Abstract: The high temperature/high pressure (HTHP) reactor of the present invention includes a main cylinder, a reversible piston inside the cylinder that divides the main cylinder into a pressurization chamber and a reactor chamber, a thermal jacket, a lid and an end cap. The HTHP reactor of the present invention is configured to facilitate microbial growth in the reactor chamber that provides high temperature and high pressure conditions that simulate resource reservoir conditions, wherein the resource may be an oil reservoir. The HTHP chamber of the present invention is configured to receive a fluid sample from the underground oil reservoir that has been maintained at reservoir temperature and pressure throughout sampling, transfer to transport bottle, transport to the laboratory, and inoculation into the chamber.