Shengli Oilfield, China as a natural analogue of CO₂ storage: Diagenetic fluid evolution mode and potentials for carbon sequestration

Abstract

The Shengli Oilfield is discussed to investigate CO2 trapping mechanisms involved in geothermal oil reservoir systems. Diagenetic fluid evolution of the dawsonite‐bearing sandstones from the Dongying Sag, Bohai Bay Basin, China, was studied to understand the CO2 migration and accumulation and the formation of dawsonite, as a possible mechanism for mineral trapping of CO2 in geothermal fields. Information about the petrology, dawsonite‐forming fluid, authigenic mineral associations, and diagenetic fluid evolution mode was analysed using techniques including thin‐section identification, scanning electron microscope observation, X‐ray diffraction analysis, X‐ray fluorescence spectrum analysis, hydrochemistry, and stable isotope analyses. The paragenetic sequence, from early to late, is early calcite (A)–compaction–dissolution–quartz overgrowth–late calcite (B1)–ferrocalcite (A)–dawsonite–late calcite (B2)–ferrocalcite (B)–ankerite–siderite or pyrite. The current in situ formation water type of the dawsonite‐bearing sandstone strata is NaHCO3, with the typical characteristics of weak alkaline, high mineralization, and high Na+ + K+ and HCO3− content. The formation fluid from which the dawsonite forms is mainly sealed underground brine, which has been affected by atmospheric precipitation and magmatic activity, and is thus mixed with surface meteoric water and subsurface hydrothermal fluid. The fluid evolution process sequence is alkaline fluid environment prior to CO2 influx, an early micritic or sparry calcite (A) association; acidic fluid environment after CO2 influx, a quartz overgrowth and kaolinite association; alkalescent fluid environment, an association of late calcite (B1), ferrocalcite (A), and illite; alkaline fluid environment, a dawsonite and late calcite (B2) association; and weak‐alkaline fluid environment, an association of ferrocalcite (B), ankerite, siderite, and pyrite.