Abstract
Natural analcime, an aluminosilicate mineral with multiple genetic mechanisms, widely occurs in fine-grained sedimentary reservoirs rich in oil and gas. Researchers have discussed the source and formation mechanism of reservoirs and the influence of morphology formation, occurrence characteristics associated with minerals, geochemical data, and Si/Al ratio on reservoir properties. The occurrence location, particle size, automorphism, purity, and fracture development can indicate the source of analcime macroscopically. The correlation between the enrichment of associated minerals and the content of analcime indicates that the associated mineral assemblage or correlation provides a material source for the formation of analcime or effectively improves the formation environment. Geochemical data are often used to identify analcime related to primary magmatic crystallization and hydrothermal processes. The genetic source grouping scheme based on the Si/Al ratio, which is a traditional means to identify the source of analcime, has been widely used in the research on analcime. After more than 200 years of study, research has shown that analcime distributed in fine-grained sedimentary rocks was mainly formed by burial alteration of volcanic materials (V-type analcime), conversion of nontuffaceous materials (N-type analcime), hydrothermal deposition mineralization (H-type analcime), and precipitation directly from an alkaline lake or pore water (P-type analcime). Based on reservoir properties, analcime that formed before the organic acid release stage of source rocks can effectively improve the porosity through precipitation–dissolution mechanisms after the release of massive organic acid, whereas the cementation formed by the transition of the fluid from acid to alkaline during the intermediate diagenetic stage would reduce porosity to some extent.
Highlights
In terms of analcime, an important natural aluminosilicate mineral, Bradley published an article in the journal “Science” and recorded authigenic analcime from tuff and oil shale in the Eocene Green River Formation of North America [1]
Domestic and foreign scholars began to focus on transforming the reservoir by analcime during diagenetic evolution to explore and discuss the effects of improving the porosity from both positive and negative perspectives. e previous work is systematically summarized and typical research cases are compared to supplement the analysis of the occurrence characteristics, formation mechanism of analcime in fine-grained sedimentary reservoirs, and changes in reservoir properties caused by analcime
Analcime is a sodium-rich aluminosilicate mineral widely distributed in sedimentary rocks, which has an ideal chemical formula of Na16Al16Si32O96∙16H2O [2]. e structure of analcime is equiaxially homogeneous, hexagonal, or octagonal and colorless with low-negative projections under single-plane polarized light and shows full extinction under orthogonal light. e crystal structure comprises silica–aluminate lattice, channels, voids, and cations [30]
Summary
An important natural aluminosilicate mineral, Bradley published an article in the journal “Science” and recorded authigenic analcime from tuff and oil shale in the Eocene Green River Formation of North America [1]. Discussion on the occurrence, source, and formation mechanism of analcime has continued for nearly a century since that report Various zeolite minerals such as mordenite, clinoptilolite, natrolite, analcime, heulandite, and laumontite have been discovered in low-grade metamorphic and sedimentary rocks [2–5]. Analcime is widely distributed in tuff, biochemical rocks, and clay rocks, with various occurrence forms, sources, and multiple formation mechanisms. Based on these characteristics, the coexistence of different types of analcime is common under the influence of complex sedimentation, burial, and diagenesis. In sedimentary areas lacking pyroclastic material, analcime is mainly formed via three mechanisms: (1) conversion of nontuffaceous materials (clay, feldspar, and alkaline zeolite) [12, 16–20]; (2) hydrothermal deposition mineralization [21–25]; and (3) precipitation directly from an alkaline lake or pore water [7, 26–28]. Domestic and foreign scholars began to focus on transforming the reservoir by analcime during diagenetic evolution to explore and discuss the effects of improving the porosity from both positive and negative perspectives. e previous work is systematically summarized and typical research cases are compared to supplement the analysis of the occurrence characteristics, formation mechanism of analcime in fine-grained sedimentary reservoirs, and changes in reservoir properties caused by analcime
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