The recognition, correlation, and quantification of oil mixtures remain challenging in petroleum system studies. Most prolific basins have multiple source rocks that generate petroleum over wide ranges of maturity. Compound-specific isotopic analyses of alkanes (CSIA-A) and diamondoids (CSIA-D) are very effective for determining hydrocarbon mixtures. Quantitative diamondoid analysis (QDA) and CSIA-D provide a unique advantage for source correlation of thermally altered liquids or condensates and for condensate mixtures with black oil. Biomarker fingerprints, QDA, and various CSIA methods were applied to 37 oil and condensate samples to investigate the existence of deep sources and to identify and deconvolute cosourced oil mixtures. The data were used to unravel the components of mixed oil having widely diverse levels of maturity in the north–central West Siberian basin. Three oil families and their locations are recognized in the basin. One of the families appears to be composed of oil mixtures derived from two end-member families that originated from the Upper Jurassic Bazhenov and Lower to Middle Jurassic Tyumen source rocks. Our results suggest that a significant part of the gas in the giant gas fields of north–central western Siberia (e.g., Urengoi and Yamburg) is of thermogenic origin. The source of this thermal gas, which was formerly assigned to various source origins, was determined to be the Tyumen Formation. Some samples in the basin also show mixtures of noncracked Bazhenov oil with cracked Tyumen condensate. The area where prevalent oil cracking has occurred was determined from QDA.