The Sui gas field in northeast Baluchistan was discovered by a test well completed by Pakistan Petroleum Limited in 1952. After the drilling of five more wells on the large exposed dome, the reserves of recoverable natural gas were estimated to be 4-5 million million cubic feet. The gas occurs in porous Eocene limestone and contains approximately 90 per cent methane, 4.5 per cent carbon dioxide, 3.5 per cent nitrogen, with some hydrogen sulphide. In 1954-1955 a 16-inch pipeline was constructed from Sui to Karachi with a purification plant at Sui; sales of purified gas began in September, 1955, and at the end of 1958 were 35 million cubic feet per day.

The proof of large gas reserves encouraged further drilling in the region and subsequently five more big domes have been tested--Zin on the northwest, Uch and Jacobabad on the west, Khairpur on the south, all by Pakistan Petroleum Limited, and Mari on the southeast by the Pak-Stanvac Petroleum Project.

Each of the tested structures has proved gas accumulations in Eocene limestones, but not all in the same formation. The pools are nearly all large but the composition of the raw gas has shown remarkable differences. The methane content varies from 90 per cent at Sui to 12 per cent at Khairpur, and the carbon dioxide content from 6 per cent or less at Mari and Sui to 70 per cent at Khairpur.

The total volume of recoverable methane in the region is estimated to be over 10 million million cubic feet, but none of the wells drilled so far has given evidence of liquid hydrocarbons in appreciable quantities. The gas overlies saline water. Minor shows of oil have been observed and flowing gas is accompanied by a little "condensate"; bitumen is widely distributed in the Eocene rocks.

Because of the widespread distribution of shows of hydrocarbon and other gases it is not possible to reach definite conclusions regarding the source rocks and migration paths, but it is suggested that the Eocene and Cretaceous shales have been the source of much methane, and the carbon dioxide and some nitrogen may have come from Cretaceous and Jurassic limestones. Much of the nitrogen perhaps with carbon dioxide may be the result of deep-seated igneous intrusions.

The migration of the gases is studied against the background of the complex geological history of the region.