Four major factors control petroleum richness of a region: source rock, reservoir rock, seal, and trap. Assessment of undiscovered resources of oil and gas in poorly known regions should be based on comparative analysis of these factors in a forecast region and in a well-explored analog area. Three of these factors mainly reflect stratigraphic, rather than tectonic, conditions, and the fourth, the trap factor, includes both stratigraphic and tectonic aspects. The predominance of stratigraphic information in the factors indicates that the main unit used for petroleum resource assessment done by comparative geologic analysis should be a stratigraphic unit. Such a proposed unit, which is called an independent petroliferous system (IPS), is understood here as a body of rocks eparated from surrounding rocks by regional barriers to lateral and vertical migration of fluids, including oil and gas. Stratigraphically, an IPS is essentially homogeneous. It includes source rocks, reservoir rocks, traps, and a regional seal, and thus, it is a suitable unit for comparative analysis of the factors and petroleum genetic studies. For oil and gas resource assessment in poorly known regions, an IPS has certain advantages over a basin or play as an assessment unit. The concept of an IPS can also be used in statistical methods of resource appraisal and can increase reliability of these results.