Indus basin off Pakestan contains few wells

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

• Resevoir objectives, Indus basin off Pakistan [17807 bytes]

The U.N. Conference on the Law of the Sea reaffirmed sovereignty of nations over 22 km (12 nautical miles) of territorial sea, a 370 km (200 nautical miles) Exclusive Economic Zone (EEZ), and rights over the continental shelf to at least 370 km (200 nautical miles) and out to 648 km (350 nautical miles) or beyond under specified conditions. ¹

With a coast line of about 990 km (615 miles), the EEZ for Pakistan extends over an area almost 240,000 sq km (69,960 sq nautical miles), or 40% of the land sedimentary area, in which two distinct geological provinces, the Indus Offshore and the Makran Offshore, have been defined.

The provinces are separated by the Murray ridge. The Indus Offshore basin straddles the continental crust of the extension of Sind platform and Karachi trough and oceanic crust of the Arabian Sea east of Murray ridge-Owen fracture zone plate boundary. The continental shelf between Murray ridge and the Indo-Pak marine border is 120-150 km wide, cut in the southeastern corner by the submarine canyon of the Indus river. The structural pattern is comprised of three parts:

1. A platform, west and south-southwest of Dabbo Creek of 1,304,800 hectares at Cretaceous-Tertiary level.
2. A shelf margin basin that changes from north-west to north-south orientation with southwestern margin bounded by gentle uplift of 1,323,200 hectares.

3. Shale-diapiric regions in the northwestern and western-southwestern parts of approximately 4,356,600 hec- tares in deepwater area.

The whole area is superimposed by the intricate drainage pattern of many mouths of present and proto-Indus, with extension of new and paleocanyons, channels, and deepsea alluvial fans into the Arabian Sea. ² Time stratigraphic records, based on drilled wells, are shown in Fig. 1 [49168 bytes] and Fig. 2 [63970 bytes].

Tectonics, structure

The structural elements of the region are the culmination of 225 million years of geologic history. 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 northwestern margin of the Indo-Pak subcontinent, followed by a Tertiary age compression.

In the onshore Badin trough/Sind platform the intra-Cretaceous extension faults and associated folds responsible for petroleum traps are products of cover rock shearing above crustal tear zones. These are probably linked through the Kutch and Cambay rifts to the Bombay rifts, and other crustal extension zones to the south.³ The Sind platform extensional tectonics created traps of Cretaceous-Sembar and Lower Goru reservoirs in tilted fault blocks, juxtaposed to marine Sembar shale source rocks. These appear to extend into the offshore, providing ground for other favorable overlying plays. Many oil and gas fields are producing in this basin.

The Kutch basin of India is separated from the Lower Indus basin of Pakistan by an east-west running basement high. It is an extra-continental downwarp (small ocean basin). A complete sequence of Mesozoic rocks is exposed in the uplifted part of the basin. Paleocene to Pleistocene unconformably overlie the Deccan Trap basalts. They thicken offshore and are represented mainly in a carbonate facies. According to 1991 statistics, offshore well results have been encouraging (Fig. 3 [59334 bytes]).

The Bombay Offshore basin is the southerly extension of the Cambay basin and is on a broad shelf believed to be the largest continental shelf in the world and site of Bombay High, a giant oil field. Nearly half of India's oil reserves are located in this field. The major producing horizons are of Middle Miocene age. Eocene, Oligo- cene, and early Miocene sequences are also oil/gas bearing in many structures.

Three possible source rocks are:

• Pre-Middle Miocene shales in the Surat depression.

• Shales in the shelf margin basin.

• Shales alternating with limestones of the Bombay platform.

The well correlation section off western India (Fig. 4 [54124 bytes]) shows the correlation of Bombay basin wells with the GKH 1 well, the latter pertinent to Indus offshore petroleum products.

Exploration history

Exploration began near shore with Sun Oil Co. in 1961-62. Conducting seismic surveys were Wintershall AG in 1969-72, Shell International in 1973, Phillips Petroleum (deep water) in 1977, Husky Oil in 1976-78, OGDC-Norad in 1982, OGDC-Petro-Canada in 1986, Oxy in 1988-89, Canterbury Resources/Sceptre (transition zone) in 1990.

The wells drilled off Pakistan are shown in Table 1 [26341 bytes]. Source rocks are shown in Table 2 [15705 bytes].

The geochemical analysis carried out in the Karachi South A-1 well indicate that the shales of the Kirthar, Ghazij, and Dunghan formations have oil prone kerogen and are mature for the generation of oil. The TOC values range from 0.46% to 1.6%. The Ranikot and Mughal Kot formations have a mixed type of kerogen with potential for both oil and gas. The TOC values range from 1.0% to 1.22%. Occasionally they are as high as 2.21%. These formations are mature for condensate and gas.^3-4

In PakCan-1 all samples indicated the presence of Type III kerogen. TOC values indicate fair to good source in the Miocene Gaj formation. Maturity analysis indicated an immature zone down to 2,800 m and early mature to mature from 2,800 m to TD 3,701 m. However, there is no correlation between the results of the DST and the source rock. The condensate and the gases recovered from DST samples are of peak maturity levels. As such these hydrocarbons have migrated from a deeper source.

In Sadaf-1 TOC values indicate very lean source. Throughout the well the TOC values are below 0.5%. Few values are 0.5-0.6%. The kerogen type is III, thermally immature.

Play types

The following play types have been observed on the seismic:

• Seismic anomalies that appear to represent reef development, at edges of Oligocene carbonate platform, Paleocene carbonate platform, and Cretaceous carbonate platform, of Oligocene, Paleocene, and Cretaceous ages, respectively, with superimposed/parallel reefal ? anomalies of younger ages.5

• Rollover anticlines against growth faults in shelf margin basin.

• Wrench fault structures and anticlines.

• Structures associated with distribution and continuity of sandstone reservoirs and channel sands/turbid- ites related to submarine canyon fan systems.

Opportunities

The density of drilling in the Indus Offshore basin has been one well per 6,985 sq km and one well per 2,628 sq km on the platform and shelf margin basin of 0-200 m water depth, placing the basin in partial to frontier category.

Of the 20 wells drilled, three could not reach objective due to technical obstacles, while at least three appear to have been located on inadequate traps.

None of the apparent reefal anomalies have been tested by the drill to date. Geochemical studies have indicated that oil prone Paleocene and Early Miocene source rocks are present.

The basin is surrounded by oil producing basins. Many seismically delineated traps await the drill in the Indus offshore basin.

References

1. Petroleum Economist, Vol. XLIX, No. 9, September 1982, p. 366.

2. Quadri, V.N., Petroleum and the Pakistan offshore, Petroleum Institute of Pakistan, 1985, pp. 1-11.

3. Robertson Research for Husky Oil, Pakistan, A petroleum geochemical evaluation, interval 6,000-11,000 ft of the Husky Oil Karachi South A-1 well, off Pakistan, 1978.

4. Shuaib, S.M., Geology and hydrocarbon potential of offshore Indus basin, AAPG Bull., Vol. 66, 1982, pp. 940-946.

5. Quadri, V.N., and Quadri, S.M.G.J., Reefal petroleum prospects possible in Pakistan, OGJ, Mar. 25, 1996, pp. 85-90.

The Authors