A Scanning Electron Microscope Study of Porosity in the Eagle Ford Shale of Texas

The Eagle Ford Shale accumulated in intra-shelf basins at relatively low latitudes on a southeast facing margin during early part of the late Cretaceous. A scanning electron microscope study on drill-core samples from LaSalle County was conducted to understand the development of petrographic features and porosity.

Samples were argon ion-milled and examined with a field emission scanning electron microscope. Because organic matter is liable to develop additional pores due to heating by the argon beam, extensive testing was undertaken with variable beam intensities and sample-stage temperatures to identify potential milling artifacts and to adopt procedures to minimize them. Ion milling artifacts to consider include shrinkage cracks in organic matter and clays, chemical interactions at pyrite-organic matter contacts, and enlargement and coalescence of pores in organic matter.

The bulk of the visible porosity is found in organic-carbon-rich matter (OM) in framework pores, in the mineral matrix, and in fossil tests. The OM shows an abundance of very small pores in the 10-50 nm range that give it a "foam-like" texture, and superimposed on this "foam" are larger pores on the 100-1000 nm size range. That the latter are interconnected can be demonstrated with stacks of successive ion milled slices, and careful study of the "foam" pores strongly suggests that these are interconnected as well.

These samples also showed several different types of pores that collectively amount to several percent of "SEM-visible" pores (>10 nm). The bulk of the Eagle Ford Shale comprises mainly calcareous fecal pellets in a more silicate-rich groundmass (clays, quartz, etc.). The fecal pellets consist of biogenic carbonate debris that delineates framework pores that in turn may be filled by OM. The groundmass contains a mixture of pores that are either defined by carbonate grains, or by phyllosilicate frameworks (that also can contain OM).

Foraminifera and calcispheres are abundant; they are either randomly scattered due to bioturbation, or are concentrated into beds by bottom currents. The open spaces in these fossil tests are filled with authigenic clays, calcite, and OM. Authigenic clay fills define phyllosilicate framework pores that are filled with OM.
 

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California