PETROSTRUCTURAL QUARTZ OF SAND RESERVOIRS OF KRAPIVINSKOYE OILFIELD (TOMSK REGION)

The subject of investigation is the oil-bearing sandstones of Krapivinskoye oilfield. The core samples of sandstones were taken from the oil-bearing well at the depth of 2589-2684 meters and were space oriented by the paleomagnetic method. The primary objective was to determine quartz orientation by the internal structure and shape with the help of micro-structural methods. It was done to diagnose the anisotropy of sand reservoirs, to determine the specific facial conditions of sediment accumulation and to estimate the influence of tectonic processes. The analysis of the allocation of elongated quartz grains in the test specimens let determine their preferred orientation in the bedding plane and natural orientation to it. The petrostructural analysis of distribution patterns of optical axes of quartz grains showed that they are characterized by low anisotropy. The anisotropy manifests itself in the occurrence of local maximums, which reflect the natural orientation of optical axes of quartz grains within small groups - domains. Every domain has its own orientation of optical axes; therefore the rock at large can have a practically isotropic pattern. There has been observed a certain relation between the optical orientation of quartz grains and their elongation, which manifests itself in the correlation of maximums location on the diagrams of optical axes and the predominant elongation on rose-diagrams. To determine common mechanisms in all test specimens the assembled diagrams of quartz optical axes and its grains elongation were made and analysed. Their comparison proves that the oil-bearing sandstones of Krapivinskoye oilfield were formed under sedimentation followed by the tectonic influence. The influence of sedimentogene factors manifests itself in the predominant distribution of the quartz optical axes at the descending vertical angles not less than 40-50°. Such distribution is conditioned by the orientation of the elongated quartz grains at the angle of 10-15° to the horizon under the influence of sedimentation. Being such the quartz optical axes coincide with the elongation or have a 40° deviation from it due to the initially prolate form of quartz grains and/or their preferable prism and rhombohedron splitting. The tectonic influence led to the formation of maximum of quartz optical axes, which demonstrates their gentle immersion with 330° azimuth alignment. Such maximum is probably due to the one-sided sub-horizontal compression to the northwest. It is proved by the results of the analysis of predominant distribution of low-amplitude fracture directions at the deposit. The probable cause of the tectonic low influence on the orientation of the quartz optical axes is the bed low temperature - about 100°С.

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