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Pore Types Across Thermal Maturity within the Eagle Ford Formation, South Texas


Scanning electron microscopy of Ar-ion milled samples shows that character and evolution of porosity is strongly affected by type, abundance and distribution of organic matter (OM) within the Eagle Ford Formation, South Texas. Samples were collected, imaged and quantified to provide insight into pore types and distributions across a range of thermal maturities. Low maturity samples contain pore networks dominated by relatively large coccolith-hosted primary intergranular pores with a mean equivalent circular diameter of ∼110 nm, ranging up to ∼2 μm. Primary intragranular pores are observed within coccolith fragments, coccolith-bearing fecal pellets, foraminifers, phosphate clasts, and other skeletal debris. OM-associated pores at low maturity are dominantly large pores at boundaries between organic matter and mineral surfaces with a mean equivalent circular diameter of ∼100 nm. Smaller pores within clay-associated OM are observed, with a mean equivalent circular diameter of ∼30 nm. In contrast, high-maturity samples show porosity dominated by secondary pores within OM. OM consisting of smaller equant pores grading into larger pores with more complex and irregular shapes. Measured OM-hosted pores range in equivalent circular diameter from ∼4 nm to ∼400 nm with a mean of ∼22 nm within high maturity samples. Mineral hosted pores are also present at higher maturities, many associated with clay minerals or dolomite, but are much smaller, with a mean equivalent circular diameter of ∼60 nm ranging up to ∼850 nm. In addition, fecal pellets and skeletal grains are observed to contain OM that pervasively fills intra-particle pore space, which suggest that porosity is reduced through incursion of mobilized bitumen. Both detrital kerogen and diagenetic bitumen are present at both high and low maturity, and cause porosity loss both through deformation of ductile kerogen with compaction, as well as incursion of primary pore space with mobilized diagenetic bitumen. As thermal maturation increases, bitumen is mobilized filling intraparticle pore space, and secondary pores develop within OM.