A petrographic study of the genesis of anomalous sections in the Bazhenov Formation, the Imilor deposit, Western Siberia

Abstract

Research subject. Anomalous sections present in the oil- and bituminous-bearing Bazhenov Formation (BF), in the central areas of the West Siberian basin, are considered to have emerged due to the injection of plastic pulp masses from the sedimentation slope into weakly lithified basal deposits. The aim of this research was to evaluate the as-emerged rocks by analyzing their composition and structure at the macro- and micro- levels. Materials and methods. We investigated reference well sections in the Imilor deposit to elucidate the interaction of the rocks with sandy-clay landslide pulp, their volumetric weight, ductility or brittleness, caused by the presence or absence of a continuous mineral frame. Results. According to core and well logging data, nine regional packs were distinguished in the BF stratotype. The maximal depth of the natural hydraulic fracturing of the rocks is explained by their high bulk density. The lateral movement of the main slump mass is found to have occured on the stratigraphic level of the first and second BF packs, since these layers possessed zero-buoyancy with regard to the injected pulp under their approximate equality in terms of bulk density. The traces of such active movements were revealed in rock samples from the walls of horizontal cracks in the form of clay intrusions having a thickness of about 0.1 mm and strict orientation of all micro-crystals along the walls and the flow. It is shown that inclusions of the rocks under investigation (bazhenits) in BF are generally characterized by their initial stratigraphic sequence, which is confirmed by the sequential disintegration of proto-bazhenit plates during the process of BF up-floating on a growing sedimentation slope. During the disintegration of the lithified and non-lithified layers of radiolarits, the former crumbled due to alternating loads, while the latter - mobile radiolarites - were represented by the hydraulic fracturing liquid, along with quicksand. Conclusion. A detailed petrographic study of core samples from intact deposits of the Bazhenov and George Formations and their anomalous sections have allowed their layer-by-layer comparison to be confirmed the landslide hypothesis and to be derived the information on the degree of lithification of the BF strata during the formation of its anomalous sections.