The research was aimed to study the pore water volume and chemical composition in the isolated core samples of the Bazhenov source rock formation taken from four different oil fields in West Siberia, Russian Federation. The approach is based on the complex laboratory methods of the investigation such as Dean-Stark and evaporation methods, the X-ray spectrometry, mass-spectrometric study (ICP-MS), water and Pfeffer's solution extracts. It was found that the samples contain three types of pore water: free groundwater, physically bounded water, presumably capillary water, and chemically bounded water. By using direct laboratory techniques quantity of residual pore water was found to vary within 0.51÷3.49 wt.%. The quantitative results of the microcomponents (Li, V, B, Ba, Th, U, Sr etc.) and rare earth elements (REE) in the pore waters and the rock samples were also determined. Determined values of rock cation exchange capacity vary within 3.98÷19.50 meq/100g of rock, which exceeds previously published data by factor of two, but integrally correspond to composition of rock clay fraction of the analysed samples. Since start of Bazhenov oil production in 1968 no free water has been produced and therefore Bazhenov formation was considered to be water-free. The results of the research show substantial amount of formation water in the Bazhenov rock samples and provide a solid basis for re-evaluation of hydrocarbon resources and reserves of Bazhenov formation, as well as improving quality of well log petrophysical calibrations, especially for electric, nuclear-magnetic resonance and dielectric methods. The research workflow also included laboratory studies of hydrogen and oxygen isotopic compositions for three Bazhenov water samples, which were extracted from rock samples in a retort at 200°С for 5 hours. Isotopic composition of hydrogen was found in the range δ2H VSMOW values from –64.5‰ to –63.8‰, of oxygen δ18O VSMOW values from -2.0‰ to +1.4‰. These ranges are in a good agreement with values characteristic for formation waters and brains associated with petroleum reported for many sedimentary basins. Isotope analysis of water from flowback fluid co-produced with oil in one of Bazhenov wells showed that water genesis is different to that of Bazhenov formation water. The studied flowback fluid is very close in isotopic composition to artificial fluid pumped in the Bazhenov formation interval to enhance oil recovery. With δ2H VSMOW values -74‰ and -76‰ for hydrogen and δ18O VSMOW values -7‰ and -9‰ for oxygen respectively, both fluids correspond to formation waters of West Siberia overlying the Bazhenov formation. The results suggest no detectable contribution of Bazhenov formation water in flowback fluid co-produced with oil in investigated Bazhenov well.