Structural characteristics and thermal evolution of nanoporosity in shales

Key words:  shale  nanoporosity  pore type  thermal evolution

Abstract:

Samples from the lower Paleozoic marine shales and Mesozoic continental shale in the middle and upper Yangtze region were taken to investigate the structural characteristics and the thermal evolution of nanoporosity in shales using argon-ion milling technology, field emission scanning electron microscopy (FE-SEM) and low-pressure N₂ adsorption experiments. The results indicate that there are four major pore types in the shales, including organic nanopores, interparticle pore, intraparticle pores as well as microfractures. TOC and Ro are the key factors controlling the development of nanoporosity volumes. For high thermal evolution shales, the order of the development extent of organic nanopores in different types of kerogen is type Ⅰ>type Ⅱ>type Ⅲ. Quartz and clay minerals have indirect effects on the development and distributions of nanoporosity by controlling changes in TOC. Pore-size distributions and relative proportions of micropores, mesopores, and macropores are affected by thermal maturity evolution. During the high-over mature stage of shale, micropores and mesopores associated with organic matter have continuously increased with the raising thermal maturity. Under the condition of extremely high thermal maturity, the macroporous in the shales are transformed into mesopores and micropores, and the organic nanopores decrease, causing nanopore volume to be displayed a decreasing trend after the first increase with increasing thermal maturity. Thermal cracking gas and methanation of kerogen and soluble bitumen in shales can be the main reasons for organic nanoporosity formation.