Abstract Geological models for the effects of clay diagenesis on shale/mudstone properties are under evaluation for their use in predicting the distribution of hydrocarbon occurrences, and risk of biodegradation. Traditional clay petrology studies show that the onset of reactions, often described as smectite to illite diagenesis, begins at 60° to 80°C in sedimentary sequences. Petrophysical models based on the precipitation of diagenetic clay within shale pore systems indicate rapid and severe permeability reduction at the onset of this reaction. Evaluation of global petroleum systems in a variety of basin settings, including the North Sea and the Gulf of Mexico, indicate an exponential increase in probability of formation fluid overpressure and hydrocarbon occurrences at and above these temperatures, along with concurrent reduction in the risk of biodegradation of reservoired petroleum. The mechanism by which these risks are controlled in both cases can be related to clay diagenetic induced permeability reduction, which when combined with diagenetic porosity reduction models and geological factors restricting lateral drainage, increases the probability of vertical hydrocarbon migration via hydrofracturing of low-permeability shale units to create new petroleum systems/plays. The industry fails to appreciate the exponential increase in exploration risks as a function of reservoir temperature and often drills beyond the optimal entrapment zone of 60° to 120°C, with corresponding reductions in efficiency. Hydrocarbons which migrate to shallower depths and lower temperatures show an increased risk of biodegradation. The proposed migration mechanism is consistent with the distribution of global conventional oil resources at the basin scale. Geologically, however, spill-fill/lateral migration, mainly in foreland basin settings, has resulted in extensive heavy oil/tar sands deposits, which eluded entrapment at optimal conditions for recovery.

Abstract At temperatures higher than c . 80 °C, thermally driven, isochemical diagenetic porosity loss in siliciclastic sediments leads to a thinning of the sediment column. This process, termed thermochemical compaction, results in surface subsidence and generation of sediment accommodation space. The diagenetic reactions driving thermochemical compaction will operate regardless of the initial mechanism of basin formation and in addition to any externally controlled processes causing continued subsidence. Thermo-chemical subsidence rates are an inverse function of geothermal gradients. The total rates of subsidence at the surface, including isostatic and mechanical compaction effects, may reach several tens of metres per million years, and may have been important in driving Tertiary subsidence in basins west of Ireland. Unlike the exponentially decaying subsidence caused by tectonic thinning and rifting of the crust, thermochemical basin subsidence is a selfregulated intrabasinal process, which proceeds at a relatively high constant rate over geological time. If not arrested by extrabasinal or tectonic events, the overall effect can ultimately result in sediment metamorphism and granitization.