Morphology, Distribution, and Genesis of Nanometer-Scale Pores in the Mississippian Barnett Shale
Robert M. Reed1, Robert G. Loucks1, Daniel M. Jarvie2, and Stephen C. Ruppel1
1Bureau of Economic Geology, University of Texas at Austin, Austin, TX
2Humble Geochemical Services, Humble Instruments & Services, Inc., Humble, TX
The Mississippian Barnett Shale from the Fort Worth Basin of Texas predominantly consists of black, clay-mineral-poor, calcareous and siliceous mudstones. Siliceous mudstones from two wells have been examined to characterize pores. A few primarily intragranular micropores >500 nm in diameter are present but they are isolated and numerically insignificant. Nanometer-scale pores (nanopores) are the dominant pore type. Use of Ar-ion-beam milling provides surfaces without topography related to differential hardness which are suitable for examination of nanopores.
Nanopores are primarily found in three locales within the samples. Carbonaceous grains host the majority of nanopores with many of these grains containing hundreds. Other nanopores are found in bedding-parallel wisps of largely organic matrix material. The remaining, less common, nanopore locale is within extremely fine-grained matrix areas not associated with organic material. Intragranular nanopores tend to be larger, and less regularly shaped than intergranular nanopores; which tend to be more elliptical to elongate, smaller, and less complexly shaped.
At least two distinct morphologies of nanopores have formed in carbonaceous grains. In one type, nanopores are more elliptical and do not have a clear distribution pattern. In the other, nanopores are more rectilinear and form parallel linear arrays. This latter pattern may be controlled by original structure in the grains. Median pore diameters vary from grain to grain, but a typical diameter is ~100 nm. Intragranular porosities up to 20.15% have been measured. All intragranular nanopores in carbonaceous grains may result from devolatilization of the organic material during hydrocarbon maturation.
AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas