Impact and implications of hydrothermal fluids migration in the Frolov hydrocarbon province in West Siberia

Abstract

Both the prolific source rocks of the Bazhenov Formation and the reservoir-prone units of the Abalak Formation cover most of the West Siberian region. Hydrothermal fluids circulation has been suggested to affect hydrocarbon potential and reservoir properties at some localities of this vast hydrocarbon province. However, integrated studies are currently lacking. Here we present an extensive multidisciplinary study combining regional studies with core logging properties and sample analyses (i.e. geochemistry, petrography, organic geochemistry) to reveal processes related to the hydrothermal activity that affected both formations. We also focus on the factors and implications for secondary unconventional reservoir formation within the deposits from the Kamennaya summit of the Frolov oil and gas region. Three boreholes corresponding to type sections located in different parts of the study area (elevated, transitional and deep) were investigated. The elevated and transitional parts of the basin are adjacent to the weakened contact zones of a granite massif. These tectonic discontinuities acted as conductors for hydrothermal fluids circulation on the area. Lithological and stable isotope studies revealed the presence of intensive hydrothermal secondary alterations in various types in rocks, including massive metasomatic carbonatization, sulfatization, sulfide mineralization and leaching processes. This hydrothermal fluid activity resulted in reservoir formation within siliceous radiolarite layers, while in the deep part of the basin potential reservoirs were completely healed. Organic matter maturity varies in different parts of the basin. We identify several mechanisms related to the various hydrothermal processes that influenced rocks composition, structure and organic matter maturity: (1) A weakened zone located on the periphery of the granite massif in the basin basement is characterized by extensive fault damage zones facilitating the migration of high temperature fluids along deposits during post-sedimentation stages; (2) The thickness, mineral composition of the rocks overlying the basement control fault attenuation and fluid workflow; (3) The fluids composition, their acidity/alkalinity, prompted various changes in rock structure, especially in secondary porosity formation. The identification of various hydrothermal processes and their influence on pore space allowed to determine the approach for the spatial distribution prediction of unconventional reservoirs in the studied formations, and subsequently improve the efficiency and effectivity of the exploration process not only at the Kamennaya summit, but also on the other similar West Siberian basin areas.