An analysis of the composition and amount of steranes, terpanes, alkanes, and metal porphyrins in crude oils from the Koltogor rift and its adjacent regions showed that these oils either had been generated predominantly by marine organic matter accumulated in suboxidizing and reducing environments or are due to the organic matter of lacustrine and coastal facies deposited in oxidizing, suboxidizing, and reducing environments. There are low-maturity and mature oils among these crudes, as well as oils thermally transformed to a substantial extent.