ORGANIC-MATTER-HOSTED PORES IN MUDROCKS: HETEROGENEITY OF MORPHOLOGY AND MICROSCALE SPATIAL DISTRIBUTION

Pores that are present in the organic matter of mudrocks are thought to play an important role in methane storage and flow. Samples have been examined from a number of units, including Mississippian Barnett Shale, Devonian Woodford Shale, Jurassic Haynesville Shale, Jurassic Bossier Shale, Mississippian-Devonian New Albany Shale and Cretaceous Eagle Ford Shale. Pore morphology and development are complex, and many questions remain about their evolution in organic-rich mudrocks. Observations suggest that pore development is driven primarily by the thermal maturation of organic matter, with pores beginning to form around vitrinite reflectance values of 0.8%.

Differences have been noted in both shape and distribution of pores within organic matter—spheroidal to elliptical outlines are present in some organic grains. Other organic matter grains show typically larger, more complex shapes that are perhaps the result of merging of simpler forms during pore development. Organic matter pore diameter varies from nanometers to hundreds of nanometers, with the more complexly shaped pores generally being larger. Pores with complex shapes appear to be more interconnected than those with simpler shapes. Some grains show linear arrangements of pores that appear to be related to either underlying original structure of the organic matter or phyllosilicate inclusions in the organic matter. Ratios of pores to organic matter vary from low to high in different grains. Most samples, even if thermally mature, contain at least some organic matter that does not develop pores.

Pore distribution and morphology in organic matter are more homogeneous in samples with disseminated intergranular webs of organic matter. In samples with more isolated, individual grains of organic matter, pore development is typically heterogeneous, with differences in morphology, distribution, and areal development occurring even between adjacent grains.