Print this pageAAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Spectrum of Pore Types in Siliceous Mudstones in Shale-Gas Systems
Robert
Loucks1; Stephen C. Ruppel1; Robert
M. Reed1; Ursula Hammes1
(1) University of Texas at Austin, Austin, TX.
Pore networks in siliceous mudstones of shale-gas systems are variable and complex. A spectrum of pore types has been identified on the basis of analysis of a number of shale-gas systems, including Devonian Woodford Shale, Mississippian Barnett Shale, Pennsylvanian Atoka Shale, Jurassic Haynesville/Bossier Shales, Lower Cretaceous Pearsall Shale, and Upper Cretaceous Eagle Ford Shale. Each shale-gas system has its own combination of pore types, depending on the mineralogy, texture, and fabric of the siliceous mudstone.
Pore sizes in the analyzed suite of siliceous mudstones range from approximately 5 nm to several microns. The pore types can be classified as (1) interparticle pores (between particles), (2) intraparticle pores (within discrete particle boundaries), and (3) organic-matter intraparticle pores. Primary interparticle pores between grains are related to original mudstone pore space. These pores make up the primary pore system that is generally connected. Interparticle pores occurring between soft grains are commonly compacted. Intraparticle pores can be primary or secondary pores, but they occur within a discrete particle, such as a pyrite framboid or a porous phosphate particle, or as molds of fossils, crystals, or grains (i.e., feldspars). Organic-matter intraparticle pores are related to thermal maturation of organic matter during hydrocarbon generation.
Pores observed in siliceous mudstones suggest that a pore network may have one dominant pore type or a complex combination. Mudstones from the Barnett Shale in the gas-producing area of the Fort Worth Basin have a pore network dominated by organic-matter intraparticle pores, whereas the Pearsall Shale appears to have a pore network dominated by intraparticle pores.
Many research questions remain regarding pore types and pore networks in siliceous mudrock shale-gas systems: (1) How do pores and pore networks evolve from initiation at the surface through burial? (2) Can porosity and permeability be predicted from mineralogy, temperature, pressure, and time as can be done with sandstones? (3) Is there a relationship between pore types and permeability, and how do we measure this relationship? (4) Do different types of organic matter produce different numbers of organic-matter intraparticle pores? (5) How accurate are our measurements of petrophysical properties of mudstones? (6) Does the preservation state (condition of the core sample) of the mudstone affect petrophysical measurements?