The results obtained during 1999ś drilling plan in Loma La Lata field, related to the bitumen and the reservoir behavior through new information coming from production logs, NMR, and multipolar acoustic logs, formation tester, open hole completions, core analysis, pre-existent geochemical studies and detailed diagenetic descriptions; allow considering the postulate of bitumen generation related to an early migration from Los Molles Formation like more consistent. This bitumen has a wide and regional distribution and its influence should be contemplated in all the units older than Vaca Muerta Formation. It is considered convenient logs that register the effective porosity to the correction of the conventional ones by some distribution law. In this sense the density of bitumen seems to be sufficiently high as for not affecting the NMR tools. The porosity of Sierras Blancas Formation is secondary and generated in a second phase of dissolution, being the permeability controlled by rock type and abundance of pore bodies; acquiring importance again the magnetic resonance whose permeabilities and porosities show adjusted values with the petrophysics measurements. Some layers of the reservoir could behave as tight gas, and the gas would be flowing to wells for channels of high permeabilities in the main layer of the field. These variations in the units would be related with an intra-Tordillo unconformity and they demand the fracture of the same ones to accelerate their production.