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HYDROTHERMAL MODELING OF SECONDARY RESERVOIR FORMATI ON IN SILICEOUS LIMESTONS (H2O–CO2 SYSTEM)

M. Yu. Zubkov

West-Siberian Geological Center, Tyumen, Russia

We have discussed the first results of hydrothermal modeling of the formation of secondary reservoirs (hydrothermal silicites) in siliceous limestones, which are widespread in the Pre-Jurassic complex of West Siberia, namely, in the H2O-CO2 system, in which the mole fraction of CO2 varied from 0.0 to 1.0, and the temperature from 215 to 410оС. It has been found that in this system there is a predominant dissolution of carbonates and, first of all, dolomite, as a result of which the surface of siliceous limestone is silicified and secondary porosity is formed in them. In some samples, quartz microcrystals with varying degrees of perfection of crystal faceting are shaped. It is found that pyrite is also unstable under these hydrothermal conditions and decomposes forming iron-containing carbonates at its sacrifice. Its transformation into pyrrhotite and copper and nickel sulfides is also observed. Biogenic quartz dissolved by a hydrothermal fluid is released from it either in the form of microcrystalline quartz, or in the form of quenching phases represented by cristobalite and/or amorphous silica. It is also found that at temperatures above 360°C, instead of iron-containing carbonates, chlorite is formed due to pyrite, while kaolinite also decomposes along with pyrite. It was experimentally established for the first time that at a high temperature (410°C), carbon dioxide acquires oxidizing properties and, as a result of its interaction with copper, oxide and red oxide of copper are formed. In addition, under these conditions, pyrite passes into iron-containing carbonates rather than chlorite. The main regularities of the formation of secondary reservoirs, i. e. hydrothermal silicites, in the Paleozoic siliceous limestone have been revealed.

Keywords: siliceous limestones, hydrothermal fluids, H2O-CO2 system, secondary reservoirs, hydrothermal silicites.

DOI 10.20403/2078-0575-2020-3-43-58