Cretaceous of Pakistan, India should hold more promise

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

The main oil and gas fields of Arabia, Iran, Central Asia, Pakistan, and India are concentrated in three regions:

1. The Caspian-Central Asian region of Stocklin's Northern Domain,¹ essentially of continental character but including the Caspian Sea of oceanic crust, a relict of Paleo-Tethys;
2. The Persian Gulf region of the Southern Domain.
3. The Indo-Pakistan region of the Southern Domain.

In the Southern Domain, the main Gondwana fragments, the Arabian and Indo-Pakistan shields fringed by broad stable Mesozoic-Tertiary shelves covered by thick sequences of shallow water sediments, show strikingly similar Paleozoic records but significant divergent post-Triassic sections, with the Indo-Pakistan shelf displaying less stable conditions than the Arabian shelf. ¹

Fig. 1 [202,970 bytes] is a regional tectonic map of the sedimentary basins of Pakistan and neighboring basins of India.

Cretaceous deposition

In the Persian Gulf region, the Cretaceous isopach pattern suggests a new diastrophic development in the history of the synclinorium ( Fig. 2 [111,315 bytes]). This instability is manifested in a renewal of uplift in western areas of the platform and erosion of uplifted Paleozoic-early Mesozoic sandstones to supply sands for the important Cretaceous reservoirs in the supergiant oilfields.

These sandstones form an eastward bulge into the Persian Gulf as far as Cyrus oil field, and in the south beyond Qatar Peninsula. East of the areas of sandstone concentration is a lateral facies change to shale that is the principal elastic lithofacies in the Cretaceous of Iran and Oman.²

The Cretaceous system in Pakistan extends over a large part of the Lower/Middle Indus, Upper Indus (Kohat and Potwar) basins (Fig. 3 [106,882 bytes] and Fig. 4 [117,613 bytes]). The Cretaceous outcrops extend from Waziristan (Kurram formation) in the north to Kutch in the south, and the Jaisalmer arch in the east to Axial Belt and beyond in the Ras Koh range of the Balochistan basin.³

In the Indus basin of Pakistan and the Jaisalmer, Kutch, Saurashtra, Narmada, and Cauvery basins of India, the formations recognized in Cretaceous are tabulated in Fig. 5 [64,562 bytes]. Fig. 6 [234,177 bytes] is a generalized stratigraphic correlation of Pakistan's Sui and Mari wells with the Jaisalmer basin of India. The early Cretaceous sediments are well developed in the Middle and Lower Indus basins.

The clastics of the Sembar and Lower Goru formations represent a complete pattern of sea level rise deltaics and sea level fall turbidites. The transgression in Coniacian time brought an end to the deltaic sedimentation, and interbedded marls and limestones of the Upper Goru formation were deposited.

The overlying Park formation is composed of cream to light grey micritic limestone. The Fort Munro formation represents a thick, shallow water carbonate platform system extending farther basinward than the underlying Parh limestone. The Mughalkot is the basinal equivalent of the Pab.

The thick, interbedded quartzose sandstones and shales of the Pab formation were deposited in the Middle and Lower Indus basins, which represent delta plain/ shallower water environments, while the dark grey limestones and marls of the Kawargarh formation were deposited in the northern Potwar area in deepwater conditions.

On the Indian side, there was a continuous deposition from Jurassic to mid-Eocene in the craton-margin Jaisalmer basin. The extra continental downwarp Kutch basin was formed in Early Jurassic, followed by the divergent margin Cambay basin in early Cretaceous and Narmada in Late Cretaceous.

Thus there is a stratigraphic section from mid-Jurassic to Early Cretaceous in Kutch, Lower Cretaceous in Saurashtra, and Upper Cretaceous in Narmada. The lowest of the three structural stages of the Cambay basin is represented by Mesozoic rocks, but for all practical purposes it is a Tertiary basin with the Deccan Trap representing the basement. Likewise Cretaceous is regarded as missing in the Punjab basin on the northern slope of the Indian platform.

The rift basin of Cauvery extending into Palk Bay offshore, the latter shared by India and Sri Lanka, was born in early Mesozoic. It has a thick sedimentary section ranging in age from early Cretaceous to Tertiary.⁵

Potential prospects

The supergiant oil fields producing from Cretaceous reservoirs in the Persian Gulf region include Burgan (Kuwait), Manifa (Saudi Arabia), Safina Khafji (offshore Saudi Arabia), Rumaila, Zubair (Iraq), Sabria, Murban-Bab (Abu Dhabi), Darius-Kharg (Iran), and Fahud (Oman).

Tremendous quantities of oil have accumulated in this region, and many authors have reflected on the causes of this abundance. Kamen-Kaye2 suggested that the main factors contributing to the great productivity are:

• Tremendous aggregate pore volume of reservoirs, confined in closure, and preserved largely in long anticlines;
• Closer association of original source and reservoir rocks, mainly by intercalation; and
• Preservation of large volumes of original porosity before adverse effects of diagenesis could set in, due to early migration from source to reservoir.

The most significant and successful hydrocarbon discovery in Pakistan has been in the Lower Indus basin, in the Lower Cretaceous (Lower Goru) sandstone reservoirs in rift-related fault block traps, vertically and laterally sealed by Upper Goru transgressive shales, and sourced by fault juxtaposed or underlying Sembar organic shale. The Sembar and Lower Goru contain prodigious oil and gas prone source rock sequences, with realistic total Sembar oil yields in the range of 65-155 million bbl of oil/sq km, which at optimum maturity can source giant fields; with the Talhar shale of Lower Goru capable of generating up to 21 million bbl oil/sq km.

Several medium and small oil/gas/condensate fields are producing from Upper Sands, Middle Sands, and Basal Sands of Lower Goru (the majority from Upper Sands). The Sembar has so far sourced only one giant gas field, Sui, with Tertiary reservoirs; however the Lower Goru and Sembar have not been drilled to date in this very large structure located in the Middle Indus basin.⁶

Other producing Cretaceous reservoirs are the Parh limestone and Mughal Kot at Jandran and the Pab sandstone at Dhodak condensate and gas field and Loti, Pirkoh, and Rodho gas fields.

In the Indian Palk Bay and Madnam (Cauvery basin), light oil and gas have been discovered in Cretaceous horizons which are hosts to both source and reservoir rocks. Oil has been tested in the offshore Kutch basin.

Potential hydrocarbon plays can be envisaged in the Lower Goru basal sands and Sembar reservoirs. Distal Lower Sembar sediments were deposited in a faulted Upper Jurassic (Chiltan) sea bed. Middle Sembar, a prograding beach/barrier system, is characterized by a thick clinoforming wedge of fine-medium grained sands and siltstones. After cessation of Middle Sembar sedimentation, drowning occured, and distal sedimentation continued in the west, of probably transgressive sands reworked from Middle Sembar clinoforms developed against the last clinoform paleoslope.

Sembar differentially eroded before deposition of basal sands of Lower Goru, with the complete section of distal sediments preserved in the west but only the Lower Sembar and part of prograding proximal Middle Sembar preserved in the east. Plays envisaged are:

• Structural highs and convergent faults within distal grabens,
• Stratigraphic entrap-ment within distal grabens,
• Stratigraphic entrap-ment in transgressive sands onlapping the last Middle Sembar clinoform paleo- slope; and
• Slump mounds deposited at the toes of clinoform paleoslopes.

The first play is the least risky, and structural prospects within Chiltan grabens just west of the last Middle Sembar clinoform paleoslope are recommended to be sought.

In the Kutch basin, the Cretaceous sequence of deltaic deposits is mainly sandstone, with minor shale, characterized by rapid sedimentation with marine intertonguing beds, which formed favorable environments for hydrocarbon generation and entrapment. Requisite heat generation due to Deccan Trap volcanism in Late Cretaceous/Early Paleocene greatly aided the maturation process, which further increases Cretaceous prospects of Western Kutch.

In the Saurashtra basin, the Mesozoic thickness below Deccan Trap is 1,000-2,000 m. Fluvial deposits, including channel levees and flood plains as well as alluvial fans, demonstrate the excellent heterogeneity of facies. In the Narmada basin, Cretaceous sediments (Nimar) show excellent reservoir characteristics. The Saurashtra basin is comprised mainly of sandstone and shale, with minor development of limestone and in a few places development of black shale. Fig. 7 [62,137 bytes] is the hydrocarbon prospect map for the Kutch, Saurashtra, and Narmada basins.

Conclusion

Most of the oil and gas fields of the Lower Indus basin are producing from Upper Sands of Lower Goru. The wells in these fields have not been valid tests for Middle and Basal sands, which are shifted with respect to Upper Sands traps. The Lower Goru and Sembar have not been drilled on some large structures. ⁶

It is probable, too, that wells drilled within the prograding Middle Sembar zone have failed due to lack of lateral seal and source immaturity or were not drilled within Top Chiltan (Upper Jurassic) closures. Lower Goru Middle and Basal sands and Sembar prospects hold great promise.

The Pab sandstone and other Upper Cretaceous reservoirs are recommended to be tested in structural traps on the western folded flanks and in stratigraphic entrapment on the eastern flanks of the Sind and Punjab platforms.

Mesozoic rocks hold good potential for hydrocarbon occurrence in the Kutch basin, especially in the offshore. Mesozoic rocks of requisite thickness are likely to be present in the northwestern and southwestern parts of the Saurashtra basin on either side of the basement arch.

These regions warrant continued efforts for Cretaceous exploration, as well as in the western part of the Narmada basin up to estuaries of Narmada⁵ and in the Cauvery basin on both the Indian and Sri Lankan sides.^{5 8}

References

1. Stocklin, J., Main structural zones recognized in the Alpine ranges of Iran, Afghanistan, Pakistan and Pamir-Karakorum, West Himalayan region, UNDP, 1976.
2. Kamen-Kaye, M., Geology and productivity of Persian Gulf synclinorium, AAPG Bull., Vol. 54, No. 12, December 1970, pp. 2,371-94.
3. Kadri, I.B., Petroleum geology of Pakistan, published by Pakistan Petroleum Ltd., Karachi, October 1994, pp.. 93-108.
4. Kemal, A., Balkwill, H.R., and Stoakes, F.A., Indus basin hydrocarbon plays, International Petroleum Seminar, Islamabad, 1991, pp. 85-90.
5. Biswas, S.K., and Deshpande, S.V., Geology and hydrocarbon prospects of Kutch, Saurashtra, and Narmada basins, Petroleum Asia Journal, November 1983, pp. 111-126.
6. 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, p. 92.
7. White, R.N., Depositional history and prospectivity of Sembar formation, OGDC Archives, November 1992.
8. Ceylon Petroleum Corp., Prospecting for hydrocarbons in Sri Lanka, 1986.

The Authors