Sedimentary Geology, 40 (1984) 211-215 211 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
UPPER JURASSIC BLACK B ITUMINOUS SHALES IN WESTERN S IBERIA
N. KRYLOV and M. KORZH
Institute of Geology and Exploitation of Fossil Fuels, Fersman St. 50, P.O. 117312, Moscow (U.S.S.R.)
(Accepted for publication January 13, 1984)
Krylov, N. and Korzh, M., 1984. Upper Jurassic black bituminous shales in western Siberia. In: L.F. Jansa, P.F. Burolett and A.C. Grant (Editors), Basin Analysis: Principles and Applications. Sediment. Geol., 40: 211-215.
The Bazhenovskaya Formation, of Volzkski (Tithonian)-Berriasian age, is one of the most interesting and unique sedimentary formations of the Mesozoic section of the western Siberian platform. It is a shaly sequence characterized by a uniformity of lithoiogy, high content of organic matter, broad regional distribution (in excess of 1 >(106 km2), and relatively uniform thickness. The formation shows high electrical resistivity (up to 5000 f~), and its natural radioactivity exceeds background radiations by 10
mes or more. Because of these features, the Bazhenovskaya Formation is the leading marker horizon in the sedimentary cover on the west Siberian platform.
Regionally, the main area of b i tuminous shales of the Bazhenovskaya Format ion
is located in the centre of the paleobasin and is surrounded by a zone of non-bi -
tuminous or low bituminous deposits (Fig. 1). The latter zone covers an area of
about 400 km 2. Towards the margin of the paleobasin, these deposits are replaced by
thicker, shal low-water terrigenous sediments. Structurally, the Bazhenovskaya For-
mat ion occurs at depths of 1000-1200 m in the western part of the Siberian
platform, and at depths of 3000-3500 m in the central and northern parts of the platform. The average thickness of the formation is 35 m, with the maximum
thickness occurring in depressions in the northern part of the platform, and the min imum thicknesses, associated with uplifts, in the Middle Ob region (Fig. 1).
Lithologically, the formation is composed of greyish-brown to black bi tuminous shale (Fig. 3a), with intercalat ions and lenses of biogenic siliceous rocks (radiolarites,
Fig. 3b), marls, and l imestones (Fig. 2). The shales in the upper part of the formation are thinly bedded, in the lower part they are massive. Some intercalat ions are
enriched in syngenetic calcite and glauconite. There is an abundance of radiolar ian
fossils (Fig. 3b), nodozar ian foraminifera, pyrit ized ammonite trace fossils, pelecy-
0037-0738/84/$03.00 1984 Elsevier Science Publishers B.V.
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Fig. 1. Lithofacies map of the Late Jurassic-Early Cretaceous sedimentary deposits of western Siberia. I = limit of Volgian-Berriasian deposits; 2 = lithofacies boundary; 3 = limit of occurrence of the Bazhenovskaya Formation; 4= boundary of the non-bituminous facies, Bazhenovskaya Formation; 5 = isopachs of the bituminous facies; 6 = mudstones with less than 107,, sandy siltstones; 7 = mudstones
with 11-30% sandy siltstones; 8 = mudstones with 31-50% sandy siltstones; 9 = sandy-sihstones with 31-50% mudstone; 10= regional distribution of bituminous sediments; I I = area where the Bazhenovskaya Formation is missing due to uplift; 12 = direction of sediment transport; 13 = boundary of the Siberian platform.
pods, and significant amounts of phosphatic fish debris (Fig. 3c) and coc- colithophorids in the rocks. In some sections (Omski and Bolsherechenski regions) accumulations of aquatic plants amount to 30% of the organic matter. The forma- tion is highly silicified, with an average silica content (of mainly biogenic origin) of 15%. Both pyrite and silica attain their highest content in the central areas of the western Siberian platform. The high content of organic matter is the outstanding feature of the rocks of the Bazhenovskaya Formation. The maximum values of organic carbon content (17.7%) occur in the central part of the platform (Mansy syneclise and Koltogorsky depression). The organic matter of the Bazhenovskaya Formation is mainly of a sapropelic type.
Fig. 2. General ized strat igraphic co lumn for the Upper Jurass ic -Lower Cretaceous sedimentary sequence
of the western Siberian platform. I = sandy siltstone; 2 =sha le ; 3 = marl; 4 = dolomite; 5 = l imestone;
6 = siderite; 7 = bi tuminous rock; ./2 = Middle Jurassic; Js = Upper Jurassic; K 1 = Lower Cretaceous.
The main component of the Bazhenovskaya Formation mudstones is up to 65% of clay minerals, comprised by a mixture of mixed-layer minerals (hydromica- montmorillonite), beideillite, and chlorite. The sedimentary basin is distinctly di- vided into areas of different clay mineral associations. The central part of the platform is characterized by occurrence of hydromica and beidellite, the western side
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Fig. 3. a. Bituminous silicious argillite. Note thinly laminated character of the rock and parallel orientation of sedimentary particles (silica: white) and bituminous matter (dark). Unidentifiable microfos- sils near the center of the photograph. Thin section, normal light, magnification 90. b. Bituminous radiolarite. The radiolarians are moderately preserved and have silica overgrowths. The matrix is siliceous clay saturated by bitumen. Thin section, normal light, magnification 60 . c. Bituminous argillite with high concentration of phosphatic debris composed of fish bones. Note parallel orientation of the particles and the microlaminated character of the rock, suggesting deposition in a hydrodynamically quiet environment.
by hydromica-kaolinite, the northern areas of the platform and the eastern and southern sides by hydromica with admixtures of kaolinite, chlorite and beidellite.
The bituminous clays of the Bazhenovskaya Formation were deposited in an epicontinental marine basin of normal salinity, during a period of maximum transgression in the Late Jurassic. The climate was warm and humid (+ 20C), and luxuriant organic life flourished in the basin. Sedimentation was characterized by tranquil hydrodynamic conditions. Sedimentation sometimes took place under reducing conditions as indicated by intensive pyritization. In terms of the gas regime of the sedimentary basin, however, hydrogen sulfide contamination occurred only in local areas, otherwise the benthos could not have existed. The accumulation of terrigenous material along the margins of the paleobasin reflects peneplanation and intensive processes of chemical disintegration in the source area. The clay minerals and products of chemical disintegration entering the basin were excessively enriched by the biomass of dying organisms.
We consider the Bazhenovskaya Formation to be a typical deposit of non-com- pensated depression and foredeep. It represents the stage of sedimentation related to maximum transgression and low terrigenous input, which effected the inner depres- sion of the paleobasin where rates of subsidence are approximately equal to rates of sedimentation.
The importance of the Bazhenovskaya Formation is that it is regarded by the majority of geologists to be the main Mesozoic source rock for hydrocarbon accumulations in western Siberia. In recent years it has produced commercial oil (e.g., the "Bolshoi Salym" field) from shaly reservoirs which developed through fracturing; there are no porous reservoir units in the formation. Analysis of the oil prospects discovered to date indicates that reservoir distribution relates to zones of fracturing associated with the major tectonic features of the region.