Devitrification-driven pore formation in the tight tuff from the Tiaohu formation in the Santanghu Basin, Northwest China

The formation of oil-bearing pores in tight tuff has attracted considerable attention from petroleum geologists since the discovery of industrial oil.Devitrification may be an important cause for the formation of these pores; however, the relevant geological circumstance for devitrification still remains unclear. This study tries to decipher the formation of devitrification pores in the tight tuff in the Tiaohu Formation,the Santanghu Basin, Xinjiang, NW China. The result shows that the oil-bearing pore size in tuff is mainly in the range of micrometers to a few nanometers, and the porosity is mainly distributed between 0.10% and 26.71%; the permeability is mainly distributed between 0.17 and 1.20 mD. After high-temperature soaking, the oil-bearing tight tuff illustrated devitrification under both acidic and alkaline circumstances, with glassy tuff showing the greatest variation in porosity, followed by crystal pyroclast glassy tuff, while the mudstone tuff and silicified tuff show relatively small variations in porosity. The 140 °C threshold marks the optimal thermal window for devitrification-driven porosity: it coincides with the smectite-illite transition and the main hydrocarbon-generation stage in the Santanghu Basin. Porosity in all tuff varieties peaks at this temperature, recording a net gain of up to 16.31%; above 140 °C, porosity declines progressively. Devitrification proceeds in three successive stages: (1) neo-mineral nucleation, (2) metasomatic replacement, and (3) dissolution.