Early generation of oil from Monterey and equivalent source rocks may well be the primary controlling factor on oil quality in the coastal basins of California. Commercial generation has occurred at reflectance levels as low as 0.3% R₀. The wide range of chemical characteristics of these oils is comparable to source extracts from samples at low levels of organic metamorphism (LOM). At the same time, gross maturity parameters, such as extractable hydrocarbon concentration and hydrocarbon-to-organic-carbon ratios, indicate that sufficient petroleum has been generated to consider these source rocks as commercially generative. Oil quality is a function of both source facies and of relative level of maturity. Production from these basins is considerable and demonstr tes that sections with oil-prone kerogen can be commercially significant at levels of organic metamorphism much lower than the conventional threshold of 0.6-0.7% R₀. Evaluation of such areas should be based on direct chemical maturity parameters as well as LOM measurements.