Controls on the Salinity of Sedimentary Basinal Fluids Under Constant Chemogravitational Potential Conditions

• Published in: Geochemistry, geophysics, geosystems : G3 Vol. 24; no. 3
• Main Author: Yoshimura, Shumpei
• Format: Journal Article
• Language: English
• Published: Washington John Wiley & Sons, Inc 01.03.2023; Wiley
• Subjects: Advective flow; chemogravitational potential; Depth; Distribution; fluid; Fluids; Halite; Hypotheses; Lithology; Natural resources; Salinity; Salinity variations; Sediment; sedimentary basin; Sedimentary basins; Sodium chloride; Solutes
• ISSN: 1525-2027; 1525-2027
• DOI: 10.1029/2022GC010628

Fluids in sedimentary basins exert a crucial influence on various geological phenomena including natural resource formation. Worldwide drilling projects have revealed that the salinity of sedimentary basinal fluids generally increases with depth, irrespective of lithology, age of sediments, or the presence of a halite bed. However, how these vertical salinity variations are produced and what controls the salinity remain unclear. This work examines a new hypothesis that downward‐increasing salinity variations are a natural outcome of the constant chemogravitational potential condition. In a static environment, the salinity is distributed such that the chemogravitational potential of the solute is constant with depth. Once formed, such a distribution would be maintained because no further migration of the solute would occur. To test the hypothesis, a constant chemogravitational potential distribution model was constructed for NaCl–H2O fluids in the sediment column, and NaCl content at each depth was calculated. The results showed that NaCl content monotonically increases with depth, and the variations are similar to the trend of measured data. However, the data were not necessarily completely reproduced by the model, and deviated in some parts from the calculated profile. Such deviation may indicate fluxing of external fluid occurring in these parts, as the constant chemogravitational potential is vulnerable to an advective flow. Therefore, it is proposed that the constant chemogravitational potential condition is a possible endmember theory, influencing natural salinity variations in a static environment. Key Points Vertical variations in salinity of sedimentary basinal fluid were simulated under constant chemogravitational potential conditions The simulation showed that the salinity increases with depth and the salinity gradient is positively dependent on the geotherm The constant chemogravitational potential distribution is roughly coincident with the data of natural sedimentary basinal fluids