In-situ thermal simulation experiment of single organic matter pore evolution in shale

Abstract

Organic matter pores is the most important shale reservoir space in South China, and the types of organic matter influence the formation and evolution process of organic matter pores. In order to achieve in-situ observation of the pore evolution process of single organic matter and reveal the pore evolution process during the pyrolysis of the macerals of the Lower Paleozoic shale, this paper focuses on the low-maturity Upper Ordovician shale from Ohio, USA and the Lower Silurian shale in the eastern Baltic Sea, Europe. By using femtosecond laser, cooling-heating stage, microscope and SEM techniques, the in-situ observation of pore evolution of the single organic matters in shale can be realized, and the pore development process and evolution law of different types of organic matters can be distinguished. The in-situ observation results of pore thermal evolution process of low-maturity "gas-prone" graptolite and "oil-prone" lamalginite show that: (1) The initial temperature of obvious volume change of graptolite is higher than that of lamalginite, which infers that the hydrocarbon generation of graptolite is later than that of lamalginite. (2) Both graptolite and lamalginite generated obvious shrinkage cracks during thermal evolution process, but the scale of shrinkage cracks of graptolite is smaller than that of lamalginite, indicating that the hydrocarbon-generating transformation ratio of graptolite is lower than that of lamalginite. (3) There is an obvious difference between graptolite and lamalginite in the evolution of internal pores: the intrinsical biopores of graptolite were enlarged and some new internal pores were formed during the high temperature stage, but no new internal pores of lamalginite were observed during the whole process of thermal evolution, which infers that "gas-generating window" is the main stage of organic matter pore development. Moreover, the difference in the composition and structure of organic matters may be the main reason for the difference in pore evolution of graptolite and lamalginite.

Keywords

• single organic matter
• pore evolution
• in-situ observation
• thermal simulation experiment