Abstract:

Pore structural analysis of the Longmaxi Formation shale in Laifeng (Hubei) was conducted based on geochemistry, petrophysics, and combined experiments on high-pressure mercury injection and low temperature N₂ adsorption-desorption. The Longmaxi Formation shale is over-matured with 0.41% to 2.35% TOC (total organic carbon) and 3.09% to 3.42% R_o (vitrinite reflectance). Mineral contents include mainly quartz (29.9% to 49.3%) and clay minerals (17.8% to 49.6%). Major pore types include inorganic inter-/intra-granular pores, micro-cracks and organic pores. The combined experiments show that the pore size is mainly micro-/meso-porous, pore throat is small and complex, and that the pore morphology is mainly fine-necked wide-body ink bottle shape. Based on the FHH model, the low relative pressure (P/P₀≤0.5) fractal dimension D₁ (2.73 to 2.76, average 2.74) and the high relative pressure (P/P₀>0.5) D₂ (2.80 to 2.89, average 2.85) are obtained. By establishing the relationship between fractal dimension and other pore structure parameters, it is concluded that fractal dimension can characterize the pore size, pore complexity and reservoir property, which can be an effective pore structure characterization parameter. Our results suggest that the Longmaxi Formation shale has high gas adsorption capacity, but the pore structure is complex and the connectivity is poor, yielding high requirements for reservoir transformation measures. Further research shows that the fractal dimension is positively correlated with the contents of organic matter, clay minerals and pyrite, but negatively correlated with the quartz content. Since the organic matter of Longmaxi shale in the study area is over-mature, its adsorption capacity is reduced, and the control of organic matter abundance on the shale adsorption capacity is unobvious.

Key words: Hubei, Longmaxi Formation shale, pore structure, high-pressure Hg injection experiment, fractal dimension, low temperature N₂ adsorption-desorption experiment