The Specific Features of the Microcomponent Composition of Pore Solutions in the Rocks of the Bazhenov Formation, Western Siberia

Abstract—

The results of experimental studies of the macrocomponent and microcomponent compositions of pore solutions in the bituminous siliceous shales of the Bazhenov Formation (BF) in Western Siberia with total organic carbon (TOC) contents up to 12.63% (“normal rocks”) and the silty sandstones of the “anomalous” section of the BF with TOC contents no higher than 0.63% (“anomalous” rocks) are presented. The studies of water extracts from core samples were performed. It was found that the pore solutions in the rocks of the “normal” BF rocks are sodium sulfate–bicarbonate, whereas the solutions in the “anomalous” rocks are sodium bicarbonate–chloride. The following tendencies were noted in the pore solutions of both “normal” and “anomalous” rocks compared with the oceanic water: (a) increase in Mn, Co, Ni, Cu, Zn, Mo, Sb, Cd, Ba, V, and Cr concentrations to varying degrees; (b) elevated Cs, Pb, and Ti concentrations in pore solutions of “anomalous” rocks only; (c) reduced Li, B, Rb, and Sr concentrations. A direct correlation between Li, B, Ba, Sr, U, Mo, Cd, Sb, and Ni concentrations in the pore water and total organic carbon (TOC) contents in the rocks was established. The concentrations of V, Cs, Sb, Rb, Ti, and Mn in pore waters display a close to inverse correlation with TOC content in rocks. The content of Ba in the pore solutions of the “normal” BF rocks equals or exceeds the Sr content, which is abnormal compared with formation water in most oil fields and with ocean water and is attributable to the specific features of organic-rich mudstone accumulation, considering that the Sr content in the “anomalous” rocks of the BF is two times higher than the Ba content, which is closer to their usual ratios in most natural waters.