Failure-to-success targets may lie in Pakistan basins

Viqarun Nisa Quadri, S.M.G.J. Quadri
Petroleum Consultants
Riyadh

Failure-to-success stories abound in worldwide oil and gas exploration history.

The fame of the Dammam "Lucky" No. 7, which launched Saudi Arabia's gigantic petroleum era, is well known. This well was spudded following a series of drilling disappointments elsewhere on a dome. It flowed steadily from 1938-89 and would have continued to produce 1,800 b/d of oil had it not been plugged.

A previous study of the anatomy of success in Pakistan has revealed that re-evaluation of structures based on state of the art seismic and drilling techniques has yielded positive results.¹ In this article we shall relate a few failure-to-success stories with implied recommendations that some structures be drilled again.

Upper Indus/Potwar basin

Pindori field

The Chak Beli Khan structure is located in the East Potwar basin (Fig. 1 [223,067 bytes]). It is a narrow, steep anticline 35.2 km by 4.4 km at the surface, eroded to Miocene Chinjis.

Pakistan Oilfields Ltd. drilled CBK-1 in 1951 to 433 m (minus 381 m) and abandoned due to high pressure water sands that could not be controlled with weighted mud.

The same company drilled CBK-2 to 256 m in the same year about 305 m northeast of CBK-1 and abandoned it after it was believed to have crossed a fault, resulting in a repeat section. POL and Pakistan Petroleum Ltd. drilled CBK-3 in 1963-64 to 3,933 m (minus 3,468 m) to the Early Miocene Murree formation and abandoned it for mechanical reasons without reaching the objective reservoir.

The drilling results showed crestal axis displacement between surface and subsurface. Some reflection and refraction seismic data were acquired, but they did not yield a clear picture and the structure was forgotten.

After more than 21/2 decades, multifold seismic showed the structure as a faulted anticline with 2,710 ha areal extent and 215 ms (approximately 410 m) vertical closure to the 2,375 ms contour. The seismic data show dip closure to the north, south, and east and a reverse fault to the west, juxtaposing the sealing Murree formation against the Eocene reservoir objective.

The structure was given a new name, and Pindori-1 was drilled in 1991 down to 4,243 m (minus 3,751 m) to the Early Permian Sardhai formation. It flowed 2,912 b/d of 42.8° gravity oil and 8 MMcfd of gas on a 32/64 in. choke from the Eocene Sakessar formation. This well unfortunately blew out and was completed as an observation well to the shallow aquifers. The field was producing 1,941 b/d of oil and 5.65 MMcfd of gas as of May 1997.

Chak Naurang field

The Chak Naurang structure is reflected at the surface as a narrow anticline covering 6.4 km by 1.6 km with 305 m of amplitude, eroded down to Chinjis. It is associated with a major E-W thrust fault and on trend with Karsal, Balkassar, and Joya Mair oil fields. There is a crestal thrust fault and a minor adjustment fault creating a horst block.

Burmah Oil Co. drilled Chak Naurang-1 in 1952-53 about 122 m low to the crest, on the downthrown side of the adjustment fault. The well reached 2,374 m, into the Sardhai formation, and tested 3 b/d of oil on a drillstem test at 2,176-2,270 m from the Eocene Bhadrar formation.

Seismic data showed the subsurface structure near Top Eocene level to cover 7 by 1.75 km in area with 144 m of vertical closure.

OGDC drilled the structure 33 years later, in 1986. Its Chak Naurang-1A produced about 150 b/d of oil. OGDC drilled Chak Naurang-2 in 1986-87 to 2,828 m in Cambrian Khewra sandstone. It was completed as a 3,200 b/d oil well in Eocene Chorgali and was producing 647 b/d of oil in May 1997.

Recommendations

Several other structures exist in the Potwar basin that need to be drilled again.

Karsal prospect

This is a relatively gentle structure created by salt pillowing. Its surface expression is a low relief flexure on the same trend and west of Balkassar, Joya Mair, and Chak Naurang oilfields, on Landsat Image 162/37.

Four wells were drilled from May 1955 to May 1957 without seismic. These wells were probably located on northeast pitch as shown by one seismic line shot in 1980 across the flank.

Karsal-1 and 3 produced 163,000 bbl of 27° oil from 1959-65 from Bhadrar. The deepest well, Karsal-1, reached TD 3,889 m in Permian. All four wells found the top of Eocene between 3,399-3,614 m.

Subsurface areal extent at Eocene level is assumed to be 3.55 by 2 km with 38 m of amplitude.

A 4,180 m deep well should be drilled southwest of Karsal-1 if seismic confirmed the subsurface crest to be in this position. Eocene and Paleocene are the main objectives, while Early and Late Permian and Khewra sandstone are secondary objectives.

Kotsarang prospect

This prospect is located on Landsat Image 162/37. From seismic at the Top Bhadrar level this structure covers 5.7 by 3.2 km with 80 m of vertical closure.

OGDC drilled a 4,153 m well into Late Permian in 1966. It produced oil for a short time from fractured Early Eocene Lockhart formation. Jurassic sandstone, the producing reservoir at Toot, Dhulian, and Meyal oil fields to the northwest and north, was absent in the well.

This prospect deserves to be reviewed with more seismic extending westward for possible tracing of a Jurassic pinch out. Tentatively a second well location is 4.2 km southwest of Well No. 1, subject to confirmation by more seismic.

Central Indus/Sulaiman basin

Domanda to Savi Ragha

Along the first exposed line of folding in the northern part of the Sulaiman basin (Fig. 2 [159,927 bytes]) lie four large surface structures: Nar Ghar (51.5 by 12.9 km), Mughal Kot (79 by 17.6 km), Darazinda (22 by 11 km), and Domanda (35 by 4.4 km).

The area is characterized by considerable folding and steep angle reverse faulting as a result of late Tertiary Himalayan orogeny. Seeps of light oil occur in the Cretaceous Mughalkot formation along its contact with the overlying Late Cretaceous Pab sandstone formation in Toi Nallah just west of the Mughal Kot fort.

PPL-POL drilled the Domanda-1 well in 1959-60 on the surface structure Domanda eroded down to Eocene Ghazij shale, which remained in the same formation from surface to 3,408 m with minor gas shows.

After more than 3 decades, British Gas in 1994 drilled the Savi Ragha-1 well in this area to TD 2,977 m in Mughalkot. This well, south of the Domanda surface anticline and east of the Mughal Kot surface anticline, flowed at maximum rates of 16 MMcfd of gas and 700 b/d of condensate from Early Eocene Ranikot sandstone. The gas has a high BTU value and contains only 1% carbon dioxide.

Nandpur field

On the Punjab platform flank of the Sulaiman basin, Amoco drilled the Sarai Sidhu-1 well in 1973-74 on a seismically delineated prospect to 3,280 m in Infracambrian. It found low quality gas (55-70% nitrogen and CO ₂) in Cretaceous and brackish water in Paleocene and Early Eocene.

OGDC in 1984 drilled the Nandpur-1 well 1 km southwest of Sarai Sidhu-1. It went to TD 2,110 m and discovered gas in Jurassic and Cretaceous sediments.

Recommendations

Prospects to be drilled again in the Sulaiman basin include Zindapir and Kotrum.

Zindapir prospect

This prospect is located on Landsat Image 162/39 on the western folded flank of the Sulaiman foredeep.

Like Sui and Uch, it is a recent feature as indicated by upturned gravel along their flanks. These are carried on thrusts whose leading edges are buried beneath alluvium of the Indus plain, as suggested by the fold axes being parallel to but in front of the exposed thrusts.² Production to date is limited to the fringes of the thrust belt in areas like Dhodak, Sui, and Uch.

Source rock maturity may be at peak oil generation around the thrust margins, as indicated by seeps and fields. The prospect was covered by 60 km of seismic in 1980. Closure is assumed to be 22 by 4.32 km, with 103 m of amplitude at Cretaceous level and 44 sq km by 500 m at the top Early Jurassic level, respectively.

East of Zindapir may lie an underthrust structure, Vidor, presently covered by Neogene Upper Siwalik sediments.

OGDC drilled the Zindapir-1 well on the bases of geological analysis and inadequate seismic, and it was dry. Quantitative structural mapping of satellite imagery 162/39 and 163/39 is recommended to locate more seismic profiles to locate the next drilling site. A strategy similar to that used at Savi Ragha can help.

Kotrum prospect

This is a seismically delineated structure located on Landsat Image 162/39 in the Sub-Sulaiman foredeep en echelon to the Sakhi Sarwar structure. Amoco drilled the latter in 1976 to 4,581 m in Paleocene. At the Eocene Habib Rahi level it covered 30 by 7.5 km with 350 m vertical closure. At Pab sandstone level it had an area of 27.5 by 5 km with 200 m vertical closure.

OGDC drilled a well in 1979 to 4,797 m, penetrating about 373 m of Pab sandstone, without encountering any oil/gas horizon and with many drilling problems.

It is recommended that a second well be drilled north of the first well, subject to confirmation by a few more seismic profiles.

Lower Indus/Kirthar basin

Badin trough

The wells Digh (1956), Talhar (1957), Mirpur Batoro (1957), Nabisar (1958), and Badin (1961), drilled by SVOC were all located on the basis of single fold seismic (Fig. 3). The seismic did not provide a reliable pre-Tertiary subsurface picture due to the presence of Paleocene basalt blocking energy and giving rise to multiples whose configuration was erroneously assumed to be that of Mesozoic.

The tilted fault blocks created by extensional tectonism during Cretaceous revealed by multiple fold seismic have an entirely different configuration than the Tertiary.

Thus the failure-to-success story in the Badin trough started with the failure of Digh-1 and ended with the success of oil in the Union Texas Khaskeli-1 in 1981, with the Early Cretaceous fault block play opening up this major oil/gas producing province on the Sind platform. Even with multifold seismic, success came with the fifth well drilled.

Khairpur to Kandra

Burmah Oil had drilled the Khairpur-1 well in 1925 on the Khairpur dome that proved to be a failure due to its off-crest location. PPL drilled Khairpur-2 in 1956 to 3,035 m (minus 2,976 m) into Paleocene, with only a slight inflow of gas from Late Eocene Kirthar.

Premier Oil drilled Kandra-1 in 1990 on the eastern dome of the NW-SE trending fault on the same faulted anticline 6 km east of Khairpur-2, which tested 7.44 MMcfd of gas from Early Eocene Sui Main limestone.

Badhra to Bhit

Hunt-PPL drilled Badhra-1 on a convergence/wrench-related Badhra surface anticline on the folded edge of the Kirthar foredeep in 1958 to 1,332.6 m (minus 413 m) into Ranikot. This well experienced severe lost circulating.

Just north of Badhra lies the Bhit structure, a gentle, slightly asymmetric surface anticline bounded by reverse faults on both flanks, eroded down to Middle/Late Eocene limestone.

Lasmo drilled Bhit-1, TD 1,839 m, which it junked, and had gas shows, followed by Bhit-2, 200 m west, to 2,067 m in 1996-97. The latter resulted in a gas discovery in Pab sandstone. Net gas pay is 150 m, flow rate 17.7 MMcfd, and gas composition 80% hydrocarbons, 20% nitrogen, minor CO₂, and no H2_S.

Recommendations

Prospects that can be candidates for a second probe in the Kirthar basin include the Benir structure and Lakhra-Sann arch.

Benir prospect

This is a structure located in the Karachi trough on Landsat Image 163/42 covering 14 by 4 km at surface with 400 m of vertical closure.

OGDC drilled the Benir-1 well to 2,347 m (minus 1,910 m) during 1967-68. It penetrated Upper Ranikot in unfavorable structural conditions in an overthrust block. It never reached Lower Ranikot, productive at Kothar and Hundi gas fields, or Pab sandstone, gas reservoir in the Bhit-2 well.

A geological reconstruction suggests that in the underthrust, a major part of the favorable Lower Ranikot to Pab interval forms a fairly large and relatively simple fold. The second well should definitely be located on the basis of seismic.

Lakhra-Sann prospect

These adjacent highs are located on the western folded flank of the Sind platform on Landsat Image 163/42 and are similar to the Jacobabad high but of longer evolution.

The Lakhra surface high is 68 by 16 km with 914 m of vertical closure in several separated closures of restricted size, gentle, symmetric, with no major faults, with one normal longitudinal fault.

Five wells were drilled, one by Burmah Oil in 1948 and four by Hunt in 1957-58, on the basis of single fold seismic of very poor quality.

Hunt's Lakhra-1 was assumed to be located on the highest closure, with BOC's well on the west flank. Lakhra-2, 3, and 4 were drilled to shallow depths to evaluate the gas shows in Hunt's Lakhra-1. That well had an oil show in Early Cretaceous Lower Goru, the producing reservoir in Badin trough oil fields. These wells failed to test Mesozoic traps and at best were Tertiary tests.

According to OGDC's seismic surveys of 1980-81 and 1983, subsurface areal dimensions of the Lakhra high at base Tertiary are 5 by 3 km with 85 m vertical closure, and areal dimensions at top Lower Goru are 17 by 3 km with 80 m vertical closure.

The Sann subsurface is 13 by 8 km with 80 m vertical closure at top Lower Goru level, and its subsurface crest is about 120 m lower than Lakhra's at this level. However, these results may be unreliable because OGDC's field corrections were reported to be inaccurate.

Fresh seismic will be required to locate another well. The proven excellent source rock Sembar may be gas prone in this area, but shales of Lower Goru are in the light oil/condensate window. Pab sandstone, which produced salt water and gas in Lakhra-4, and Jurassic limestone of secondary porosity due to vugs and fractures, which produced strong formation water flow in BOC's Lakhra well are potential reservoir objectives.³

Conclusion

Many structures in Pakistan have been discarded and shelved on the basis of a single unsuccessful hole.

A comprehensive analysis is required to determine the anatomy of failure; the faulty or inadequate seismic or the drilling foul-ups have to be ruled out before a dry hole is accepted as a prospect's epitaph.

The failure-to-success stories provide enough encouragement to probe such prospects again.

References

1. Quadri, V.N., and Quadri, S.M.G.J., Anatomy of success in oil and gas exploration in Pakistan, 1915-94, OGJ, May 13, 1996, pp. 92-97
2. Prost, G.L., Recognizing thrust faults on remote sensing images, World Oil, September 1990, pp. 39-45.
3. Quadri, V.N., and Shuaib, S.M., Hydrocarbon prospects of southern Indus basin, Pakistan, AAPG Bull., Vol. 70, No. 6, 1988, pp. 730-747.

The Authors