Advances in the understanding of the generating capacity of the Fm Pozo D-129: lithological and geochemical aspects. Golfo San Jorge Basin, Argentina The Pozo D-129 Formation is a lower Cretaceous lacustrine unit which is the main source rock in the Golfo San Jorge Basin. The unit thickness varies from a few hundreds of meters to more than 1500 meters in the basin center. Up to now Pozo D-129 Formation has been considered having an homogeneous generation capacity throughout the whole section in basin modeling and volumetric estimations. Exhaustive analyses of recently acquired and revisited geochemical and lithological data from 335 wells unravel a more complex scenario. In the San Bernardo fold belt area three main sections were recognized: Lower, Middle and Upper. The Upper Section is composed by organic matter rich black shales with measured TOC up to 5.5% and HI values typical of type I to I/II kerogens, which we interpret to have sourced most of the hydrocarbons produced in the basin. Secondarily, only thin and isolated black shale layers were identified in the Lower Section with poorer quality organic matter and modest hydrocarbon generation potential. Finally, the Middle Section, despite of being the thickest, shows null potential for hydrocarbon generation. The Upper Section was also recognized in northern and southern flanks of the basin with some slightly different hydrocarbon generation potential related to variations in the organic composition we summarize in this article.