Characterization of gas shale pore systems by porosimetry, pycnometry, surface area, and field emission scanning electron microscopy/transmission electron microscopy image analyses: Examples from the Barnett, Woodford, Haynesville, Marcellus, and Doig units

The nanometer-scaled pore systems of gas shale reservoirs were investigated from the Barnett, Marcellus, Woodford, and Haynesville gas shales in the United States and the Doig Formation of northeastern British Columbia, Canada. The purpose of this article is to provide awareness of the nature and variability in pore structures within gas shales and not to provide a representative evaluation on the previously mentioned North American reservoirs. To understand the pore system of these rocks, the total porosity, pore-size distribution, surface area, organic geochemistry, mineralogy, and image analyses by electron microscopy were performed. Total porosity from helium pycnometry ranges between 2.5 and 6.6%. Total organic carbon content ranges between 0.7 and 6.8 wt. %, and vitrinite reflectance measured between 1.45 and 2.37%. The gas shales in the United States are clay and quartz rich, with the Doig Formation samples being quartz and carbonate rich and clay poor. Higher porosity samples have higher values because of a greater abundance of mesopores compared with lower porosity samples. With decreasing total porosity, micropore volumes relatively increase whereas the sum of mesopores and macropore volumes decrease. Focused ion beam milling, field emission scanning electron microscopy, and transmission electron microscopy provide high-resolution (∼5 nm) images of pore distribution and geometries. Image analysis provides a visual appreciation of pore systems in gas shale reservoirs but is not a statistically valid method to evaluate gas shale reservoirs. Macropores and mesopores are observed as either intergranular porosity or are confined to kerogen-rich aggregates and show no preferred orientation or align parallel with the laminae of the shale. Networks of mesopores are observed to connect with the larger macropores within the kerogen-rich aggregates.

[1]  M. Lamberson,et al.  Coalbed Methane Characteristics of Gates Formation Coals, Northeastern British Columbia: Effect of Maceral Composition , 1993 .

[2]  Bernhard M. Krooss,et al.  Experimental characterisation of the hydrocarbon sealing efficiency of cap rocks , 1997 .

[3]  F. Javadpour.  Nanopores and Apparent Permeability of Gas Flow in Mudrocks (Shales and Siltstone) , 2009 .

[4]  R. Loucks,et al.  Morphology, Genesis, and Distribution of Nanometer-Scale Pores in Siliceous Mudstones of the Mississippian Barnett Shale , 2009 .

[5]  R. Sigal,et al.  Laboratory NMR Measurements on Methane Saturated Barnett Shale Samples , 2011 .

[6]  Philip H. Nelson,et al.  Pore-throat sizes in sandstones, tight sandstones, and shales , 2009 .

[7]  D. Dollimore,et al.  An improved method for the calculation of pore size distribution from adsorption data , 2007 .

[8]  R. Marc Bustin,et al.  Lower Cretaceous gas shales in northeastern British Columbia, Part I: geological controls on methane sorption capacity , 2008 .

[9]  R. Marc Bustin,et al.  The importance of shale composition and pore structure upon gas storage potential of shale gas reservoirs , 2009 .

[10]  Roger Wepf,et al.  3D geometry and topology of pore pathways in Opalinus clay: Implications for mass transport , 2011 .

[11]  G. Lu,et al.  Surface area development of sewage sludge during pyrolysis , 1995 .

[12]  R. Bustin,et al.  Characterizing the shale gas resource potential of Devonian–Mississippian strata in the Western Canada sedimentary basin: Application of an integrated formation evaluation , 2008 .

[13]  Paul Marschall,et al.  Characterisation of Gas Transport Properties of the Opalinus Clay, a Potential Host Rock Formation for Radioactive Waste Disposal , 2005 .

[14]  M. Mastalerz,et al.  Meso- and Micropore Characteristics of Coal Lithotypes: Implications for CO2 Adsorption , 2008 .

[15]  P. E. Potter,et al.  Mud and Mudstones: Introduction and Overview , 2005 .

[16]  Dennis Beliveau.  "Honey, I Shrunk the Pores!" , 1993 .

[17]  R. Marc Bustin,et al.  The organic matter distribution and methane capacity of the Lower Cretaceous strata of Northeastern British Columbia, Canada , 2007 .

[18]  R. Bustin,et al.  Shale Gas Potential of the Lower Jurassic Gordondale Member, Northeastern British Columbia, Canada , 2007 .

[19]  H. Marsh.  Adsorption methods to study microporosity in coals and carbons—a critique , 1987 .

[20]  Q. Fisher,et al.  U-Pb systematics of an Upper Carboniferous black shale from South Yorkshire, UK , 2003 .

[21]  D. Avnir,et al.  Recommendations for the characterization of porous solids (Technical Report) , 1994 .

[22]  Christopher R. Clarkson,et al.  Variation in micropore capacity and size distribution with composition in bituminous coal of the Western Canadian Sedimentary Basin: Implications for coalbed methane potential , 1996 .

[23]  R. Bustin,et al.  On the effects of petrographic composition on coalbed methane sorption , 2007 .

[24]  D. Jarvie,et al.  Unconventional shale-gas systems: The Mississippian Barnett Shale of north-central Texas as one model for thermogenic shale-gas assessment , 2007 .

[25]  P. Hall,et al.  Pore Structure of the Argonne Premium Coals , 1995 .

[26]  Robert L. Folk,et al.  Petrology of Sedimentary Rocks , 1974 .

[27]  P. Buseck,et al.  TEM investigation of Ca-rich plagioclase: Structural β uctuations related to the I1-P1 phase transition , 2007 .

[28]  E. Barrett,et al.  (CONTRIBUTION FROM THE MULTIPLE FELLOWSHIP OF BAUGH AND SONS COMPANY, MELLOX INSTITUTE) The Determination of Pore Volume and Area Distributions in Porous Substances. I. Computations from Nitrogen Isotherms , 1951 .

[29]  John F. Unsworth,et al.  Moisture in coal: 2. Maceral effects on pore structure , 1989 .

[30]  Hailiang Dong,et al.  Tem Observations of Coherent Stacking Relations in Smectite, I/S and Illite of Shales: Evidence for MacEwan Crystallites and Dominance of 2M1 Polytypism , 1996 .

[31]  H. Rietveld.  Line profiles of neutron powder-diffraction peaks for structure refinement , 1967 .

[32]  Philip L. Walker,et al.  Nature of the porosity in American coals , 1972 .