Two series of samples, grain series (GS) and block series (BS), with different particle size (between 0.5-1.0 mm and 1.5 x 1.5 x 5.0 cm) and rock geometry (irregular and prismatic, respectively), were obtained from an oil shale with Type II kerogen from Cantabria (Northern Spain). Both series were pyrolyzed at different temperatures between 50 and 500-degrees-C by the open-medium pyrolysis system. Petrological and geochemical analyses were carried out to determine the organic transformation level, to characterize the resulting products and to establish a relation of maturation between the two series. Results show that the main processes of organic diagenesis occurred at a temperature interval ranging from 250 to 450-degrees-C in both series. However, differences in evolution parameters show that the diagenetic processes do not occur in the same way in both series as they depend on the physico-morphological characteristics of the rock that is being pyrolyzed. This is reflected in the greater level of oil production and expulsion during the pyrolysis process that is more important in BS than in GS. Expulsion seems to be favoured by the fissure system developed during the thermal evolution in BS which does not occur in samples with smaller particle size. At the same time, in BS a greater retention of some polar components (asphaltenes) leading to a differential impregnation of vitrinites is produced. This explains the differences in reflectance found between both series after the primary cracking. The results of this study confirm and supplement previous work about the importance of confinement in pyrolysis experiments aiming to reproduce sedimentary organic diagenesis. In addition to qualitative and quantitative variations in kerogen degradation products, the present study conducted on rock samples shows that the confinement also enhances oil expulsion through cracks and fissures developed in the rock in the course of heating. Although these results are probably not directly applicable to natural series, they help to interpret the origin of discrepancies between evolution parameters which appear to be primarily related to confinement.