Although the Caspian Sea region is one of the oldest oil-producing provinces on earth, it still has striking promise for future discoveries.

The Caspian Sea region contains a wealth of oil and gas deposits, thanks to nature's combination of source rock, reservoirs, traps and seals. Of the many sedimentary basins scattered throughout the area, three immense provinces hold exceptional potential for hydrocarbons: the PriCaspian, South Caspian and Amu-Darya basins.
Perhaps the highest profile basin in the region is the PriCaspian. Most of this 500,000sq km basin lies in Kazakhstan, while a much smaller portion falls in Russia. This basin contains the bulk of the undiscovered oil in the region. The PriCaspian Basin is a classic salt province that has captured the imagination of the world's oil community. People are intrigued with its potential because the giant onshore fields Tengiz and Karachaganak both reside within its boundaries.
Until about 30 years ago, exploration focused on salt-dome structures in the Emba area. These prospects are akin to the suprasalt Gulf of Mexico plays where fields are developed around salt domes and other salt-controlled features, as well as fault-controlled structural closures and four-way dip closures. Most of the suprasalt reservoirs are sandstones of Middle Jurassic and Lower Cretaceous age.
By the early 1970s, however, subsalt plays became the primary targets. Explorationists realized that extremely large accumulations could occur in the PriCaspian beneath its thick layer of Permian salt, somewhat analogous to the subsalt play in the Gulf of Mexico. The presalt Paleozoic play is a deep and complex one; the reservoirs are found at great depths and the formations below the salt are overpressured and contain significant amounts of H2S.
Nonetheless, drilling in the 1970s yielded discoveries at Astrakhan, Zhanazhol and Karachaganak. The latter, which lies on Kazakhstan's northern border with Russia, is estimated to contain recoverable reserves of 2.4 billion bbl of oil and condensate, plus 16 Tcf of natural gas.
The PriCaspian's other supergiant, the Tengiz field, was discovered in 1979. Estimates peg Tengiz's original oil in place at 26 billion bbl, along with 67 Tcf of gas, and recoverable oil reserves of up to 9 billion bbl.
The Tengiz reservoir lies at depths between 13,000 and 19,000ft, its reservoir pressure is 12,000 psi, its reservoir temperature is 230° F, and its gas contains 16% H2S.
The possibility that more fields like Tengiz exist has propelled many foreign firms into the PriCaspian subsalt play. Below the thick Kungurian salt deposit, Middle and Upper Carboniferous reefs developed on a belt of uplifts that ringed the basin margin. Periods of exposure enhanced the reservoir qualities in these reefs, yielding tremendous porosities and permeabilities. Some sandstones in the Devonian and Lower Carboniferous also serve as subsalt reservoirs, as well as certain Lower Permian carbonates.

South Caspian
The geology of the South Caspian Basin, another enormous sedimentary basin that covers the southern portion of the Caspian Sea and adjacent onshore areas, is quite different from that of the PriCaspian.
The 400,000sq km South Caspian is Azerbaijan's key petroleum province, and also extends into northern Iran and western Turkmenistan.
It is characterized by very thick sequences of very young sediments. The rocks are mainly of Mesozoic and Cenozoic age; most of the hydrocarbon reservoirs are Upper Pliocene sandstones. Formation pressures are quite high, as evidenced by abundant mud volcanoes.
Structurally, the basin is quite simple, with elongated anticlines locally complicated by mud volcanoes, shale diapirs or faults. Hydrocarbons are trapped in many of these anticlines, and local seals are often interbedded with the productive formations. On some structures, as many as 20 productive intervals have been found. The lack of regional seals and the size of the structures limits the amount of oil that can be contained in each field; while the targets are quite attractive, accumulations the size of Tengiz are unlikely in this basin.
The onshore sectors of the South Caspian have been heavily explored throughout the oil industry's long tenure in the region, but that's not the case with the Caspian Sea portion. While dozens of large structures have been drilled on the Azerbaijani continental shelf area in waters less than 815ft (250m) deep, many undrilled features still exist in shallow as well as in deeper waters.
The gas-prone Amu-Darya Basin, largely in eastern Turkmenistan and western Uzbekistan, also offers potential for significant accumulations. This 360,000sq km basin has an Upper Jurassic evaporite layer that covers about 300,000sq km, allowing for exploration plays within that area in both suprasalt and subsalt sequences.
The largest field found to date is the giant Dauletabad-Donmez in southern Turkmenistan, containing recoverable reserves of 60 Tcf of gas. Amu-Darya's source rocks are very mature, and oil is found in only minor amounts. Traps are generally structural in the basin, and most established production is from Cretaceous sandstones. Above the salt section, Cretaceous structures are the main targets. In the area of the evaporite basin, the subsalt Upper Jurassic carbonates offer the best potential for future major finds.
According to a 1998 energy study sponsored by the Center for International Political Economy and the James A. Baker Institute for Public Policy, the Caspian nations of Azerbaijan, Kazakhstan, Turkmenistan and Uzbekistan collectively contain estimated possible reserves ranging between 55 billion and 137 billion bbl of oil and between 303 Tcf and 664 Tcf of gas.
On the conservative side, this potential puts the Caspian region on a par with other major hydrocarbon supply areas of the world such as the North Sea and the Far East.
Despite the current reticence of some operators to fully embrace the region's potential, this is only due to political and financial rather than physical challenges. Most know that eventually the Caspian will fulfil its potential.