Abstract

Pore system and matrix constituents of the Eagle Ford Shale are described from 86 representative samples from southwestern and eastern Texas. Argon-ion-milled samples were imaged using a field emission scanning electron microscope (FE-SEM) to document matrix constituents and pores. Samples were chosen based on lithology, x-ray diffraction mineralogy, matrix porosity, and effective permeability measurements. The Eagle Ford Shale is characterized by a complex pore system, reflecting variations in depositional environment, composition, and diagenesis, resulting in heterogeneous reservoir properties. Interparticle pores are the most common nonorganic pore type and typically occur between coccolith fragments. Limestone intervals contain less organic matter and may be recrystallized. The porosity and effective permeability associated with intercrystalline and interparticle pores within limestone intervals is strongly controlled by the degree of recrystallization. As the degree of recrystallization increases, porosity and permeability decrease. Organic nanopores, when present, are most abundant and display the best-developed pore systems within fecal pellets exhibiting open fabric and in organic-matter-filled interparticle pores between coccolith fragments in the matrix. Organic nanopores are also observed in organic-filled foraminifer chambers and commonly occur in conjunction with intracrystalline pores between authigenic clays, which form localized complex pore systems. Although there appears to be a relationship between the abundance of organic nanopores, predominant pore type, pore connectivity, and effective permeability, common vertical and lateral variability in the presence and abundance of organic porosity, even in similar microfacies, suggests that the parameters controlling organic nanopore development are not fully understood and deserve additional detailed studies. Regional differences in pore abundance observed along the Texas Eagle Ford Shale trend are the result of variations in depositional environments and clastic (clay) input. In general, the Eagle Ford in southwestern Texas has higher quality matrix properties because of the higher abundance of interparticle and intraparticle pores present in marlstones compared to lower reservoir quality associated by the more clay-rich Eagle Ford and Maness shales in east Texas.