This study provides a comprehensive sedimentologic analysis of the J2 prospective horizon (Middle–Late Bathonian) in the central West Siberian Basin. Based on the extensive dataset, it was established that structural complexity of this horizon is largely caused by a variety of depositional environments. Sedimentary facies of this horizon pass upward from widespread continental at the base, through deltaic or coastal-continental, to marginal-marine at the top. Change in the paleohypsometry of the study area reflects the distribution of depositional environments, in terms of the proportions of continental, transitional, and marine deposits. The study also shows that facies variability and change in depositional settings can have a significant control on reservoir quality, as well as its vertical and lateral distribution. Using a combination of detailed GDE reconstructions, structure contour mapping on the top and base of the J2 horizon, analyses of well test data, probability of reservoir presence, distribution of average porosity and permeability within different subenvironments, a composite map showing variations in the reservoir effectiveness in the J2 horizon was generated.
—We study the morphology and sedimentary facies of trace fossils formerly identified as Arctichnus found at the base of the Jurassic section near Cape Airkat (northern Siberia). They are most often found in shoreface silty sand and are similar to Rosselia socialis ichnospecies in morphology, taphonomy, and depositional environments. On the basis of this similarity, the Airkat trace fossils should be identified as Rosselia socialis Dahmer, 1937. Analysis of the type collection of Arctichnus arcticus has revealed new morphological features of the taxon.
An integrated lithofacies analysis of the Middle–Upper Bathonian petroleum horizon J2 in the northeastern part of the latitudinal Ob’ region was carried out. The new data were used to supplement and refine the available data. The recognized lithofacies and their associations clearly mark off the boundaries of reconstructed sedimentation conditions. A petrographical study of the silt–sandy rocks of the horizon was carried out with a detailed calculation of their grain size and petrographic compositions. Grain size coefficients were analyzed with the use of genetic, dynamic, and dynamogenetic diagrams. The new data permitted a considerable refinement of the reconstructed sedimentation conditions of the horizon. It has been found that the transgressive change of sedimentation conditions is reflected in regular changes in some characteristics of the studied sediments: a decrease in the total number and thickness of coal interbeds, an increase in the degree of sediment bioturbation, a change of ichnofossils, and an increase in the amount of pyrite. More detailed paleogeographical schemes have been constructed for the formation of the lower (continental), middle (transitional), and upper (littoral marine) parts of the horizon. They provide a better understanding of the regularities of development of the Middle–Late Bathonian sedimentary basin in the study area.
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