Paleogeographic and litho-facies formation conditions of MidUpper Jurassic sediments in S-E Western Siberia (Tomsk Oblast)

This paper describes the criteria to identify Tumen (lower Bathonian) and Naunak (upper Bathonian-Callovian- Oxfordian) suites within S-E Western Siberia (Tomsk Oblast). The specific paleogeographic and litho-facies formation conditions of sediments and numerous vegetable remains and ichnofossils indicated the fact that this territory was the location of sedimentogenesis transition during Tumen and Naunak suite formation. Based on integrated survey oil-gas potential litho-facies groups were defined in Mid-Upper Jurassic sediments within S-E Western Siberia.


Introduction
Mid-Upper Jurassic sediments in S-E Western Siberia are the most potential oil-gas deposits. However, it should be noted that due to existing polyfacies cross-sections it is rather difficult to identify complex-structured reservoirs [7]. Mid-Upper Jurassic sediments in S-E Western Siberia include Vasugan horizon embracing marine Vasugan suite and coastal-marine Naunak suite [4] being underlain by Tumen suite. Today, there are two controversial questions which are under discussion: what are the distinct and visible division between these suites and how to trace the reservoirs within these suites. Hard-to-identify stratigraphic units are conditioned by the lenticular lithological bodies and insignificant suite rock component contrast intensity.
Traditionally, production logging data is applied in sectional layering. However, log data interpretation of geological bodies with different facies features scarcely ever seems to be distinct. In this case, direct methods are more significant in the biostratigraphic control [3].
Backstripping and effective forecasting of natural petroleum and gas reservoirs were based on core sampling involving paleobotanic (macro-plant remains), lithological-petrographic, geochemical, fluorescence microscopy and X-ray diffraction methods, as well as logging interpretation.

Results and discussion
Based on the integrated analysis the lithological-petrographic and palaeontological criteria were established for the diagnosis of Tumen and Naunak suites.  The fragment material composition in sandstones and silt-sandstones was predominately quartz and feldspar of average roundness and sorting. The cementing matter included mainly hydromica, chlorite, muscovite (more often plastically deformed), which, in its turn, indicates the fact of intensive catagenesis. The postdiagenesis alteration in Tumen suite was more intensive than in Naunak suite (intensive cataclasis, fracturing, flatness of fragments alongwise the bedding plane). Numerous and various plant traces pertaining to upper Tomsk phytohorizon (Lower Bathonian) could be observed on the bedding surface [5]. This flora included equisetum (horsetail), fern and ginkgophyta, of which the predominate type was Coniopteris (fern). The following could also be found throughout Tumen suite profile: Equisetites sp., Coniopteris vialovae, С. simplex, Raphaelia diamensis and Nilssonia urmanica. Based on detailed studies of the taxons several ferns such as Coniopteris vialovae and Raphaelia diamensis were revealed аs limited stratigraphic bands. This, in its turn, could be applied in stratigraphic breakdown and correlation of the continental sediments in S-E Western Siberia.

Characteristic features of Naunak suite
Naunak suite is mainly composed of thick medium-coarse grained sandstone and silt-sandstone interbeds ( fig.3). The terrigenous varieties in Naunak suite comparable to those in Tumen suite involve weakly sorted and less rounded fragment material. The cementing matter includes not only micaceous minerals but also carbonates (siderite, calcite) and authigenous glauconite. Post-diagenesis alteration predominately embraces the corrosion and regeneration of the quartz fragments and feldspar kaolinization; however, to a lesser extent, it also involves cataclasis and fracturing.
A few horizon cross-sections revealed a high bitumen content being confined to the pores and cementing matter. Naunak phytohorizon plant traces (upper Bathonian-Callovian-Oxfordian) could be observed on the bedding surface involving an insignificant paleoflora composition comparable to underlying Tomsk phytohorizon. This composition predominately includes gymnosperms (nakedseeded): czekanowskiales, cycadales, ginkgoales and coniferales. Lower ranking sequence involves ferns. Sporadically equisetum types can be found. Czekanowskia types are few in number-only 3-4. However, they form monodominant accumulations composing interbeds within terrigenous sediments. The

Conclusion
Chemical-petrographic study of coals from Tumen and Naunak suites via Rock-EVAL method did not only indicate different biochemical coal formation environment conditions but could also be applied in identifying the stratigraphic units.
The analysis of geophysical data based on gamma ray, high-frequency induction logging 35, neutron logging and induction electrical logging in correlation with complex lithological and biofacies studies made it possible to isolate the sedimentation cycles in Tumen and Naunak suites. The most potential petroleum-gas reservoirs (irrespective of age) are sandstone sediments of channel facies formed during TST [1]. Horizons enriched by migrated bitumen of greasy-resinous (light yellowbrownish luminescence) and greasy (light yellow luminescence) composition were identified under the fluorescent microscope. Such horizons having porous and cemented bituminous structure could be potential hydrocarbon reservoirs.
Therefore, detailed integrated study of hard-to-identify Mid-Upper Jurassic sediments in S-E Western Siberia furthered possible stratigraphic breakdown and intraregional correlation, as well as the prediction of potential reservoirs [10].