In the Sonda de Campeche offshore region of Mexico, the Tithonian sedimentary sequence is the most important source of hydrocarbons that occur today in Paleocene traps. The maturity of both source rocks and petroleum in reservoirs is known to increase from northeast to southwest across the region. This is manifested at the molecular level and in bulk petroleum properties such as API gravity and gas-oil ratio (GOR). We have analyzed a selection of source-rock samples from across the area, covering the entire maturity spectrum, by pyrolysis gas chromatography. These data give insights into the GOR of yet-to-be-generated petroleum for each maturity stage. A mass-balance model based on these same data and complementary data from laboratory experiments (MSSV pyrolysis) provided cumulative GOR as a function of generation stage (transformation ratio). Regional field GOR trends are consistent with instantaneous rather than cumulative GOR predictions, thereby supporting the notion of mainly localized vertical migration avenues in association with a late timing of trap formation.