Petroleum system analysis: impact of shale diagenesis on reservoir fluid pressure, hydrocarbon migration, and biodegradation risks

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.