Abstract:

Microscopic pore structures and their primary controlling factors on the Lower Paleozoic Longmaxi Shale in the upper Yangtze area were investigated using a fieldemission scanning electron microscope, highpressure mercury intrusion, lowtemperature nitrogen adsorption and carbon dioxide adsorption. Pore morphology and pore size distributions from macropores to micropores were successfully characterized. Combined with the geochemical parameters and mineral composition, the factors influencing the nanoscale pore structure were analyzed. The results indicate that the pores in the shale reservoirs are generally nanoscopic and can be classified into four types: organic pores, intraparticle pores, interparticle pores and microfractures, of which the most common are organic nanopores and interparticle pores between clay particles. The combination of the highpressure mercury intrusion, lowtemperature nitrogen adsorption and carbon dioxide adsorption curves enabled the description of the pore size distributions for micro, meso, and macroporosity. The pore size distributions are multimodal including not only predominant mesopores (250 nm) but also a certain amount of micropores (<2 nm). The nanoscale pores primarily consist of slitshaped pores with parallel plates and inkbottle type pores. Micropores and macropores account for most of the pore volume, whereas those with a diameter less than 5 nm account for most of the specific surface area. The TOC and clay mineral contents are the primary factors controlling the nanoscale pore structure characteristics, whereas micropores and macropores are mainly controlled by the TOC content. Mesopores and macropores are primarily determined by the content of clay minerals and brittle minerals, respectively.

Key words: Longmaxi Formation, pore morphology, pore size distribution, main controlling factors