Siltstone sub-complex. These sediments have approximately the same distribution in the Paleogene section as do the sands. They occur at greater distance from the shore of the basin in beds 0.5-10 m thick. Abundant siltstones are present in the upper Paleocene, lower and middle Eocene, and Oligocene. Grain size is variable, and relatively pure varieties are rare. Montmorillonite predominates among the clays, and zeolites among the authigenic minerals.
Clay sub-complex. These sediments are represented by fine-grained muds deposited in relatively deep-water parts of the marine basin or in shallow-water estuaries with low-lying coasts with a calm hydrodynamic regime. They occur largely in the south of the study area in the form of lenses and individual beds at various stratigaphic levels. They are transitional to siltstones and sands, biogenic siliceous rocks (diatomites, radiolarites), and carbonates (Coccolithophoraceae and foraminifera). They make up 5-10 percent of the section in the north and 30-80 percent in the south. The mineral composition of these pelitic rocks is largely montmorillonite; also present are hydromica, chlorite, and kaolinite. Non-clay components are organogenic calcite, quartz, some zeolites, cristobalite, siderite, and pyrite.
Composition of these clays indicates deposition in relatively deep-water parts of the marine basin with normal salinity and low rates of accumulation. This was conducive to intensive development of limy nannoplankton, accumulation of which in the clayey oozes on the floor of the basin created a deficit of oxygen and an increase in carbon dioxide, hydrogen sulfide, calcium, silica and other components involved in diagenesis.
Siliceous clays in these Paleogene sections are associated with siliceous rocks (diatomites, opokas) and are transitional to clays proper.
These sub-complexes pass from one into another forming vertical and lateral sub-formations with extensive development of mixed types. This results in interbedding of sandy reservoirs and silt-clay seals, thereby providing for migration and concen- tration of hydrocarbons. Such combinations of sediments are best represented in the following regions:
Northwest and west parts of North Caspian depression: lower Paleocene - 20-120 m; upper Paleocene - 20-200 m; middle Eocene - 50 130 m; Oligocene - 500 m.
Southwest border of North Caspian depression: lower Paleocene - about 250 m; upper Paleocene - 50-783 m; middle + lower Eocene - 50-130 m; Oligocene - 30-140 m.
These Paleogene clastics are thus favorable for oil and gas both according to thickness and distribution of reservoirs and seal. The lithologic factor of risk in these regions is low. As for structural and geochemical criteria for oil-gas potential, further investigations are required.
Taken from Akhlestina, Malyshev, and Ivanov, 2001;
digested in Petroleum Geology, Vol. 37 (Planned Annual Volume).
Copyright 2003 James Clarke. You are encouraged to print out
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