Characteristics, genesis, and significance of laumontite in sedimentary rocks of non-marine petroliferous basins in China

Abstract

Laumontite is a calcium-rich zeolite that typically occurs as an authigenic mineral in sedimentary rocks. Due to its specific formation conditions and its high instability in acidic environments, laumontite provides a key geological indicator for analysing sedimentary-diagenetic system and identifying high-quality reservoirs in middle-deep strata. This study offers a comprehensive review of the distribution, occurrence, chemical composition, genesis, controlling factors, and the impacts on hydrocarbon reservoirs of laumontite in the sedimentary rocks of typical non-marine basins in China, such as the Junggar Basin, the Ordos Basin, and the Sichuan Basin. Previous research indicates that laumontite commonly develops as a continuous or patchy cement, fracture filling, and replacement product within vertically stacked deltaic subaqueous distributary channels. Sandstones enriched in plagioclase or volcanic material are considered ideal host rocks for laumontite formation. During eodiagenesis (<85 °C), laumontite extensively fills intergranular pores, with individual crystals typically exhibiting a long prismatic habit, and commonly occurring in association with clay minerals, quartz, and heulandite. During mesodiagenesis (85–175 °C), laumontite occurs as patchy intergranular fillings, with individual crystals progressively developing a short prismatic morphology, and is mainly associated with calcite, illite, quartz, and albite. Laumontite in sedimentary rocks is typically characterised by a low Si/Al ratio (2.00–2.20), and its chemical composition shows no systematic variation with temperature or occurrence. Formation mechanisms of laumontite include the albitisation of plagioclase, transformation of volcanic material, and alteration of early-formed zeolites. Incomplete transformation of plagioclase may result in a higher Si/Al ratio in laumontite. Fluid inclusion homogenisation temperature data indicates laumontite in sedimentary rocks primarily forms within a temperature range of 60–140 °C. The transformation of plagioclase and volcanic material to laumontite can proceed throughout this interval, while alteration of heulandite to laumontite generally requires temperatures above 90 °C. High pH, low pCO₂ and Ca-rich pore fluids are key factors controlling laumontite formation, while the presence of Na⁺ lowers the equilibrium temperature of laumontite-forming reactions. Although early-formed laumontite occupies primary pores, it also contributes to compaction resistance. Owing to its well-developed cleavage and large internal pore volume, laumontite dissolves more readily in acidic fluids than K-feldspar, albite, or other aluminosilicates. Its dissolution zone provides a favourable environment for hydrocarbon accumulation. Additionally, the internal cavities within laumontite crystals possess a large specific surface area, which enables strong physical adsorption of methane molecules.