Petrogenetic typing of volcanics from the top of the oil-bearing pre-Jurassic complex in the Elizarovsky trough of the West Siberian Plate

The results of petrogenetic typing of acidic volcanic rocks (rhyolites and dacites) of the top of the oil-bearing pre-Jurassic complex of the field located in the Elizarovsky trough of the West Siberian plate are considered. These volcanic deposits have a complex structure and, according to the results of well sampling, are characterised by extremely high variability of rock productivity, irregular watering, and, probably, the presence of several levels of water-oil contacts. The study of core material (about 850 linear m) and rocks in thin sections (1320 samples), as well as the application of high-precision research methods (e.g. microprobe and X-ray phase analyses) made it possible to identify 32 types of rocks based on composition, textural and structural features, which were united into 7 groups of similar deposits. As a result of genetic interpretation of the rock structure, the presence of lavas, lavobreccia/clastolavas, lavoclastites, tuffs, lake sediments without and with pyroclastics, and metasomatites was established. The formations of the first four genetic types predominate in this rock complex. In the studied volcanics no reliable signs of the development of weathering horizons were not revealed. The volcanic rocks are characterised by different degrees of secondary transformation, in which the formation of improved reservoir properties was mainly due to leaching, which occurred under the action of solutions at depth, as well as the appearance of fracturing. The best reservoir properties are characterised mainly by effusives with pearlite texture, volcaniclastics and amygdaloid effusives. Volcanic rocks with improved filtration-capacity properties are characterised by porosity values up to 25–28% and permeability values up to the first tens of mD.