Thermal maturity is often considered as the main factors controlling the formation of OM-hosted pores in gas shale systems. Nonetheless, this process remains poorly understood and controversial. The use of artificial thermal maturation could be a key to better understand the formation of shale porosity during oil and gas production. However, the faster experiment with higher pyrolysis-temperature compared to the slower kinetic condition of natural systems may modified the chemical transformations of OM and thus porosity after maturation. The effect of these opposite experimental conditions on shale porosity need to be quantified. For this purpose, confined pyrolysis of immature organic-rich shales (Kimmeridge Clay, UK) were performed under two experimental conditions: 325°C during 3 days and 283°C during 104 days. Porosity after maturation was measured by low pressure nitrogen adsorption and evaluated as function of thermal maturity, OM properties (Rock Eval) and pyrolysis duration. The OM chemical transformations were investigated by GC/MS analysis of oil and gas generated during pyrolysis experiments. First results confirm that both experiments have reached same degree of thermal evolution: the beginning of the oil window. Samples matured during 104 days exhibit lower gas concentrations, higher production indexes and higher concentrations of hopanes, n-alkanes, steranes and aromatic steroids. A longer maturation time seems thus to favour a more efficient transformation of polar components into saturated and aromatic hydrocarbons (HC). Two major effects can be noted on porosity: (i) a less important filling of mineral-linked porosity by non-extractible organic components leading to a greater mesopore volume after maturation, (ii) the formation of fine mesopores (<10 nm) after maturation which do not exist for the shorter maturation time. Furthermore, the difference of HC concentrations between the two experiments, and the size of pores form after maturation seems strongly influenced by the OM quality. Experimental conditions of confined maturation and OM properties seems thus greatly influence the formation of shale porosity at the beginning of the oil widow.