Further research on chlorite rims in sandstones: evidence from the Triassic Yanchang Formation in the Ordos basin, China

摘要

Chlorite rims are the most common type of authigenic chlorite in sandstones, and attract particular attention because they are always found in high-quality sandstone reservoirs. This study researches chlorite rims in sandstones of the Chang 9 oil-bearing member of the Triassic Yanchang Formation in the Ordos basin, China. Using casting thin sections, a scanning electron microscope and porosity-permeability data, the following issues were addressed: (1) the formation stage of chlorite rims, (2) the relationship of chlorite rimformation to other diagenetic features, and (3) the relationship of chlorite rims with sandstone reservoir quality. From the results, the following conclusions can be drawn: (1) chlorite rims form during the A-B phase of eodiagenesis, (2) chlorite rims cannot enhance the mechanical strength of rocks and their ability to resist compaction, (3) the reason that chlorite rims inhibit quartz secondary enlargement is a change in pH and silica concentration leading to the cessation of quartz growth. However, chlorite rims have no relationship with the formation of later idiomorphic authigenic quartz; (4) chlorite rims cannot enhance the reservoir quality of sandstone, but can be an indicator of high primary intergranular porosity.