부산대학교 도서관

The present study investigates the processes responsible for Middle Eocene dolomite occurrences in Kachchh Basin in the western Indian Continental Shelf. Three types of dolomites are identified. They exhibit euhedral to subhedral crystal boundaries and a core of calcite or amorphous organic matter. Non-stoichiometric dolomite is the major component, while calcite, pyrite, glauconite, palygorskite, chlorite and quartz are minor constituents. The presence of algal micritization, oxidized organic matter, framboidal pyrites and filamentous- to net-like microbial covering around dolomites suggests an organogenic origin. The studied dolomite sustained marine–phreatic, meteoric–phreatic and meteoric–vadose diagenesis. The marine–phreatic diagenesis is evidenced by algal micritization, pyritization, matrix-replacive early dolomitization and marine cementation; the meteoric–phreatic diagenesis by neomorphism or crystal coarsening, shell dissolution and pore-filling sparitic cement and the meteoric–vadose diagenesis by secondary porosity and permeability, partial filling of meteoric–vadose cements and ferruginization, reflecting regression of seawater. The δ18O (− 1 to 1.7‰ VPDB) values of the studied dolomites infer modified seawater as the diagenetic fluid for dolomitization. The δ18O-based diagenetic temperature varies between 18° and 32 °C. The diagenetic temperature, inhomogeneous texture and point to tangential contacts of allochems reflect a shallow burial origin. The extremely negative δ13C values (− 26 to − 39‰ VPDB) reflect mixing of normal marine carbon with a carbon reservoir strongly depleted in 13C, most likely oxidation of methane or biodegradation of crude oil. Methane would have been supplied through upward seepage of thermogenic methane from underlying hydrocarbon reservoir(s). [ABSTRACT FROM AUTHOR]