Porosity evolution during in-situ conversion process of low-maturity shale in Songliao Basin

The low-maturity shale oil and gas resources exhibit vast reserves and tremendous development potential, particularly in Songliao Basin with resources up to 323 600 Mt. However, current research on pore size and morphology evolution during in-situ heating of low-maturity shale in northern Songliao Basin remains insufficient. Taking the low-maturity shale in northern Songliao Basin as object, this study employs thermal simulation experiment combined with CT scanning, high-pressure mercury injection, low-temperature nitrogen adsorption, and scanning electron microscopy (SEM) to investigate the pore development characteristics during heating process, and analyze the evolution process and mechanism. The results show that at 350-375℃, the organic matter undergoes significant pyrolysis, generating abundant gas pores and organic pores; at 400-500℃, pyrolysis approaches completion, developing extensive intergranular pores and microfractures, with increased porosity and average pore size. The total pore volume and average pore diameter display a synchronized increase-decrease-increase trend. The pore morphology is dominated by slit-shaped pores with narrow openings at 0℃ and 300℃, transformed to slit-shaped pores with wide openings at 350℃, primarily open-ended cylindrical pores at 375℃, and reverting to slit-shaped pores with wide openings at 400-500℃.