Serpentinization of ultrabasites in Pre-Jurassic series: an example of Uvatsky petroleum district

The author presents the results of investigations of serpentinous ultrabasites making a part of basement rocks in the Gerasimovsky License Area situated in the Uvatsky Petroleum District (southern part of the Tyumen Region). Basement composition and structure influence the behaviour of magnetic and gravity fields. In the Gerasimovsky area, serpentinous peridotite massifs occur at a depth of about 3 km, they do not cause bright positive anomalies, so it is rather difficult to delineate plutons. The basement topography determines thickness of the weathering crust, and composition of the basement bedrocks predetermines composition of a clastic part and clay components in sedimentary rocks of the cover. There is also correlation between basement rocks composition and structure and lithological features of deposits in the basal horizons of sedimentary cover, which is important to take into account when selecting focus areas for hydrocarbon fields exploration and prospecting. Studies of basement will allow interpreting geological and tectonic history of the region, as influence of many different tectonic factors on oil and gas occurrence may be manifested in paleogeography, lithological and geochemical features of promising deposits. All the results of core studies presented in the paper are original. The paper presents petrological characteristics of structural features of the rocks that compose a large ultrabasic massif, comparison with serpentinite of the neighbouring Katyssky area, describes typical mineral compositions of serpentine and serpentinous ultrabasite varieties, describes special features of chemical composition, and demonstrates thin section microphotographs. The correct methodology of serpentinite varieties identification involves the use of precise methods to validate light-optical diagnostics. According to the results of reservoir properties investigation, serpentinous peridotites and serpentinites of the Gerasimovsky area belong to the rocks having low porosity and permeability, namely: porosity is 0.2 to 5.2 %, permeability - (0.01-0.06) · 10-3 µm2. Density of the rocks is 2.62 to 2.63 g/cm3. In accordance with thin section descriptions and ferruginosity, serpentinites of the Gerasimovsky License Area correspond to netted (reticulate) protobastites. It was supposed that serpentinite of the Gerasimovsky and Katyssky areas are not separate bodies, but large zones of serpentinite melange, and their formation is based on the process of protrusive injection of serpentinites. As a result of these studies, new data on basement rocks were obtained, which allow improving the accuracy of geophysical data interpretation within the framework of geological mapping of the Uvatsky region