Evolution of sedimentary basins Geochemical heterogeneity of the Aptian-Cenomanian sediments of Western Siberia on the example of the Pokurskaya suite of the south Varieganskian bar

Afonin I.V.

Tomsk State University, Tomsk, Russia

heaven05@list.ru

The Nizhnevartovsk oil-and-gas bearing region has been investigated during 30 years. The southern part of the Varieganskian bar is considered as the most prospective object, of which the Cretaceous deposits enclose over 44 oil and gas pools. These deposits are stratigraphically confined to the Pokurskaya Suite characterized by the complicated structure and the inconsistent lithological composition.

At present, there exist several models of composing this structure, however they do not enable the precise correlation to be performed and prognoses to be made. Hence, the problem of subdividing these formations is pressing at the moment. The seismo-stratigraphic method was basic in studying these deposits, however in view of the complexity in the data interpretation, it does not always allow the exact correlation of sequences to be carried out. So, methods of litho-geochemistry have been proposed for searching reference indicators.

Rock mass PK_1-2 was selected as an object for studying; argillites and siltstones dividing beds of sandstones were picked out from this rock mass. Geochemical characteristics of these rocks were studied on the base of 112 determinations of petrogenic oxides content with the RFA and microelements composition using ICP-MS. The studies were carried out in the Centre of Shared Access (CSA) "Analytical centre of the geochemistry of nature systems" of Tomsk State University (TSU). For the lithogeochemical typification of these rocks, the following techniques were applied: hydrolyzate module (HM), titanium module (ТM), femic module (FM), normalized alkalinity (NKM), geochemical coefficients (La/Yb, Fe/Mn, Sr/Ba, Ce/Ce*, Eu/Eu*), as well as a number of binary diagrams Sr/Ba- Fe/Mn, La/Yb-∑TR.

Relying on the performed investigation, the studied samples were divided into five geochemical groups. The first group includes rocks characterized by a low level of the accumulation of the rare-earth elements (43.6-63.3 g/t) on the background of elevated values of ratios La/Yb and Sr/Ba being respectively 12.98-15.83 and 0.7-0.83, thus suggesting the formation the studied deposits in lagoonal conditions.

In rocks of the second group, the progressive increase was marked in the accumulation of rare-earth elements (111.1-155.2) in combination with values 9.9-12.4 for ratio La/Yb, 0.44-0.54 for Sr/Ba, 93-163 for Fe/Mn. This indicates to the maritime conditions of sedimentation (maybe, conditions of the shallow-water gulf).

The third group includes samples characterized by variations in the rare-earth elements content in the range of 108-198, which, if combined with other indices (La/Yb 9.8-16.4; Sr/Ba 0.13 – 0.37; Fe/Mn 27-99) are indicative of forming the rock mass in the weakly salinized shallow sea basin.

The fourth group is characterized by the abrupt jumps of the lanthanoids content from 41.1 to 325 g/t, while the symbasic changes in the ratios La/Yb and Fe/Mn were respectively 7.1-17.7 and 36-193. These results point to the fact that this rock body was forming in the middle part of a shelf zone, which experienced periodical submergences. As a consequence, the facies conditions of sedimentation changed abruptly, that is much pronounced by geochemical coefficients.

The fifth group consists of cyclic sediments distinguished on the base of the CaO behaviour. They are marked for their wide variations of the geochemical indices. However, it should be noted that a potential reference indicator has been found, making possible the relative correlation of these sediments. The abnormal clayey horizon serves the purpose, whose ratio Се/Се* changes as a rule in the range 1.25-1.47 (rarely reaching the value of 1.7).

Hence, the performed investigation has allowed sediments to be divided into several geochemical groups making possible the correlation between boreholes. Besides, conditions of forming the rock bodies have been established, which are substantiated by the petrographic characteristics of rocks. The stable geochemical reference indicator has been distinguished for the cyclic sediments, providing a means for a relatively clear-cut determination of the strata boundary.

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