SEDIMENTARY AND EPIGENETICAL TRENDS AIDING THE HYDROCARBON EXPLORATION IN WEST SIBERIA

Abstract

Several observations on Jurassic and Cretaceous clastic rocks of west Siberia may be important not only for oil and gas exploration but also for other fields of interest: 1 The sand and silt percentage of the sections and the sand grain-size increase in structural uplifts. This points to synsedimentary initiation of the present structures. 2 The percentage of convexo-concave grain contacts increases with depth, this increase is greater below a depth of 2,300 m. 3 The percentage of wedge-shaped grain contacts is higher in the water-bearing sandstones from the same depth. 4 In the clay matrix of water-bearing sandstones, kaolinite is widespread, whereas mixed-layer minerals (montmorillonite-hydromica) prevail in oil-bearing sandstones. The quantitative evaluation of these and other investigations proved a relatively young oil accumulation and a still younger gas accumulation in the area under consideration. The older oils differ chemically from the younger ones. 5 The clay cover of oil- and gas-bearing structures proved to contain less silt content and sand flasers than the clay cover of water-bearing structures. It is suggested, that the sandy and silty clays were not effective in trapping the hydrocarbon accumulations, mainly because of stronger jointing (when compared to pure clays). With increasing depth, the content in expanded layers decreases in these clays. At the same time, the number of jointed covers increases.