NANOPORES IN THE MISSISSIPPIAN BARNETT SHALE: DISTRIBUTION, MORPHOLOGY, AND POSSIBLE GENESIS

The Mississippian Barnett Shale from the Fort Worth Basin of north-central Texas predominantly consists of black, clay-mineral-poor, calcareous and siliceous mudstones. The unit is notable for forming a complete hydrocarbon system: source, seal, and reservoir. Siliceous mudstones from two wells have been examined to characterize pores; a task complicated by extremely small pore sizes.

A few intragranular and intergranular micropores >500 nm in diameter are present in the siliceous mudstone samples. These pores are commonly associated with shell material, siltier laminae, or areas of incomplete early cementation (quartz, calcite, or pyrite). These micropores are not common and do not appear to have good connectivity in siliceous mudstones, but appear more common in other lithologies.

Use of Ar-ion-beam milling provides a unique opportunity to examine smaller nanoscale pores in the Barnett Shale. Ion-beam milling provides a small low-relief surface without topography related to differential hardness seen with normal polishing. SEM imaging of this surface allows unambiguous identification of pores.

Nanopores are prevalent in three locales within the shale samples. Carbonaceous grains (possibly plant fragments) host the majority of nanopores, although not all of these grains have pores. Many other nanopores are found in bedding-parallel wisps of matrix material with high organic content. The other common locale is within matrix areas not closely associated with organic material. Inside carbonaceous grains, pores tend to be larger, and less regularly shaped than intergranular pores; which tend to be more elliptical to elongate, smaller, and less complexly shaped.

Porosity values ranging from 0.1 to 7.6% have been measured using mercury porosity testing on core plugs. SEM examination of ion-milled surfaces has not found pores sufficient to account for these measured values.

At least two distinct morphologies of nanopores are developed in carbonaceous grains. In one type, pores are more elliptical and do not have a clear distribution pattern. In the other, pores are more rectilinear and form parallel linear arrays. This latter pattern may be controlled by the original structure in the grains. Bubble-like holes in grains may result from devolatilization of the organic material during hydrocarbon maturation.