The refinement of facies dismemberment and inter-well correlation in the AC12 seam deposits of the Priobskoye field

The correlation of the АС₁₂ seam sections is carried out in the south-western part of the Surgut arch according to the data of core samples investigation. The analyses are performed on the distribution and changes in the thickness of siltstones-collectors, the structure of sections, composition and stratigraphic assignment. The sedimentologic core analysis has been performed for 1350.75 m of core from 44 boreholes of the Priobskoye deposit. The detailed lithologo-facies studies of the borehole sections have made it possible to define common settings of their formation. The essential depositional environment for the majority of deposits penetrated by boreholes is the deep-sea one, which is represented by the facies associations from the turbidity current channel, channel banks of the turbidity current, the inter-channel space of the deep water slope. For revealing changes in the sandstone seams throughout the geological section, the sedimentation conditions are worked out in detail by applying a unit of less dimension than the main fraction. Several lithofacies are established on the base of the textural-structural peculiarities of sandstones and alternated sandstones-claystones characterizing the different intensity of sedimentation. In interpreting the results of the granulometric analysis, some divergence with the initial macroscopic characteristics of lithofacies is observed. The mineralogical composition of samples has enabled to supplement the macroscopic description with the additional information on the rock type, the structure of pore space, the ratio of rock-forming minerals, post-sedimentation changes, cement. For refining the stratigraphic correlation, the present author has established lithofacies characterizing the sedimentation settings within the sandstone seams. In dividing lithofacies according to the textural-structural peculiarities, possible is the incorrect determination of their dependence on a definite facies setting. More detailed consideration of granulometric and mineralogical composition demonstrates the difference between lithofacies A1 in conditions of the turbidity current channel and lithofacies A1 in conditions of the channel bank. More precise conclusions on conditions are made after dividing the large-scale facies into lithofacies corresponding to sedimentation subsettings. On the base of granulometric and petrographic analyses, the necessity is proved to establish more fractional lithofacies in the facies settings, because macroscopically identical sandstones demonstrate differences in the granularity at the microscopic level. A significant similarity is revealed in samples from the same lithofacies A1 but obtained from different boreholes. As a whole, the sedimentation settings established for sandstones-collectors are confined to the deep-water slope and marginal parts of the delta and deep-sea settings of sedimentation. The detailed correlation of seam АС₁₂ shows the lateral persistence of sandstones-collectors with pinching-out in separate boreholes; and, depending on the geographical location of boreholes, substitutions take places characteristic for the transgression or for the regression.

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