Print this pageAAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Pore Types in the Barnett and Woodford Gas Shales: Contribution to Understanding Gas Storage and Migration Pathways in Fine-Grained Rocks
(1) Geology and Geophysics, University of Oklahoma, Norman, OK.
(2) Geology, State University of New York, Potsdam, NY.
The identification of ‘organo-porosity’ (micro- and nano-scale pores within organic matter in shales), its importance to storage and perhaps transfer of gas molecules through shales, and methods for gathering 3D images of the pores [most notably through argon-ion milling/field emission scanning electron microscopy (FESEM) ]have all been well documented and discussed for unconventional gas shales, most notably the Barnett Shale of north Texas. However, there are other types of pores within the Barnett and Woodford, and probably other shales which provide both porosity and permeability. Also, there are other techniques for preparing, imaging and identifying these pores.
Floccules are common within these shales; spaces between the flocculated particles can provide significant---possibly interconnected---pores. Fecal pellets, with up to 15% porosity, are also common, and often concentrated in discrete laminae or beds which might provide pore connectivity. Fossil fragments such as sponge spicules are also common shale constituents; spicules originally have hollow central chambers, but many of these become closed during burial either by silica diagenesis or by filling with clay and organic particles. Microchannels within shale matrix, which may be the bounding surfaces of scours or micro-sedimentary structures, serve to provide permeability pathways for hydrocarbon migration. Fractures occur at a variety of scales, including across individual mineral grains, and across brittle laminae, beds, bedsets and stratal packages.
When present in sufficient quantity, these pore types offer potential gas (and oil) molecule storage spaces and permeability pathways through the shales. In addition, the occurrence of fractures which cross-cut brittle beds such as chert, but not interbeds of ductile shale, suggest that artificial fractures in subsurface reservoirs probably hold proppant in the brittle beds, but less so in interbeds of ductile shale.