Karachaganak gas-oil field is located on the northern margin of the North Caspian oil-gas basin. From Late Devonian to late Bashkirian time of the Middle Carboniferous the Karachaganak structure was an atoll. Then a system of reefs developed in the region in the Early Permian. Total amplitude of the high is 1.5 km.
The reservoir below the Lower Permian Kungurian salt is a single large carbonate massif of tectonic-sedimentational origin. The core is composed of Lower-Middle Carboniferous rock. The apex of the massif is located above the central part of the Carboniferous high and is composed of Lower Permian carbonates.
The pay zone at Karachaganak contains four main types of carbonates: biohermal, biomorphic detrital, organo-clastic, and biochemogenic.
The biohermal variety accounts for 10-60 percent of the total. These are gray to beige limestones and are commonly dolomitized. The rock is 70-90 percent recrystallized. The biomorphic detrital variety is the most common of these rock types. It contains 30-90 percent organic remains. Varieties recognized are crinoidal-foraminiferal, crinoidal- bryozoan, bryozoan, coral-crinoidal-foraminiferal, and polydetrital.
The organo-clastic rocks are subordinate in importance. Among them are braccias with coarse-clastic texture. Detritus makes up 5-50 percent of these rocks.
Biochemogenic and chemogenic varieties do not have a wide distribution. Mud and detritus are commonly intermixed.
Reef facies of Visean-Serpukhovian age form the atoll. They are largely recrystallized algal limestones up to 250-300 m thick. At the crest of the high in the interior lagoon they are replaced by shallow-water marine organo-clastic limestones 450 m thick. The Lower Permian is 800 m thick and is more complex, consisting of a greater variety of rock types. The various rock types present indicate a reef core, terraces, flank, and foot of the reef.
The reservoirs are 48 percent pore types, 19 percent cavern-pore, 21 percent fracture-pore, and 12 percent fracture-cavern-pore.
In the Lower Permian sediments maximum effective thickness and porosity are characteristic of the reef core. There is a clear tendency toward decrease on the terraces and flanks, and a minimum at the foot of the reef. The Carboniferous reservoir is relatively thick, and porosity and permeability are best in the lagoonal sediments.
Taken from Kononov and others, 1986; digested in Petroleum
Geology, vol. 23, no. 7/8, one structural facies map.
Copyright 2003 James Clarke. You are encouraged to print out
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