In this topic, we investigate the dynamics of petroleum fluid compositional evolution, thermal or biologic alteration, and retention - expulsion behaviour. A significant change in scale, for example where basin scale processes overlap with biogeochemical activity, is the main distinction as compared to the other topics we investigate. Within petroleum reservoirs we investigate the mechanisms of biological degradation of hydrocarbons, and the factors responsible for causing physical and chemical heterogeneities that are manifested as discrete production compartments. Within petroleum source rocks we analyse their properties with respect to their capacity for generating, expelling, or retaining petroleum. A central multidisciplinary project GASH focuses on the investigation of unconventional natural gas resources in the impermeable mudstone horizons of Europe, or “gas shales”.