Experimental simulation of the process of hydrothermal fluids interaction with the Jurassic deposits of the West Siberian basin (Н2О-СО2 system)

The authors discuss the results of hydrothermal simulation of epigenetic mineral associations and secondary resevoirs formation in different types of the Jurassic sedimentary rocks in Western Siberia in Н2О-СО2 system. The experiments were conducted at temperatures between 235 and 400 °C, pressure of 70 MPa, and mole fraction СО2 varying from 0 to 0.5. The duration of the experiments ranged from 5 to 19 days. In most experiments, pyrite charge was added as a buffer. It is found that in the experiments in Н2О-СО2 system the intensive bitumen and volatile components formation from organic matter present in the initial samples was observed. Process of bitumen and volatile components generation was accompanied by rock fluid fracturing and secondary porosity formation. Under these conditions, carbonates, acid plagioclase and, less often, kaolinite were found unstable. Formation of hydrothermal ferrigenous chlorite was observed instead. Both dissolution of the unstable mineral phases by hydrothermal fluid and formation of naphtides from plant detritus resulted in formation of additional secondary porosity. In addition, silicification of sample surface occurs. As СО2 concentration increases, an increase of kaolinite and siderite stability is observed. With a rise in temperature of hydrothermal fluids, СО2 starts to actively exhibit the oxidizing properties. As a consequence, formation of scheelite, bunsenite, cuprite, and tenorite occurs in the presence of elements making a part of autoclave, and also carbon oxide (СО). Presence of pyrite charge boosts the formation of hydrothermal ferrigenous chlorite. At the same time, formation of pyrrhotine, millerite, and chalcosine is reported. It is shown that in addition to methane, a gas phase remaining in autoclaves after hydrothermal experiments are finished contains significantly more C2+ hydrocarbons. By the example of the Jurassic deposits, the authors substantiate the participation of tectonic and hydrothermal processes in formation of hydrocarbon pools within the West Siberian Mesozoic series