ACTION OF HYDROTHERMAL FLUIDS AS A FACTOR IN THE FORMATION OF SECONDARY POROSITY IN ACIDIC VOLCANICS FROM THE TOP OF THE OIL-BEARING PRE-JURASSIC COMPLEX IN THE ELIZAROVSKY TROUGH OF THE WEST SIBERIAN PLATE
Yu.V. Rostovtseva1, S.V. Tomasenko2, V.D. Nemova2,3
1Geophysical Centre of the Russian Academy of Sciences (GC RAS), Moscow, Russia
2LUKOIL-Engineering, Moscow, Russia
3Gubkin University, Leninsky Prospekt, Russia
Keywords: Acidic volcanics, secondary porosity, characteristics of hydrothermal fluids, leaching, chloritization, top of the pre-Jurassic complex, Elizarovsky trough, West Siberian plate.
Abstract
Volcanics of dacite-rhyolite composition from the Permian-Triassic top of the oil-bearing pre-Jurassic complex in the Elizarovsky Trough of the West Siberian Platform were studied using a set of methods. It was established that, alongside fracturing, two main types of secondary porosity are distinguished in these rocks, formed on the felsitic (main) mass of the rocks and phenocrystals of feldspars. The alteration of rocks by weakly acidic-neutral chloride fluids led to the dissolution of the felsitic (main) mass of the volcanics, leaching of silica, and the precipitation of chlorite and idiomorphic crystals of albite at temperatures ranging from approximately 300–350°C down to about 175°C. The formation of chlorite occurred on average at around 291–310°C. As a result of acid leaching, porosity was developed along phenocrystals of feldspars, which usually contain abundant microscopic residual TiO₂ inclusions. The chemical composition of volcanics with secondary porosity shows differences in SiO₂, Al₂O₃, TiO₂, P₂O₅, Cl, and Ba compared to other studied rock types. Volcanics with perlite structure exhibit the highest filtration-capacitive properties. The study revealed the heterogeneous action of hydrothermal fluids, manifesting differently in the near-vent zone of the paleovolcano and in its distal parts.
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