The Influence of Sandstones with a High Concentration of Heavy Minerals on the Distribution of Hydrocarbons in a Reservoir Using the Example of a Field in Western Siberia

The article presents the results of special core studies in sandstone intervals with abnormally high values of natural radioactivity. Combining the obtained data with sedimentological analysis and actual field information made it possible to explain the position of the fluid contact and, accordingly, the distribution of hydrocarbons in the reservoir.
In two wells, 16 samples were selected for lithological and mineralogical study in places characterized by anomalous values of natural radioactivity according to gamma ray logging data. Laboratory studies of the core were carried out: profile spectral gamma ray, X-ray phase analysis of the mineral composition of the rocks, petrographic description of thin sections, study by scanning electron microscopy.
The studied rocks are represented by sandstones. Their texture is due to numerous layers of enrichment in heavy minerals, the minimums and maximums of which correspond to the minimums and maximums of profile radioactivity on the core. Among the accessory minerals, the main ones associated with an increased content of radioactive elements are zircon, thorite, fluorapatite and orthite. Their content in the rock can reach 15–40%.
As a result of the work performed, a conceptual model was built that describes the mechanisms of formation of layers with high concentrations of heavy accessory minerals. From the point of view of sedimentation, such layers are a marker of transgression and have low filtration and capacitance properties according to laboratory data. This explains the different positions of fluid contacts in closely spaced wells and clarifies the internal structure of the productive formation.

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