The most important element of the structure of hydrocarbon resources is their distribution by accumulations of different sizes. The formalism of this direction is developed in detail. Estimates of the resource structure of the largest oil and gas provinces of Russia are carried out. However, the lack of a substantiated theoretical model for the formation of the distribution of accumulations by size gives rise to a number of problems, including practical ones. This concerns the forecast of the number of small accumulations and hydrocarbon resources concentrated in them in well-studied oil and gas systems, the forecast of distribution for poorly studied systems, and the identification of the relationship between distribution parameters and the geological characteristics of the oil and gas system. The explanation of some empirically established regularities, in particular, the stable nature of the value of the distribution parameter ~ 2, is of independent theoretical interest. The paper presents the current state of this complex of problems and some possible directions for their solution.