Saturation evaluation for fine-grained sediments

Accurate quantification of the gas hydrate content in the deep sea is useful for assessing the resource potential and understanding the role of gas hydrates in the global carbon cycle. Resistivity logging data combined with Archie's equation are often used to calculate gas hydrate saturation, but the reliability is dependent on the rationality of the empirical parameter cementation factor and saturation index. At present, an increasing number of fine-grained hydrate-rich sediment regions have been discovered world-wide through drilling efforts, and the reservoir types and hydrate distribution are diverse, which differs greatly from that of coarse-grained reservoirs of hydrate-bearing sediment. This results in vertical variations in m and n through stratigraphy. At present, the saturation evaluation effect of these reservoirs cannot be improved. In this work, a theory for the determination of the cementation factor and saturation index was first proposed to obtain reliable and variable values of the empirical parameters. Then, a hydrate saturation evaluation technique with variables m and n was formed based on the well logging data. This technique was used to evaluate complex fine-grained hydrate-bearing reservoirs in several regions worldwide. It was found that the highest n could be 16, and the log calculation results were more consistent with the core hydrate saturation. Additionally, the cause of the excessively high n values was explained from physical principles, and the result was verified with actually well log data. In future eval-uations of the amount of hydrate resources in fine-grained sediment reservoirs worldwide, new satura-tion estimation methods should be taken into account to advance hydrate research. (c) 2023 China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V.This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/ licenses/by-nc-nd/4.0/).

Automated summary

Accurate quantification of gas hydrate content in the deep sea is essential for assessing resource potential and understanding its role in the global carbon cycle. Current methods relying on resistivity logging data and Archie's equation have limitations due to empirical parameter reliability. This study proposes a theory to determine cementation factor and saturation index, and develops a new technique for evaluating complex fine-grained hydrate-bearing reservoirs using well logging data. The results show improved consistency with core hydrate saturation, supporting the need for new saturation estimation methods in future evaluations of hydrate resources in fine-grained sediment reservoirs.