EXPLORATION Reefal petroleum prospects possible in Pakistan

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

Fig. 2: Pakistan Seimentary Basins [51,830 bytes]
Fig. 3: Indus Basin Generalized Stratigraphy [57,180 bytes]
Fig. 5: Surface Samples [201,553 bytes]
Fig. 6: Surface Samples [205,352 bytes]

Carbonate buildups including reefs and banks have proven to be prolific hydrocarbon reservoirs in the U.S., Canada, Mexico, North Africa, Southeast Asia, and the Middle East.

Seismic interpretation, particularly of high quality marine data, and geological analyses, including petrographic studies of selected formations, reveal a broad spectrum of possible reefal prospects in formations of different ages in Pakistan.

However, the region with good seismic that provides good recognition of possible reefal buildups remains to date devoid of well confirmation. These prospects, if scientifically explored and exploited, seem to the authors capable of turning Pakistan into a net oil exporter.

Oil & Gas Development Corp. of Pakistan formed the technical services department in 1976 for a systematic scientific review of all exploration data and basin analysis. In 1985 this department was renamed offshore department, with responsibility for all offshore exploration. This article is based on the department's work in collaboration with geoscientists from Russia, the U.S., Norway, and Canada covering the area shown in Fig. 1 [43,730 bytes].

Four major types of carbonate buildups, easily recognized from seismic interpretation,1-3 include:

Barrier buildups that are linear with relatively deep water on both sides during deposition;
Pinnacle buildups that are roughly equidimensional and were surrounded by deep water during deposition;
Shelf margin buildups that are linear with deep water on one side and shallow water on the other; and
Patch buildups that form in shallow water either in close proximity to shelf margins, or over broad shallow seas.1

Regional setting

Pakistan's four onshore basins-Upper Indus (Potwar), Central Indus (Sulai-man), Lower Indus (Kirthar), and Baluchistan-and two offshore basins-Indus and Makran-belong to the southern domain of Stocklin's4 main structural zones between Iran and Central Asia5 (Figs 2, 3).

The structural elements of Pakistan are the culmination of 225 million years of geologic history. The Kirthar-Sulaiman mountain belt, the northwestern margin of the Indo-Pak subcontinent, was an Atlantic type margin from late Paleozoic until Cretaceous, when it became a plate boundary.

Thus there was a rifting phase from Triassic to early Jurassic and a later drifting stage, probably beginning in early Cretaceous involving the subsidence of the margin, followed by a Tertiary age compression. The intra-Cretaceous extension faults and associated folds responsible for petroleum traps are products of cover rock shearing above crustal tear zones in the Badin trough of the Lower Indus basin. These are probably linked through the Kutch and Cambay rifts to the Bombay rift, site of giant Bombay High offshore oil field, and other crustal extension zones to the south.6

The first recorded marine transgression occurred on the western margin of Indo-Pakistan during early Jurassic, when the Kutch basin opened as a result of counter-clockwise rotation.

The Chiltan limestone of late Jurassic age is the first marine unit in the Lower Indus basin. The Pak-Indian plate's drift history shows it to have remained between 30 S. and 30 N. Lat. from middle to late Cretaceous 80 million years ago until Mio- cene 20 million years ago.

In late Cretaceous the Pak-Indian plate crossed a mantle hot spot at 7 S. Lat. and 72 E. Long., providing an extrusive volcanic source for the widespread Deccan trap basalt.7

During this period the northwestern continental margin experienced a regional uplift and regression as evidenced by the nature of sediments in the Kutch and Cambay basins. Mid-Jurassic carbonate (reefal) and mid-Miocene to Paleocene carbonate sediments are objectives for exploration in the offshore extension of the Kutch basin,7 which covers at least the lower half of the Indus offshore basin of Pakistan.

During Phanerozoic, while there were times when no reefs formed, these were very short like Middle and Upper Cambrian.

During most of Phanerozoic there were structures that some workers call reefs, some mounds, and some banks. These reef mounds or rather incomplete reefs occur in three preferred habitats: downslope on gently dipping platform margins; in deep basins (island arcs?); and spread widely in reef lagoons or wide shelf areas.

During the TA2 supercycle between the Intra Thanetian/Late Paleocene 58.5 million years ago and Base Lutetian/early Eocene 49.5 million years ago sequence boundaries8 are noted wide transgressions and a sharp decrease in the supply of clastic material into marine basins surrounding the Pak-Indian shield.

Accumulation of carbonate and pelagic shale predominated, and favorable conditions were created for development of organic buildups. TA2 is a second order global cycle of relative change of sea level, incorporating an assemblage of rocks with distinctive stratigraphic and structural attributes. The same supercycle witnessed the formation of Australian reef systems and the Laccadive Islands along India's western margin.

Globally the distribution of Cenozoic reefs in time and space is a direct result of changes in oceanic circulation patterns brought about by the progressive blockage of the Tethyan seaway by plate interaction.

Although there are reefs of Paleocene age, most of the corals in them are survivors from the Cretaceous.9 Within South Asia in general and Pakistan in particular, Paleogene reefs are mainly algae-foraminiferal and to a lesser extent formed by hermatypic coral genera.

Geological analyses

In the Indus basin, TA2 limestones are gas reservoirs at Sui, Uch, Zin, Kandhkot, Khairpur, Jacobabad, Maza- rani, Loti, and Sari gas fields (Fig. 1 [43,730 bytes]); they are known as Sui Main limestone, Upper Ranikot, and Dunghan and overlain by Ghazij shale, which in turn is overlain by Kirthar limestone.5

The first suggestion that the TA2 carbonate in Sui may be an example of a large shallow water algae-foraminiferal carbonate bank was made in 1959.10 From the petrographic analysis of cores of Sui Well 1 carried out by OGDC, the Sui Main limestone and limestone of Upper Ranikot is highly fossiliferous alveolina limestone, partly bioclastic, containing shell fragments, dolomitic, comprised of carbonates with mosaic and cavernous textures.

An indirect confirmation of the reefogenous nature of these limestones is their inconsistent but good reservoir porosities of 28-30%. The zone of contact of the light colored shallow-shelf chalky algae-foraminiferal limestone with the dark grey to black argillaceous bituminous limestone is characterized by maximum thickness of the lower carbonate TA2 strata, and the thinning of overlying clayey strata at Sui and Mazarani gas fields (Fig. 4 [119,813 bytes]).

The Sui Main limestone is a thick, shallow water platform carbonate with a well developed shelf margin with pronounced backsteps to the north and west. Detailed correlations in the Loti field area about 35 km north-northwest of Sui suggest seaward side of this margin may be characterized by pinnacle reefs.6

Petrographic analyses of selected formations carried out at the H.N. Fisk Laboratory of Sedimentology, University of Cincinnati11 indicate that in the Lower Indus basin reefal buildups of Cretaceous and Jurassic age can also form petroleum prospects. Reproduced in six elements as Figs. 5 and 6 are examples of petrographic analyses of some samples from Jurassic to Miocene, collected from surface sections at Bara Nai, Samulji, and Gaj River (Fig. 1 [43,730 bytes]).

During a field trip12 in May 1987, OGDC-Offshore Department and Canadian geologists noted an excellent biostromal mound unit with large 0.5-1 m "brain" type high energy coral mounds of early Miocene age at Pir Mango section (Fig. 1 [43,730 bytes]).

A Paleocene unit south of Kalrhi Lake appears to be a near front reef environment with shell and algal/coral laminations. It is inferred that shelf margin reefal banks can occur in Upper Paleocene, Eocene, and Oligocene formations while potential for patch reefs exists in Upper Cretaceous, Upper Paleocene, Lower Eocene, and Lower Miocene formations.

Seismic analysis

Seismic sections collected onshore sadly lack the refined statics required for recognition of carbonate buildups. Attempts using "dim spots" (onlap and seismic facies change), isochron maps and interval velocities, although not particularly reliable, have allowed a demarcation of a shelf edge carbonate bank similar to Sui and Mazarani in the Lower Indus basin.5

Possible reefal complexes (Figs. 7 [94,269 bytes], 8 [169,228 bytes], 9 [115,794 bytes]) have been observed on seismic sections of good quality data collected in the Indus offshore area (Fig. 1 [43,730 bytes]). All direct criteria including boundary outline with onlap of overlying reflections and seismic facies changes as well as indirect criteria of drape, velocity anomalies, spurious events, and basin architecture,3 indicate reefal prospects of late Cretaceous to Miocene ages along trends at the edges of Cretaceous, Paleocene, and Oligocene carbonate platforms.

The megasequence boundaries of TA2 and TB3 supercycles have been identified on the basis of seismic stratigraphic techniques.13 However, only two wells, one for younger and the other for older, could provide confirmatory seismic control.

The seismic stratigraphic sequence of Haq8 and Vail et al.,13 TA1 is equivalent to the geochronologic unit early Paleocene, TA2 to late Paleocene and early Eocene, TA3 to Middle Eocene, TA4 to late Eocene and early Oligocene, TB1 to late Oligocene and early Miocene, and TB2 to early and Middle Miocene, respectively.

It is interesting to note that one of these wells, Karachi South A-1, has been drilled between older and younger apparent reefal buildups, missing them both.

The extra thick Oligocene and Miocene limestone development of 2,408 m in this well made its biostratigraphic analysis to place it in a "different" facies distribution during the limestone platform buildup. The limestone occurrence throughout this interval has been described as a "lime mud buildup rather than a reefal development sensu stricto." Another encouraging factor was the presence of several good oil source rocks in the section between Upper Cretaceous and Oligocene in this well.

The term "reefal buildup" applied to these anomalies remains speculative as none of these has yet been confirmed by the drill. Further, no sophisticated studies involving color displays of "instantaneous phase," "image processing with amplitude codification," etc., have been carried out yet. The "karstified? areas" at the top of some of these features have likewise not been ascertained by amplitude analysis as a function of offset.3 Still, the size of these features-up to 220 sq km-accessibility to the drill, and presence of good source rocks nearby make them very attractive prospects.

Average maximum thickness as inferred from isochron maps for the buildups of TA2 interval range between 600-750 m. This presents an interesting as well as encouraging analogy with the thickness of 710 m drilled at Sui gas field and 780 m drilled in the Mazarani well of the lower carbonate, Upper Paleocene to Lower Eocene carbonate strata.

The PakCan-1 well drilled in 1985-86 flowed 3.7 MMcfd of gas and some condensate from Miocene, while an Indian well KD 1 drilled almost on the Pak-Indian marine border is reported to have tested 173 b/d of oil from an Eocene formation, thus confirming generation and accumulation of hydrocarbons in the Indus basin off Pakistan.

Conclusion

The geological and seismic data obtained in Pakistan are ample enough to provide factual material to locate reefal petroleum prospects.

In areas of good seismic in the Indus offshore the presence of very interesting trends is indicated, supported by petrographic analysis of rocks from exposed and drilled sections. These prospects deserve a concentrated exploration and exploitation endeavor.

References

1. Bubb, J.N., and Hatledid, W.G., Seismic stratigraphy and global changes of sea level, Part 10; Seismic recognition of carbonate buildups, in C.E. Payton, ed., Seismic stratigraphy-application to hydrocarbon exploration, AAPG Memoir 26, pp. 185-204.

2. Wilson, J.L., Carbonate facies in geologic history, Springer-Verlag, New York, 1975, 471 p.

3. Fontana, J.M., et al., Seismic interpretation of carbonate depositional environments, AAPG Bull., Vol. 71, No. 3, 1987, pp. 281-297.

4. Stocklin, J., Main structural zones recognized in the Alpine ranges of Iran, Afghanistan, Pakistan, and Pamir-Karokorum West Himalayan region, UNDP, 1976.

5. Quadri, V.N., and Shuaib, S.M., Hydrocarbon prospects of southern Indus basin, Pakistan, AAPG Bull., Vol. 70, No. 6, 19886, pp. 730-747.

6. Kemal, A., Balkwill, H.R., and Stoakes, F.A., Indus basin hydrocarbon plays, Interenational Petroleum Seminar, Islamabad, 1991.

7. Biswas, S.K., Western rift basins of India and hydrocarbon prospects, OGJ, Apr. 19, 1982, pp. 224-232.

8. Haq, B.U., Hardenbol, J., and Vail, P.R., Chronology of fluctuating sea levels since the Triassic, Science, reprint series, Vol. 235, Mar. 6, 1987, pp. 1,156-67.

9. James, N.P., Reef, in Scholle et al., eds., Carbonate depositional environments, AAPG Memoir 33, 1983, pp. 346-462.

10. Tainsh, H.R., Stringer, K.V., and Azad, J., Major gas fields of western Pakistan, AAPG, Vol. 43, No. 11, 1959, pp. 2,675-2,700.

11. Pryor, W.A., Kirthar basin petrographic study, H.N. Fisk Laboratory of Sedimentology, University of Cincinnati, OGDC-TSD, Pakistan, 1984.

12. Boleantu and Shuaib, personal communication.

13. Vail, P.R., et al., Seismic stratigraphy and global changes of sea level, in Payton, C.E., ed., Seismic stratigraphy-applications to hydrocarbon exploration, AAPG Memoir 26, 1977, pp. 49-212.

Bibliography

Movshovich, E.B., Zones of oil and gas accumulation confined to buried reefs, V/O Technoexport, OGDC-TSD, Pakistan, 1977, appendix A. 12, pp. 1-27.

Powell, C. McA., A speculative tectonic history of Pakistan and surroundings, some constraints from the Indian Ocean, in Geodynamics of Pakistan, Farah, Abul, and DeJong, Kees A., eds., Geol. Surv. of Pak., Quetta, 1979, pp. 5-24.

The Authors

Mrs. V.N. Quadri is the first lady petroleum explorationist of Pakistan. She started as an exploration geophysicist soon after Oil & Natural Gas Development Corp.'s birth and rose to become manager technical services and offshore departments and acting general manager exploration, contributing to ONGC's growth as Pakistan's leading petroleum organization. She has represented Pakistan in offshore industry workshops for developing countries sponsored by Unesco-IOC-Iocindio, Norad-NPD (Norway), and Oceanographic Commission (China). She has an MS in geophysical prospecting from the University of Michigan.

S.M.G. Jeelani Quadri has been regional manager of the public sector of OGDC. He was also exploration advisor to Marathon Pakistan Petroleum and Pakistan Shell Oil Co. After retirement from active exploration, he served in the oilfield equipment and services industry for about 10 years. He has a BS in geological and physical sciences from Queen's University and an MS in geological sciences from the University of Toronto.