Shiyou shiyan dizhi (May 2020)
Early diagenesis characteristics of biogenic opal and its influence on porosity and pore network evolution of siliceous shale
Abstract
Opaline siliceous shale from the Nenjiang Formation in the Songliao Basin and siliceous shale from the Upper Ordovician Wufeng Formation and Lower Silurian Longmaxi Formation in the eastern Sichuan Basin were selected to study the diagenetic evolution of biogenic siliceous shale and the characteristics of shale physical properties and pore structure changes during this process. X-ray diffraction, helium porosity, nitrogen adsorption, and high-pressure mercury intrusion were used to analyze mineral composition, total porosity, and pore structure characteristics. The dehydration and recrystallization of the biogenic opal occurred early, and the transition to quasi-crystalline opal CT and crystalline quartz was completed at the early diagenetic stage. During the conversion of opal-A to opal-CT, the total shale porosity decreased rapidly from more than 75% to around 30%. During the conversion to quartz, the rate of pore loss decreased rapidly, and the decrease was only about 5%, showing two stages. At the same time, the pore volume distribution of different types of pores also changed significantly. The loss of macropores was larger than the loss of micropores. The composition of pores gradually changed from macropores and mesopores to mesopores and micropores. In the early diagenetic stage of siliceous shale, the mechanical compaction and pressure solution occurred synchronously and had a strong effect on shale transformation, which reduced shale porosity, increased hardness, and enhanced the support and resistance to compaction, reducing the transformation and destruction of the early to middle and subsequent diagenesis. The characteristics of rapid diagenesis of siliceous shale in the early stage of biogenesis are the important controls for maintaining high porosity in the middle and late stages of diagenesis.
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