Organic pore heterogeneity and its formation mechanisms: Insights from the Lower Cretaceous lacustrine Shahezi shale in the Songliao Basin, NE China

Abstract

Pores associated with organic matter are well known to play a significant role in shale gas capacities. However, an extremely high heterogeneity of organic pores often impacts our evaluation of reservoir quality. In this work, we analyze the formation mechanisms of the heterogeneity based on positioning observation method using a combination of field emission scanning electron microscopy and optical microscopy. These analyses were conducted on six lacustrine shale samples at the gas window in the Lower Cretaceous Shahezi shale, which is located in the Changling Fault Depression of Songliao Basin. The results reveal that organic pore heterogeneity is mainly attributed to four controlling factors. (a) One is different hydrocarbon generation potentials among different macerals. The degree of pore development from high to low is solid bitumen, vitrinite, and inertinite. The content of carbon by the weight percentage of solid bitumen, vitrinite, and inertinite is in the opposite order, which reflects that the different hydrocarbon generation potential of each maceral is the dominant factor. (b) Another one is the remnants of primary pores in organic matter with plant cell structures. Well preserved telinite, fusinite, and semi-fusinite show cell structures, and the cells that are not completely compressed or not fully filled retain the original residual pores. (c) The third one is evolutional differences of individual solid bitumen. Not all solid bitumen developed organic pores, which is mainly attributed to the difference of solid bitumen reflectance in different solid bitumen particles. The solid bitumen reflectance of porous solid bitumen is mostly distributed between 1.6% and 2.0%, in which oil cracking to gas is dominant and porous residual solid bitumen subsequently forms. The solid bitumen reflectance of non-porous solid bitumen peaks in 1.2–1.6%, which is in the stage of kerogen transformation and oil generation with rare pore development. (d) The last one is the catalysis of clay minerals. All organoclay complexes develop abundant sponge-like pores due to catalysis during the transformation from smectite to illite. A high content of illite in the mixed layers I/S increases the specific catalytic activities, promoting the organic matter and solid bitumen to further generate hydrocarbon and form pores. Most organic–inorganic mixtures develop pores also because of catalysis from inorganic minerals.

Pore Connectivity of Deep Lacustrine Shale and its Effect on Gas‐bearing Characteristics in the Songliao Basin: Implications from Continental Scientific Drilling

“…Pores are crucial for the adsorption and storage of fluid in shale (Zhang et al, 2020;Gao et al, 2022). The type, structure, size, distribution and connectivity of pores have a significant impact on the gas-bearing characteristics of shale reservoirs and the migration of oil and gas resources (Zhang S L et al, 2018;Ma et al, 2019;Yu et al, 2021;Gao et al, 2022).…”

“…Pores are crucial for the adsorption and storage of fluid in shale (Zhang et al, 2020;Gao et al, 2022). The type, structure, size, distribution and connectivity of pores have a significant impact on the gas-bearing characteristics of shale reservoirs and the migration of oil and gas resources (Zhang S L et al, 2018;Ma et al, 2019;Yu et al, 2021;Gao et al, 2022). Porosity, permeability, specific surface area (SSA), total pore volume (TPV), and average pore diameter (APD), which characterize the reservoir and pore structure, are typically used to evaluate shale gas resource potential, exploration, and development (Zhao et al, 2017;Gao et al, 2022;Li et al, 2022).…”

“…The type, structure, size, distribution and connectivity of pores have a significant impact on the gas-bearing characteristics of shale reservoirs and the migration of oil and gas resources (Zhang S L et al, 2018;Ma et al, 2019;Yu et al, 2021;Gao et al, 2022). Porosity, permeability, specific surface area (SSA), total pore volume (TPV), and average pore diameter (APD), which characterize the reservoir and pore structure, are typically used to evaluate shale gas resource potential, exploration, and development (Zhao et al, 2017;Gao et al, 2022;Li et al, 2022). Many experimental methods have been used to analyze the pore types and structural characteristics of different sizes, including thin sections, scanning electron microscopy (SEM), gas adsorption, mercury injection, computed tomography, and small-angle neutron scattering (Loucks et al, 2009;Chalmers et al, 2012;Milliken et al, 2013;Sun et al, 2019;Liu X et al, 2020;Zhang J J et al, 2022).…”