Occurrence, composition, and origin of analcime in sedimentary rocks of non-marine petroliferous basins in China

Abstract Natural analcime, a highly brittle aluminosilicate mineral that dissolves easily during diagenesis, occurs in the sedimentary rocks of eight main non-marine petroliferous basins in China. This mineral has been well documented for the past 30 years and has proved to be significant for the formation of high-quality hydrocarbon reservoirs. The present study fully reviews analcime occurrences, chemical compositions, Si/Al ratios, origin and controlling factors, and influence on the hydrocarbon reservoir quality in Permian to Paleogene sedimentary rocks. Previous research has shown that analcime is commonly distributed within argillaceous laminae (l-type); occurs as intergranular cement (c-type), replacement (r-type), and crack filling (f-type) forms; or is closely intergrown with other authigenic/exhalative minerals (a-type). Analcime in lacustrine basins is slightly Al-rich and Na-poor, and its Si/Al ratios range from 1.95 to 2.80. The geochemical compositions of the analcime- and ankerite-bearing exhalites are characterised by enrichment in light rare earth elements and 13C and depletion in heavy rare earth elements and 18O, with negative Eu and positive Ce anomalies. Diagenetic analcime forms by the alteration of volcanic glass, clays, or zeolite precursors in saline-alkaline lakes or during buried diagenetic stages in sedimentary rocks. In addition, it coexists with other authigenic zeolite, carbonate, and clay minerals as well as albite. Hydrothermal analcime presumably precipitates from hydrothermal fluids and is not dependent on a local precursor mineral or volcanic glass; this type is commonly related to exhalative mineralisation of hydrothermal springs. The formation of natural analcime is controlled by various factors such as the composition of the parent material, temperature prevalent during crystallisation, chemistry of the fluid phase, and thermodynamic characteristics of the geological environment. As zeolites are formed in alkaline fluid phases, they become unstable in acidic pore water. Secondary pores created by analcime mineral dissolution, and intercrystalline micropores among analcime minerals, are highly significant in hydrocarbon reservoirs. Analcime-bearing mudstone, siltstone, dolostone, and exhalite are favourable lithofacies for unconventional hydrocarbon accumulation owing to its high brittleness and selective dissolution of analcime and other associated minerals.