Print this pageAAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Observations of Pores in the Cretaceous Eagle Ford Shale, South Texas, USA
(1) Bureau of Economic Geology, University of Texas, Austin, TX.
The Upper Cretaceous Eagle Ford Shale is an organic-rich, calcareous mudstone found in the Maverick Basin and adjoining areas of south Texas. The unit serves as a source, seal, and reservoir for hydrocarbon production.
More than 30 core samples from 9 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, such as plucking and gouging, which are related to mechanical polishing. Ion-milled surfaces greatly aid 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, intergranular pores dominating in most samples. However, in some samples, intragranular pores have also developed in organic matter and in grains. Calcareous globigerinid foraminifera with partly open and partly cemented chambers are a distinctive feature of this unit. Multiple pore-filling cements, including kaolinite, quartz, calcite, 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 formations, intergranular pores are still preserved in the Eagle Ford. Pores in organic matter are absent to rare in lower thermal maturity samples (VRo <0.8) but are generally well developed in higher thermal maturity samples, similar to what is observed in the Mississippian Barnett Shale of the Fort Worth Basin.