Fracture-pore networks and brittle with ductile stress-strain mechanisms: Triaxial tests on >7,600 m samples yield insights for 10,000-m deep sandstones

Basins in western China produce hydrocarbons from 8,000 m deep and have been penetrated to 10,000 m, but the mechanical and petrophysical properties of deep and ultra-deep rocks are unclear and the origins of porosity and permeability remain a mystery. Our research used core samples from a depth of 7,600 m and mechanical tests to document the likely structural and porosity evolution of sandstone due to burial to 10,000 m. During triaxial tests, we characterized microstructure evolution using micro-CT scanning images and acoustic emissions and monitored stress and strain characteristics in high-temperature and high-pressure fluid environments. Under ultra deep-burial conditions, our samples deformed by pore collapse and pore distortion and brittle and ductile fracture, independently or concurrently. Under increasing triaxial stress, temperature and fluid pressure, sandstones initially lose porosity and permeability by pore collapse and compaction then develop a network of interconnected pores and fractures. Consequently, porosity can reach 8% to 18%, possibly accounting for fluid storage and flow capacity at depths of 10,000 m. Samples from 7,600 m lack substantial quartz, calcite cement and rapid burial for our samples and rocks at 10,000 m and quartz, calcite accumulation systematics suggests that though subject to temperatures of as much as 200 °C, porosity loss and gain in sandstones at 10,000 m may be primarily due to compaction. Our tests show that due to pore collapse and grain fracture, sandstones having high initial porosity and permeability have a greater increase of porosity and permeability due to loading.