The identification and characterization of source rocks, represented by their organic richness, quality, distribution, and maturity, are important for evaluating the petroleum systems in a sedimentary basin. The geochemical characterization of the source rocks is accomplished by the analysis and evaluation of their contained organic carbon. Used parameters (total organic carbon, free hydrocarbon (S₁), potential hydrocarbon (S₂), maximum temperature (T_max), hydrogen index (HI), oxygen index (OI), production index (PI), etc.) are derived from pyrolysis studies of outcrop, cuttings, and core samples. Other tools are gas chromatography-mass spectrometry (GC-MS), isotopes of specific compounds, and carbon isotopes of surface seep material, oil, and gas that define source rock affinity, depositional environment, and maturity level. The following reional source rock systems have been identified in the southeastern basins of Mexico: Upper Jurassic Oxfordian (!), Upper Jurassic Tithonian (!) argillaceous facies (JS1), Upper Jurassic Tithonian (!) calcareous-argillaceous facies (JS), Lower Cretaceous (!) carbonate-evaporite, and lower Miocene (!) marine deltaic siliciclastic.