An extensive evaluation of organic source facies, the application of hydrocarbon geochemistry, and integrated basin modeling of the southwest portion of the Maracaibo basin in Colombia (referred to as the Catatumbo subbasin) indicate (1) multiple organic-rich horizons occur within-the Cretaceous and Paleocene sections, (2) most of the reservoired oil and gas was sourced locally from Cretaceous marine carbonate facies by means of lateral and vertical migration, (3) two subfamilies of Cretaceous oils are recognized that reflect different source facies within the Cretaceous section and different maturation and migration histories, (4) oil and gas present in the southern Catatumbo subbasin indicate a contribution from Paleocene terrestrial source facies that required westerly migration from a source area in Venezuela, possibly within the North Andean foredeep, and (5) oil generation from Cretaceous source rocks began in the Oligocene, and peak generation occurred in the late Miocene. These key conclusions are based on source rock analyses of 365 rock samples from eight wells, gas chromatography/mass spectrometry, and isotope and bulk composition analyses from nine rock extracts and seven oil samples.