Origin and Classification of Pores in Mudstones from Shale-Gas Systems

Loucks, Robert G.¹; Ruppel, Stephen ¹; Reed, Robert M.¹; Hammes, Ursula ¹; Zahm, Christopher¹
(1) Bureau of Economic Geology, The University of Texas at Austin, Austin, TX.

Pore networks in mudstones of shale-gas systems are variable and complex. A spectrum of pore types has been identified on the basis of analyzing a number of shale-gas systems, including the Devonian Woodford Shale in the Permian Basin, Mississippian Barnett Shale in the Fort Worth Basin, Pennsylvanian Atoka Shale in the Permian Basin, Jurassic Haynesville and Bossier Shales in East Texas Basin, Lower Cretaceous Pearsall Shale in southwest Texas, and Upper Cretaceous Eagle Ford Shale in south Texas. Each shale-gas system has its own characteristic combination of pore types, depending on the mineralogy, texture, and fabric of the mudstone. The pores were analyzed using Ar-ion milled samples that provide extremely flat surfaces and viewed using an field scanning election microscope that allowed recognition of pores as fine as 2 nanometers.

Pore sizes seen in the analyzed suite of 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 and are very common in shallow buried muds. These pores make up the primary pore system that is generally connected. Interparticle pores occurring between grains are reduced in size during burial by compaction and/or cementation. Intraparticle pores can be primary or secondary pores, but they occur within a discrete particle, such as in pyrite framboids, porous phosphate particles, 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 and may form a connected pore network.

Pores observed in mudstones suggest that a pore network may have one dominant pore type or a complex combination. Mudstones from the Barnett Shale have a pore network dominated by organic-matter intraparticle pores, whereas the Pearsall Shale appears to have a pore network dominated by interparticle and intraparticle pores. Organic-matter pores and interparticle pores have a better probability to be connected and form a permeable pathway than isolated intraparticle pores.

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.