The formation mechanism of laumontite and its effect on reservoir quality in Upper Triassic Chang-63 tight oil sandstones, Ordos Basin

Based on porosity-permeability measurements, thin-section petrography, SEM observations, and XRD analyses, the diagenetic features of sandstones from the Chang 63 reservoir were investigated. The results indicate that three distinct stages of laumontite cementation developed during burial. Laumontite initially precipitated as pore-filling cement during the syngenetic to early diagenetic stages, controlled by sedimentary environment, provenance, and feldspar albitisation. During mesodiagenesis, laumontite re-precipitated from alkaline pore fluids, with Ca2+, Na+, and SiO2 derived from dissolution of earlier cements and detrital minerals. Early laumontite cement inhibited compaction and helped preserve primary porosity, whereas its subsequent dissolution significantly enlarged pore spaces and improved pore-throat connectivity. The overall impact of laumontite on reservoir quality is therefore dual and strongly dependent on the intensity of dissolution processes. [Received: September 6, 2023; Accepted: December 1, 2024].