BSR and TSR occur at different depths. BSR takes place in low-temperature environments, which are shallower settings such as oil and gas fields. BSR can also take place in modern marine sedimentary environments such as inland seas, continental shelves, organic-rich deltas, and hydrothermal sediments which have intense microbial sulfate reduction because of the high concentration of dissolved sulfate in the seawater. Additionally, the high amounts of hydrogen sulfide found in oil in gas fields is thought to arise from the oxidation of petroleum hydrocarbons by sulfate. Such reactions are known to occur by microbial processes but it is generally accepted that TSR is responsible for the bulk of these reactions, especially in deep or hot reservoirs. Thus, TSR occurs in deep reservoirs where the temperatures are much higher. BSR is geologically instantaneous in most geologic settings, while TSR occurs at rates in the order of hundreds of thousands of years. Although much slower than BSR, even TSR appears to be a geologically fairly fast process.