Porosity Evolution In Lacustrine Organic-Matter-Rich Shales With High Claly Minerals Content

Pore structure is an important factor affecting the storage space and oil-bearing property in shales. The influence of mineralogy, organic matter content, and thermal evolution complicates the pore structures of lacustrine shales. In this paper, the porosity evolution of organic-matter-rich shales from Cretaceous Nenjiang Formation in the Songliao basin were investigated based on the thermal simulation experiment and in-situ scanning electron microscope analysis. Four findings were obtained: (i) The pore system of shales from the Nenjiang Formation is dominated by inter-granular dissolution pores of plagioclase and intra-granular pores of illite-smectite mixed layers, and few organic-matter pores were observed. (ii) The newly developed pores are mainly organic matter pores and clay mineral pores, with diameter of greater than 18 nm. Clay mineral pores with diameter of 18 nm-50 nm are the main contributor from low maturity to oil-generation window, and organic matter pores with diameter of greater than 50 nm are the majority of generated pores from gas-generation window to high-/over-mature stage. (iii) The porosity increased continuously with maturity, and the pore system varies at different maturity stages. The porosity evolution is controlled by illite content and organic matter abundance. The total pore volume correlates positively with illite content but negatively with organic matter abundance. These findings could provide scientific guidance for the shale oil evaluation in the Songliao Basin, and may shed light on the sweet-spotting of lacustrine shale system in China.