Authorities have so far charged 14 of those arrested after uncovering a complex, cross-border plot to steal the oil.
They have seized at least one oil tanker and millions of dollars in cash.
Court documents listed 18 dates between 24 July 2017 and 7 January 2018 when tens of thousands of tonnes of oil were stolen.
Most of the thefts from Shell's Pulau Bukom site, which is just 3.4 miles from Singapore's main island, took place during the refinery's normal working hours.
Eleven Singaporeans and three Vietnamese men were the latest to be charged on Thursday, with several granted bail.