Genesis mechanism of laumontite cement and its impact on the reservoir of siliciclastic rock: A case study of Jurassic Shaximiao Formation in central Sichuan Basin, China

Abstract

Laumontite cement's genesis and impacts on reservoir quality in sedimentary basins have long been studied in sedimentology and petroleum geology. This study aims to analyze the petrological and geochemical properties of laumontite in the Shaximiao Formation, Sichuan Basin, China, and to determine the evolutionary process behind the formation of laumontite. Approximately 175 million years ago, huge volume of volcanic materials filled the Sichuan Basin. During the burial diagenesis phase, the hydrolysis of volcanic particles and alteration of plagioclase provided the necessary ions—Ca2+, Al3+, Si4+, Fe3+—facilitating the formation of laumontite. Simultaneously, the hydrolysis of volcanic substances released ions like Ca2+, Mg2+, and Na+, elevating the PH of pore fluids, and fostering the requisite alkaline conditions for laumontite. Additionally, the dehydration of smectite to illite in nearby mudstones generated Ca2+-rich fluids, migrating into sandstones. The hydrolysis of volcanic particles combined with the Ca2+-rich fluids migrating into sandstones led to the abundant formation of laumontite within temperatures ranging from 60 to 100 °C. During this phase, the formation of laumontite accelerates the densification of the reservoir, leading to a reduction in porosity. In the meso-diagenetic period, organic acids from the underlying formations penetrate the sandstones through fractures, creating secondary pores and dissolution-related grain-edge fractures, significantly enhancing reservoir permeability.