The Aransay-Zhanatan group of sub-salt highs is located in the central part of the eastern border of the North Caspian depression. The sub-salt Permian sediments there are well known for their abundant oil shows, and commercial fields have been discovered in the Kenkiyak and Bozoba areas.
The Lower Permian (Sakmarian-Artinskian) sediments here consist of fine- to medium-grained sandstones, conglo- merates, siltstones, argillites, and thin beds of limestone. Individual beds are not persistent. A single oil field cut into blocks by a system of faults appears to be present in the Kenkiyak-Bozoba area.
Porosity in the 3835-4375 m depth interval in the Kenkiyak area is based on 110 determinations of various Lower Permian rock samples. Average for sandstones is 6.3 percent with a range from 1.7 to 14 percent. Average values for siltstone are similar to those for sandstone. Values for the limestones, however, are almost two-fold less with an average of 3.2 percent. Sandstones and siltstones of Kenkiyak area have a high carbonate content, ranging from 1.5 to 46 percent with an average at 22 percent.
Histograms for the Boboza area were complied for the Sakmarian-Artinskian sand-silt sediments without subdivision into varieties. Distribution of porosity is bi-modal with one peak at 2-4 percent and the other at 16-18 percent. Carbonate content of these rocks ranges from 10 to 45 percent with average at 25 percent.
Porosity of the sand-silt reservoir rock of Kenkiyak-Bozoba area is thus low and close to the threshold for productivity. The underlying Carboniferous carbonates are of greater interest for oil and gas. Shows have been found in these rocks at many places in the eastern part of the North Caspian depression, and a large gas-condensate field has been discovered in Zhanazhol area. Oil has been recovered from pre-Podol carbonates of Oka-Bashkirian-early Moskovian age in the Zhanazhol-Sinel'nikov area. The seal is Podol clay of Moscovian age.
The Carboniferous sediments of Zhanazhol area are sub- divided into four units: Carbonate Unit I, Clastic Unit I, Carbon- ate Unit II, and Clastic Unit II. The histogram for limestones of Carbonate Unit I is based on 147 dtereminations. Average porosity is 4.5 percent, and range is from 0.5 to 11.4 percent. A very similar distribution is found for Carbonate Unit II. These porosity values are close to the threshold for commercial productivity.
Taken from Piyp and Yedrenkin, 1986; digested in Petroleum
Geology, Vol. 23, No. 7/8, one isopach map, six histograms.
Copyright, 2003 James Clarke. You are encouraged to print
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