The evaluation of deep-ultra deep oil and gas preservation conditions should concern not only physical loss but also chemical loss, such as the oxidation of hydrocarbons into non-hydrocarbon gases. Thermochemical sulfate reduction (TSR) commonly occurs in gypsum-bearing carbonate strata. There are five main factors controlling TSR, which are paleo-oil reservoir, gypsolyte, formation water, high temperature and porous carbonate reservoir respectively. Hydrocarbon content and dry gas coefficient, non-hydrocarbon content and acidic gas index, carbon and sulfur isotopic compositions are three kinds of TSR intensity index. To a certain extent, they can qualitatively reflect the intensity of TSR and be used for qualitatively evaluating the chemical loss of hydrocarbons, but they can't meet the needs of the quantitative evaluation. Therefore, it is necessary to establish a new method of TSR hydrocarbon loss evaluation for the deep-ultra deep marine sequences.