Differences In Nanopore Development Related to Thermal Maturity In the Mississippian Barnett Shale: Preliminary Results

The Mississippian Barnett Shale from the Fort Worth Basin of north-central Texas consists predominantly of dark-colored calcareous and siliceous mudstones. Siliceous mudstones from a range of thermal maturities and burial depths have been examined in order to characterize pores, particularly nanometer-scale pores. Ar-ion-beam milling provides a low-relief surface lacking both topography related to differential hardness and surface damage that occur with mechanical polishing. SEM imaging of ion-milled surfaces allows unambiguous identification of pores down to the nanometer scale.

Samples examined thus far can be divided into two broad groups. One group is the higher thermal maturity samples, those currently buried in the range of 1,500 to 2,200 m and having vitrinite reflectances of 1.1% to 2.0%. The other category is relatively low thermal maturity samples, those currently buried less than 500 m. Vitrinite reflectance is less well constrained for these samples, but given regional trends, it should be less than 0.7% and possibly less than 0.5%.

More mature samples show well-developed nanopores concentrated in micron-scale carbonaceous grains. Large numbers of subelliptical to rectangular nanopores are present, and porosities within individual grains of as much as 20% have been observed. Shallowly buried, lower thermal maturity samples, in contrast, show few or no pores within carbonaceous grains.

These observations are consistent with decomposition of organic matter during hydrocarbon maturation being responsible for the intragranular nanopores found in carbonaceous grains of higher maturity samples. As organic matter (kerogen) is converted to hydrocarbons, nanopores are created to contain the liquids and gases. With continued thermal maturation, pores grow and may form into networks. The specific thermal maturity level at which nanopore development begins has not been determined. However, current observations support nanopore formation being tied to the onset of conversion of kerogen to hydrocarbons.