The available data on the composition of the pyrolysis products of kerogen from the Mesozoic carbonaceous strata of the Russian Plate evidence that changes in the contents of total organic carbon (TOC) lead to a regular change of the mechanisms of organic-matter (OM) conservation in sediments. Each mechanism prevails for particular TOC contents. The initial increase in the TOC content of rocks is accounted for by the fact that the higher is the biologic productivity of the basin, the higher is the portion of nonmineralized organic matter. This is due mainly to the mechanism of selective accumulation of the most stable biochemical components such as algaenan. The appearance of H2S first in the pore waters of sediment and then in the water column increases the degree of preservation of initial OM at the expense of its sulfurization. This process runs first in the lipid and then in the carbohydrate fractions of initial OM.