PORE SYSTEM MULTISCALE HETEROGENEITY IN ORGANIC-BEARING MUDROCKS

Heterogeneity of pore systems in organic-bearing mudrocks is present at several scales. Variability is pronounced not just between different formations, but also between closely spaced samples from within units and even spatially within individual samples.

Pores have been examined on argon-ion-milled surfaces, which provide a relatively artifact-free area for observation in the scanning electron microscope. Numerous mudrock samples have been studied, and units include the Mississippian Barnett Shale, the Jurassic Haynesville Shale, the Cretaceous Eagle Ford Shale, the Cretaceous Pearsall Shale, the Devonian Woodford Shale, the Devonian-Mississippian New Albany Shale, and the Devonian Marcellus Shale. Some mudrocks have few or no pores, but many samples contain a significant number of pores. Pores in thermally mature mudrocks are typically at the nanometer to micrometer scale. Pores can be divided into three general categories: intragranular, intergranular, and organic matter. Different mudrocks have different mixtures of these three types. Attempts at quantification have shown there is heterogeneity of pore development between different areas on a scale of tens of micrometers. Mudrock composition seems to be the dominant influence on pore development, closely followed by thermal maturity.

Pore-system heterogeneity has strong implications for characterization of mudrocks. Upscaling of pore quantification will be complicated by heterogeneity on a broad range of scales. Attempts to model flow in mudrocks will require complex models to approximate these heterogeneities.