The Golfo San Jorge basin (GSJB) is one of the richest oil bearing basin of Argentine. Their petroleum systems are defined by cretaceous sequences, where the Neocomian is constituted by Pozo Anticlinal Aguada Bandera and Pozo Cerro Guadal formations, dominated by black shales (source rock) and sandstones (reservoirs) respectively. Over these formations the Chubut Group is developed (Barremian?-Aptian to Maastrichtian). These sedimentary record initiate with the lacustrine deposits of the Pozo D-129 Formation (source rock), and are covered with alluvial and fluvial deposits of the Castillo and Bajo Barreal formations (main reservoirs of the basin). This contribution evaluates the subsurface lithostratigraphy with traditional cutting observations and handheld x-ray fluorescence analysis. The presence of tuff in the cutting is revalidated as a robust criterion to determine the Chubut Group base showing important modifications from the traditional position of the lithostratrigraphic limit, in consideration of the neocomian strato-type logs calibration. Chemosequences were defined from Fe, K, Ca Ti and Zr concentration values. General trends were used for long distance wells correlation, showing the independence of the measurements from the tectonostratigraphic framework and the local subsidence history. The increase of K values before the occurrence of tuff cutting at the top of neocomian deposits suggests criptotefras deposits and alteration of volcanic ash after deposition in subaqueous palaeoenvironments. The mimic-style of Fe, Ti and Zr chemologs is interpreted as a geochemical signal of the basement erosion in the sedimentary source zones. The relative decrease of these elements at the limit of the cretaceous groups is associated with the diminution of the basement exposures zones during the Late Cretaceous times. The results of this study remark the impact of the reevaluation and calibration of the lithostratigraphy in the GSJB using cutting information and state of the art technology.