Morphology, Distribution, and Genesis of Nanometer-Scale Pores in the Mississippian Barnett Shale

Authors: R. M. Reed, R. G. Loucks, D. M. Jarvie, and S. C. Ruppel

Venue: 2008 American Association of Petroleum Geologists (AAPG) Annual Convention and Exhibition, San Antonio, TX, April 19-24, 2008 oral session chaired by S. C. Ruppel and R. G. Loucks (http://www.aapg.org)

Abstract: 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.

Related NETL Project
This presentation is related to the NETL project DE-FC26-04NT15509, “Integrated Synthesis of the Permian Basin: Data and Models for Recovering Existing and Undiscovered Oil Resources from the Largest Oil-Bearing Basin in the United States.” The objectives of the project are twofold: (1) to produce a detailed, comprehensive analysis and history of Paleozoic depositional and reservoir systems in the Permian Basin, and (2) to create spatially integrated databases of depositional, stratigraphic, lithologic, and petrophysical properties for selected reservoir plays and stratigraphic horizons. These objectives will be undertaken and completed sequentially during the 3 years of the project. The overall objective is to provide Permian Basin operators with (a) outcrop and subsurface reservoir specific data, data syntheses, and models to be applied to geological-, engineering-, and completion-based redevelopment of existing reservoirs, and (b) a detailed regional stratigraphic framework for applying such models to new exploration targets.

Project Contacts
NETL – Virginia (Ginny) Weyland (Virginia.WEYLAND@netl.doe.gov or 918-699-2041)
University of Texas at Austin – Stephen Ruppel (stephen.ruppel@beg.utexas.edu or 512-471-1534)