus basin, the proven source rock is Sembar micstone and basinal muds.4

7. On the basis of chromatographic and biomarker analyses, the oils produced from random samples can be divided into three groups, with the main difference in grouping due to variations in oil maturity and extent of alteration by water washing:

• Group 1: Jurassic oil from Toot, Eocene oil from Fimkassar, Eocene-Permian-Cambrian oil from Adhi, all fields of the Upper Indus basin.

• Group 2: Cretaceous oil from Tando Alam, Laghari, and Khaskeli fields of the Lower Indus basin.

• Group 3: Eocene oil from Toot field of Upper Indus basin.

The oils from East Potwar have "heavy" isotopic signatures characteristic of marine-derived oils and cannot be positively correlated to the mature oil prone Patala source rocks from Dhurnal oil field or the mature oil prone Salt Range source rocks.5

8. The Sembar and Lower Goru formations contain prodigious oil and gas prone source oil sequences that developed under control of wave/tidal regimes-net pay greater than 300 m in or near storm wave base in pro delta and delta front regimes. Upper Sembar sequence rich in amorphous oil prone kerogen is richer than many ancient analogs due to longitudinal deposition of ocean geometry rather than the usual latitudinal orientation against well oxygenated Tethyan margins. Realistic Sembar total oil yields are in the range of 65-155 million bbl/sq km, which at optimum maturity can source giant fields.

The Basal sand unit of Lower Goru contains prograding delta sequences that contain sporadic and ephemeral source rock units with humic kerogens. The Talhar shale sequence can generate up to 21 million bbl/sq km. Its thickness varies from 24 m in southeast to 76 m in southwest of the Badin block, where it is mature.6

9. In Upper Indus (Potwar) basin, a number of reservoirs are present in the sedimentary section ranging in age from Cambrian to Miocene.

Khewra sandstone with more than 13% intergranular average porosity is gas/condensate producing at Adhi, Missa Keswal, and Rajian fields.

Tobra sandstone with more than 10% intergranular average porosity is oil producing at Adhi field, while Wargal limestone is oil producing from secondary fracture porosity at Dhurnal field.

Datta sandstone is oil producing at Toot and Meyal fields, while Paleocene packstones are oil reservoirs in some oil fields. Eocene packstones and wackstones with fracture porosity are productive in many Potwar oil fields. The Murree sands are producing at Khaur and Missa Keswal fields.

10. In the Lower Indus basin, Lower Goru marine deltaic sandstones and turbidites with 3-30% matrix porosity are oil/gas reservoirs in all Badin trough oil/gas fields. The sequence column is shown in Table 3 [13336 bytes].

The Ranikot limestone and sandstone with porosity 5-10% are gas/condensate producers at Sari, Hundi, Kothar, and Pirkoh gas fields. Sui Main/Sui Upper/Habib Rahi (Eocene) bound stone, bound stone, and limestone with vadose/fracture porosity 10-28% are gas reservoirs at Sui, Loti, Qadirpur, and Mari gas fields.

11. The drilling conditions in Upper Indus (Potwar) are extremely difficult due to abnormally high formation pressures and reversal of pressure regimes in the Siwalik formations, to a lesser extent in the north and most severe in East Potwar. Cost is therefore quite high compared to the Lower Indus basin, where drilling problems are not severe and reservoir depths are moderate.⁷

Conclusions

1. Realistic total oil yields of proven source rock Sembar in the Lower Indus basin are in the range 63-155 million bbl sq km, which at optimum maturity can source giant fields. So far only one Sembar-sourced giant field, Sui, has been discovered.

Similarly the proven source rock Patala and potential source rocks of Precambrian shale are extremely rich sources for liquid hydrocarbons in the Potwar basin. Thus at a rough estimate, only 1% of total potential resource of oil and 15% of potential resource of gas have been discovered to date. Therefore many giant potential structural and stratigraphic traps await the drill in Pakistan.

2. In places of extensive seismic grid it is recommended to determine pinchouts and seismic character, where possible, of all productive sands of Upper, Middle, and Basal units of Lower Goru sequence in the Lower Indus basin. Bari field can be used as a test case for grading future stratigraphic pros- pects. In the Upper Indus basin, an extensive sedimentological and depositional study of sandstone reservoirs is required as a prelude to search for stratigraphic traps. Horizontal drilling may be useful in locating the best fracture porosity zones of the limestone reservoirs.

3. The exploratory effort has been modest laterally and vertically. Wells drilled in the Lower Indus basin for testing Lower Goru Upper Sand unit have not been valid tests for fault closures at Middle and Basal Sand unit's levels, which are shifted with respect to Upper sands. Likewise crestal areas of deeper/older potential reservoirs in very large structures that are shifted with respect to younger horizons, e.g. in Sui and Mari fields, have remained undrilled.

4. It may be in Pakistan's interest to study the vertical concession scenario of countries like Canada to evolve a strategy in case other approaches fail in getting all of Pakistan's potential reservoirs drilled.

References

1. Ahmed, Gulfraz, An overview of oil and gas exploration and production in Pakistan., Proceedings of International Petroleum Seminar, Islamabad, Nov. 22-24, 1991, pp. 185-187.

2. Quadri, V.N., Petroleum prospects of Pakistan and oil oriented exploration guidelines, Petroleum Institute of Pakistan Symposium, Energy in the '80s, Peshawar, March 1980.

3. Kemal, Arif, Geology and new trends for petroleum exploration in Pakistan, Proceedings of International Petroleum Seminar, Islamabad, Nov. 22-24, 1991, pp. 16-56.

4. Pryor, W.A., Potwar and Kirthar basins petrographic study, H.N. Fisk Laboratory of Sedimentology, University of Cincinnati, OGDC-TSD, Karachi, 1984.

5. Palmer, Technical Report No. 130, OXY/Cities, August 1986.

6. White, R.N., Depositional history and prospectivity of Sembar formation, November 1992.

7. Kadri, I.B., Petroleum geology of Pakistan, published by Pakistan Petroleum Ltd., Karachi, October 1994.

Directorate General of Petroleum Concessions, Ministry of Petroleum and Natural Resources, A brief review, Islamabad, 1991.

The Authors