Constrains of eruption environment and hydrothermal fluid on the Permian pyroclastic reservoirs in the Sichuan Basin, SW China

Abstract

A breakthrough has been recently made in the hydrocarbon exploration of the Permian pyroclastic rocks in the Jianyang Area, western Sichuan Basin, China. With an aim to decipher the impacts of the eruption environment, the temperature of hydrothermal fluids, and the paleo-salinity on the formation of authigenic mineral assemblage and pores, this study implements comprehensive petrographic and geochemical studies through the integrated core and thin section analyses. The data presented demonstrate that the Permian volcanic intervals are intensively affected by an event of Emei taphrogeny. During basaltic magma upward migration, fractional crystallization of anorthose results in slightly-alkaline magma. The specific pyroclastic rocks are formed by the eruption of slightly-alkaline magma in the sea or a salt lake and subsequent hydrothermal alteration. During deposition and diagenesis, the authigenic mineral association is constrained jointly by the sodium-rich and high salinity water environment, and mid-high temperature, high-salinity hydrothermal fluid. Specifically, the sodium-rich hydrothermal fluid, which may sustain till the late diagenesis stage, caused pervasive albitization of pyroclastic rocks, then leading to mineral transformation and formation of a series of mineral associations. Therefore, zeolitization of volcanic glass and vesicle-infillings of zeolite is an essential condition for later mineral transformation and dissolution. Albitization of analcite, recrystallization induced by deep hydrothermal fluids, and both meteoric and deep burial dissolution expanding the micro-pore space ultimately formed porous pyroclastic reservoirs.