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Specific features of Bazhenov suite sediments in south-eastern Nurolsk sedimentary basin (Tomsk Oblast)

The specific sediment features in Georgiev (J3kmgr), Bazhenov (J3vbg) and Kulomzin (K1bkl) suites, exposed by drilling in the S-E Nurolsk depression (Tomsk Oblast), were defined and described via petrographic, X-ray diffraction and fluorescence-microscopy analysis methods. The classification of agrillites was identified, the structure-texture features, composition, voids and bitumen types and their distribution were determined. It was defined that Bazhenov suite argillites are characteristic of fine-dispersion, high biogenic silica content and scattered organic matter, enriched multi-composite syngenetic bitumen (from light to resin- asphaltine), as well as fractured surface where the migration of light bitumen occurs.


Introduction
Bazhenov suite bituminous sediments (Volgian-Early Berriasian) are today a relevant issue in terms of both the formation of source rocks [1,3] and the potential production of shale gas and shale (light tight) oil [5] within it. The suite, embracing a complex of Domanic source rock formations [8,9], is characteristic of high lithologic heterogeneity and consists of clay-carbonate-silica rocks, which, in its turn, are organic matter sources of plankton remains with silicic skeletons: radiolarian and diatoms [2]. According to the data [10], siliceous Bazhenov formation «black clay» is regarded as a source rock for more than 85 % of all oil reserves in the Western-Siberian basin. Effective core porosity for argillaceous-siliceous rocks in Bazhenov suite ranges from 3-8 % [4]. Bazhenov suite in studied area overlies Georgiev (Kimmeridgian) suite clays and is overlaid by Kulomzin (Berriasian) suite clays, involving gradation [6].

Target formation features and research methods
Investigated Georgiev, Bazhenov and Kulomzin suite sediments were taken from the core depth interval of 2822. .0 m. in Western-Kvenzersk well № 4 (figure 1), located in similar local elevation in S-E Nurol'sk megadepression within the Central Nurolsk megadepression (Western-Siberian petroleum province, Pudinsk oil-and-gas region, Tomsk Oblast). The cross-section included brownish dark-grey argillites, fine-platy, foliated areas, smooth flinty fracture and containing

Classification of sediments and lithotype characteristics
According to structure features and composition, five lithotypes were identified.  Microscopically, pelitic and lepidoblastic textures were observed in argillites as a result of formed flaky-fibrous argillaceous hydromica (64 %) aggregates. The following was observed throughout the thin section: fine-aleuritic (up to 0.01-0.02 mm.) quartz and biogenic siliceous material (20 %), composing microclusters in the primary rock matrix, and radiolarian shell skeleton remains; irregular    Rocks are carbonized (ankeriteup to 0.5 %, calcite -7.7-10.8 %) weakly pyritized and contain dissolved and recrystallized organogenic remains, often pyrite-calcite replaced: belemnites, onychites, scallop shells, radiolaria, poorly preserved sponge spicules. The organic matter is thin, evenly and layerwise dispersed in the rock matrix, tinting it reddish-brown; and this organic matter is often concentrated as lumps and inclusions. Subvertical and inclined cracks are fractured into large blocks in  Macrofauna includes fragments of variform onychite belemites, conical tubular shells, extending to 4 mm. and valva fragments; while microfauna includes desma, dissolved and/or intensely recrystallized siliceous radiolarian skeletons, their pyritized fragments with cellular texture. The rocks are organically saturated, whereas, this organic matter is correlated to either clay substance or siliceous rock constituent. The cavities in radiolarian shells are filled with this organic matter, which, in its turn, could be both dispersed and concentrated forming lumps, often, of bedding orientation. The dense rocks exhibit greasy surface and strong crude oil odor. The cores from above-described wells are splitoff layered sections.  terrigenous quartz (1 %), and remains of fragmented macrofauna , sliced-abraded plant organic detritus. In decrystallization areas of siliceous material to fine-grained chalcedony quartz aggregates, an increasing pyrite content, as thin fines and small irregular grain concentrations, as well as layerwise pyritization have been observed. Comparable to lithotypes 2-4, the organic material content in rocks of lithotype 5 decrease.

Lithotypes in cross-section
Clayey-siliceous bituminous argillites (lithotypes 2, 3, and 4) distinguished in the mid-section of the total thickness 14.3 m., comparable to underlying (lithotype 1) and overlying (lithotype 2) agrillites, exhibit more homogeneous micro-texture, smaller component particle size, enriched siliceous material (quantitatively, verging to approximate or exceeding the clay constituent content itself), sometimes enriched calcite and pyrite ( figure 7). According to the above-described specific texture and composition, Bazhenov suite is characterized by these argillites. Rocks of lithotype 1 are related to Georgiev suite, while clayey-siliceous argillites (lithotype 5) contribute to the lower section of Kulomzin suite.

Voids and bitumen
There are layer-by-layer, sub-parallel and cross-cutting bedding fractures of 0.01-0.02 mm. in the argillites. They are exclusively confined to the lithotypes distinguishing Bazhenov suite. Their formation could be associated with pressure relief, which, in its turn, is conditioned by oil-gas generation within the Bazhenov argillite thicknesses. The factual evidence of this is based on the fluorescence-microscopy analysis results which determined the presence of a wide bitumen spectrum: from light (bluish fluorescence) to resin-asphaltine (dark brown fluorescence), having dotty, uniform, scattered, spotty, and fractured distribution. The Bazhenov argillites are marked by a high content and presence of syngenetic (in rocks) and epigenetic (in cracks) composite bitumen, where resinous (lightyellow fluorescence) bitumen is predominate. The argillaceous suite rocks also contain composite bitumen; however, highly light mobile (blue and light yellow fluorescence) and greasy (yellow fluorescence) epi-bitumen are rather predominate here, having a scattered, dotty and fracture-type distribution. Redistribution of bitumen occurs not only within the Bazhenov formation (where rather light bitumen is located in the cracks), but also via the bitumen migration path: bitumen migrates from Bazhenov suite argillites to underlying and overlying rocks in Georgiev and Kulomzin suites.

Discussion
According to the widely-accepted concept, the accumulation of Bazhenov suite rocks is associated with the conditions of Late Jurassic-Early Cretaceous intensive sea basin transgression. Whereas, Georgiev suite sediments deposited under the conditions of low oxygen saturated near-bottom waters (traces of sediment-dwelling organisms); Bazhenov suite sediments deposited under the intensive restoration conditions of depositional environments and stagnant water basins, the indicators of which are the following two facts: finely-dispersed rock and a high content of the indicator mineral-pyrite. Clayey silt was enriched by biogenic and chemical siliceous and carbonate matter and dispersed organic matter as well, and resulted in the formation of the bitumen thickness of fine-washed and laminated argillite source rocks with composite syngenetic bitumen composition, which, in its turn, deposited in areas of relatively deep silt depressions. Sediments in the lower Kulomzin suite accumulated in a stable environment with low hydrodynamic activity, and under more or less deep water conditions (terrigenous impurities, plant detritus).