New Jurassic plays possible in Pakistan

Viqar-un-Nisa Quadri, S.M.G. Jeelani Quadri
Petroleum Consultants
Cincinnati

Rocks of Jurassic age are well represented in Pakistan. Jurassic rocks outcrop in a discontinuous north-trending belt extending from the southern Pab Range near Karachi to Quetta, where the belt makes a sharp turn trending eastward to Loralai, and then northeast to Fort Sandeman and the northern Sulaiman Range, through Waziristan to the Salt Range ( Fig. 1 [137,562 bytes]).

With respect to Jurassic stratigraphy (Fig. 2 [57,117 bytes]), the U.S. Geological Survey has referred to Chiltan as Sulaiman, and Takatu, the shelf equivalent of Spingwar.

As a result of the Middle Jurassic breakup of Gondwana, a wide oceanic seaway developed that was dominated by carbonate platform sedimentation. The deposition of thick shallow water platform carbonates of the Chiltan (Sulaiman) formation spanned a significant portion of the Jurassic, as is shown by its total attained thickness of more than 900 m. In marginal areas these sediments are known as Samana Suk.

The Shinawari formation is comprised of lagoon and bayfill sediments, while the clastic shoreline deposits fringe the latter. The Datta formation of Early Jurassic age is of continental origin and is widely developed in the Western Salt Range/Potwar Plateau, Kohat and Khisor Range. It is comprised of variegated sandstone, shale, siltstone, and mudstone, with occasional layers of ferruginous glass sand and fireclay.

In the Lower Indus basin,¹ the Shirinab formation had a thickness of 1,112 m in the Nabisar well. Shirinab is comprised of continental facies of consolidated sandstone with interbedded shale and siltstone with limestone interbeds in the upper part. The Chiltan formation was drilled in wells including Nabisar, Digh, Patar, and Talhar. A thickness of 420 m in the Nabisar well comprised of Chiltan shelf limestone with pisolitic to oolitic texture, increased to more than 983 m in the Digh well (Fig. 3 [186,234 bytes]).

In the Kutch basin of India the Middle Jurassic is comprised of both clastic and carbonate facies followed by mainly sandstone and shale of Upper Jurassic. The entire sequence is characterized by a major transgressive cycle followed by a regression. Carbonate facies of the Jhurio formation comprised of lagoonal mudstone/wackestone grade into biostromal rocks. It is reasonable to expect that the biostromes may develop into biohermal buildups farther down the basin.

In the eastern part of the Central Indus basin² Jurassic was encountered in the Bahawalpur East, Karampur, Tola, Sarai Sidhu, Budhuana, Kamiab, Nandpur, and Panjpir wells. It is comprised of sandstone, shale, limestone, and dolomite with a maximum thickness of 321 m. In the central part of the basin it is comprised of limestone interbedded with with shale, as encountered in the Khairpur, Jacobabad, Jhatpat, Giandari, Tadri, and Jandran wells with the maximum thickness of 2,112 m drilled in the Jhatpat well (Fig. 4 [183,995 bytes]).

Source potential

Overall source rock potential is fair to poor. In the Upper Indus basin, some Datta beds represent swampy environment as indicated by the presence of organic matter, rich black clays, and coal disseminations. These beds and part of the Shinawari formation are believed to have fair source rock potential.

In the Central and Lower Indus basin, the Loralai and Anjira members are dark colored, but source rock potential is not confirmed. At some places the Chiltan limestone is also organic rich.

In the neighboring Kutch basin, the gray and khaki shales of Jhurio and the older Kaladonger formation may form source rocks in the deeper basinal parts of the west. The Upper Jurassic, dark gray to black facies of the Jhuran formation contains sufficient amounts of organic matter required for hydrocarbon generation. Maturation will obviously be attained in the deeper part of the basin due to increased overburden.

Reservoir possibilities

In the Upper Indus basin, the Datta sandstone is producing in Toot, Meyal, and Dhulian oil fields, with thick exposures occurring at Sheikh Budin Hills in the Marwat Range. These delta front facies bear very good reservoir characteristics in thick sandstone beds that are clean and quartzose.

Average porosity is 15-17%. Some high energy facies of Samana Suk representing nearshore environment depict good reservoir potential, especially if fractured. These calcareous sandstones are gas prone in Nandpur and Panjpir gas fields of the Central Indus basin.

In the Lower Indus basin, Chiltan limestone has negligible intergranular porosity but may have secondary porosity due to fracture and vugs. In the Lakhra BOC well, strong formation water flow was encountered. In the Nabisar well, drillstem test No. 1 produced strong formation water. The Lower Jurassic contains many porous, sandy beds but is usually very deep. It has been reached only in the Nabisar and Marvi(?) wells on the platform and the Jhapat well on the Jacobabad high. In the neighboring Kutch basin, the bedded and massive sandstones of the upper member and Ketesar member of the Juran formation overlying the Upper Jurassic shale are good and readily available reservoirs.

Facies change in Jurassic

The key to the stratigraphic relationship of different Jurassic formations lies in the recognition of shelf and basinal facies. ⁴

In the Jacobabad well, the Jurassic carbonate complex is made up of shallow shelf facies, whereas in the outcrops of the Kirthar Range and Mari Bugti area, it is comprised of relatively deeper water dark facies.

These facies changes have been recognized as early as 1957 by Carter and 1959 by Woodward, who divided them into shallow water (back reef?) and deep water (fore reef?) facies. Recognition in the field was difficult because of recrystallization of the limestone. However, massive appearance, lack of bedding, and shape proved good field criteria, with microscopic examination revealing the organic nature of the rocks. This abrupt facies change is analogous to that of Jurassic carbonates in the Middle East, where the latter are prolific reservoirs.

Potential new plays

1. The authors discussed the possibility of reefal petroleum prospects in Pakistan, especially in the TA 2 seismic stratigraphic, late Paleocene to early Eocene supercycle, in an earlier article. ⁵ Examples in the world show that such carbonate banks/buildups of different ages have similar strike and lateral, usually parallel position. A Jurassic subsurface carbonate bank may exist along the contact zone between Middle Jurassic shelf and basinal facies (Fig. 5), with associated pinnacles and patch reefs (?) alongside.

An indication that favorable conditions existed for the development of Jurassic reefs are outcrops of Jurassic bioherms within the southwest end of the Sulaiman axial belt at locations 216, 158, and 50 (Fig. 5).

The bioherm at No. 158 Spinghar section is shown in Fig. 6; large bioherms are also visible at No. 216 Dara Manda section and at No. 50 Gurmai section (locations shown on Fig. 5).

Jurassic does not have proven in situ source rocks. However, in the Upper Indus/Potwar basin younger Paleocene oil has been trapped in Jurassic Datta reservoirs. The same possibility exists in the Lower Indus basin of finding Cretaceous Sembar (a proven rich source rock) oil in Middle Jurassic Chiltan reservoirs. A careful analysis of a grid of seismic data currently accessible is required to locate a more accurate fairway, especially where available outcrop data can be projected into the line of seismic sections with time-linear well logs and overlays of paleontological data, paleobathymetry, and other well information.

Later 3D seismic surveys may be required for precise location, as this technique has shown remarkable success in recent years for locating reefal prospects in relatively narrow fairways, e.g., Cisco-Canyon algal mounds in the Permian Basin of southeastern New Mexico and the Cotton Valley reef play in East Texas.

2. Of equal importance is finding stratigraphic pinchouts/plays of Jurassic sandstone facies by carrying out extensive sedimentological and depositional studies of these reservoirs with high quality seismic data. Net sand mapping of individual sand packages within the overall pay zone will be more effective as a tool for field definition or exploration than is mapping of gross sand data. Seismic data can then be used in conjunction with well data to confirm or delineate new sand plays.

References

OGDC-TSD, Oil and gas prospects of southern Indus basin, OGDC archives, Islamabad, 1984.
OGDC-TSD, Oil and gas prospects of central Indus basin, OGDC archives, Islamabad, 1984.
Kadri, I.B., Petroleum geology of Pakistan, published by Pakistan Petroleum Ltd., Karachi, October 1994, pp. 82-92.
Woodward, J.E., Stratigraphy of the Jurassic System, Indus basin, WP 64, SVOC, June 1959.
Quadri, V.N., and Quadri, S.M.G.J., Reefal petroleum prospects possible in Pakistan, OGJ, Mar. 25, 1996, pp. 85-90.

The Authors

Mrs. V.N. Quadri served Oil & Gas Development Corp. from its inception, retiring as acting general manager of exploration after nearly 30 years' service. She carried out several technical studies in collaboration with Russian, Norwegian, Canadian, and U.S. geoscientists and visited China, Norway, Canada, the U.K., and the U.S. as Pakistani delegate to Unido, Norad, and Cida petroleum workshops. She has an MS degree in geophysical prospecting from the University of Michigan as a Fulbright-Smith Mundt scholar. E-mail: [email protected]

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 BSc degree in geological and physical sciences from Queen's University and an MSc degree in geological sciences from the University of Toronto.