NANO- AND MICRO-PORES IN THE CRETACEOUS EAGLE FORD SHALE, SOUTH TEXAS, USA
More than 25 core samples from 8 wells were examined using a field-emission scanning electron microscope. Samples were prepared using Ar-ion milling—a technique that provides a flat surface free of artifacts related to mechanical polishing—greatly aiding identification of pores down to the nanometer scale. Sample depths range from 1,414.6 to 4,213.9 m. Sample thermal maturities range from 0.5% to 2.2% calculated vitrinite reflectance.
The Eagle Ford shows a variety of nanometer- and micron-scale pores. The dominant pore type is intergranular. In some samples there are also intragranular pores developed in organic matter and in grains. Calcareous globigerinid forams with partly open and partly cemented chambers are a distinctive feature of this unit. Multiple pore-filling cements, including quartz, calcite, kaolinite, pyrite, chlorite, and bitumen, indicate a complex diagenetic history for these mudrocks.
Research on the Eagle Ford is ongoing, but several trends are already evident. Unlike some deeply buried mudrock successions, intergranular pores remain preserved in the Eagle Ford. Pores in organic matter are rare to absent in the lower thermal-maturity samples (VRo <0.8) but are generally well developed at higher thermal maturity, similar to what is observed in the Mississippian Barnett Shale of the Fort Worth Basin.