Influence of multiphase carbonate cementations on the Eocene delta sandstones of the Bohai Bay Basin, China

Abstract

This study deciphers the origin and distribution of multiphase carbonate cementations and assesses their implications for reservoir quality of the Eocene delta hydrocarbon sandstones in the Bohai Bay Basin. Petrographic observation and stable isotopic analysis identified two phases of carbonate cementations: (1) eodiagenetic phase including non-ferroan calcite cement precipitated at 25–42 °C (δ13CVPDB: 1.0‰ to +5.7‰ and δ18OVPDB: 10.7‰ to −7.5‰); and (2) mesodiagenetic phase which includes ferroan calcite cement (δ13CVPDB: 11.8‰ to −5.7‰ and δ18OVPDB: 17.0‰ to −12.6‰) and ankerite cement (δ13CVPDB: 13.7‰ to −5.9‰ and δ18OVPDB: 17.1‰ to −12.4‰) precipitated at 90–134 °C. The eodiagenetic non-ferroan calcite cement that is mainly sourced from microbial methanogenesis of organic matter in adjoining mudrocks, extensively cemented at or near sandstone-mudstone contact. Mesodiagenetic ferroan carbonate cements are derived from predominantly thermal decarboxylation of organic acids and subordinate dissolution of eodiagenetic calcite cements in sandstones. These ferroan carbonates are preferentially concentrated at distances of 0.5–1.0 m from sandstone-mudstone contacts. Kinetic modelling reproduces evolution pathways of different diagenetic minerals in the Es4s sandstones (the upper part of the fourth member of the Eocene Shahejie Formation). More importantly, results of modelling reveal that the dissolution of non-ferroan calcite cements would not enhance total reservoir quality due to the precipitation of more stable ferroan calcite and ankerite cements. Therefore, the occurrence of multi-phase carbonate cementations at different sections of sandstones leads to significant destruction of porosity and reservoir heterogeneity during progressive diagenesis.