The pays in southwest Uzbekistan are Upper Jurassic carbonates. Bar reefs, barrier reefs, and solitary buildups occur in the upper (Horizons XV and XVa) and middle (Horizon XVb) parts of the section. The older bar reef system extends as a narrow belt from the Kokchi area on the west to Shurtan on the east. It contains reservoirs of the pore and cavity type 60-100 m thick. Porosity is 8-10 percent. The stratigraphically higher barrier reef system forms a belt coincident in places with the bar reef system, extending from Yuzhnyy Urtabulak on the west to Gaurdak on the east. These carbonates have porosity of 14-17 percent.
All the traps can be classified into two main groups: 1) traps on solitary buildups, and 2) traps in bar and barrier reef systems. The first of these is divided into three types: a) sealed on all sides, b) sealed in part by impermeability, and c) faulted. The second has two types: a) formed by junction of highly porous limestone reefs with anticlines, and b) faulted. The phase state of the hydrocarbons reflects the type of trap.
The gases of these retrograde gas-liquid systems are enriched significantly in heavy hydrocarbons (up to 17 percent) in traps that are faulted, independently of which group they are in. As a consequence of this, the condensate content in them is as a rule an order of magnitude higher than in pools of other types, even where overpressured.
A similar situation is found with respect to the physical- chemical properties of the liquid hydrocarbons. Condensates of the fault-associated traps are the methane type with up to 70 percent alkanes. The ratio of aromatic to methane hydro- carbons (Ar/Mn) is from 0.2 to 0.5, and the density does not exceed 0.763. The oils of these pools are also the methane type, light, and low in tar. The Ar/Mn ratio ranges from 0.27 to 0.35. In contrast the condensates in the traps sealed on all sides has much higher Ar/Mn ratios.
Liquid hydrocarbons of pools in solitary buildups sealed in part by impermeability occupy an intermediate position, as do also those of barrier reefs that have been deformed by younger anticlines.
The occurrence of various phase states in different kinds of traps shows that the Upper Jurassic organic buildups (reefs) first entered the oil window, where they became filled with free oil. Subsequent introduction of streaming gas from underlying sediments during recent tectonic activity led to dilution of the oil by this gas. The result is a mosaic of oil-gas accumulations with varying ratios of gas, condensate, and oil.
The most favorable area for new discoveries is the system of barrier reefs and adjacent solitary reefs. This belt extends for more than 250 km. Total favorable area within it exceeds 1000 sq km.
Taken from Korsun and Kushnirov, 1986, one oil-gas map;
digested in Petroleum Geology, Vol. 23, No. 3/4.
Copyright 2003 James Clarke. You are encouraged to print
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