Pore Water Characterization of the Tight Rocks of Bazhenov Formation Using the Water and Salt Extracts

Abstract

The paper presents the results of the pore water composition investigation of tight shales of the Bazhenov Formation rocks of Western Siberia, obtained by integrated interpretation of the experimental data of the water and salt extracts. The two methods of cation exchange capacity (CEC) measurement were applied – Pfeffer and Hexaamminecobalt trichloride method. Both of them showed similar results. CEC varies from 3.45 to 4.53 meq/100 g by Pfeffer method and from 3.51 to 4.35 meq/100 g by Hexaamminecobalt trichloride method. Ca, Na, Mg, K form exchange complex of all studied core samples. According to interrelation (rNa+rK)>rCa the exchange complex type is marine and was inherited from the composition of the paleobasin seawater. It was found that water extract composition depends on the core mineral content, except chlorine, which originates from the pore water. Using the thermodynamic modelling in PHREEQC program, next ratio of exchange cations was found - Na (up to 91%), Mg (up to 5.6%), Ca (up to 2 .6 %) and K (up to 0.8%). According to the calculation using the water extracts results, the pore water salinity changes from 1.23 to 21.96 g/L.