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Sem-Based Observation of Micro- and Nanopores in Mudrocks Using Argon-Ion-Beam Milled Samples

Reed, Robert M.1; Loucks, Robert 1; Milliken, Kitty 1
1 Bureau of Economic Geology, University of Texas at Austin, Austin, TX.

Micro- and nanopores are important but poorly characterized features of most mudrocks. Some commonly used sample-preparation types can create artifacts that lead to ambiguity in interpretation. For example, holes in a polished thin section may be either naturally occurring pores or the result of plucking, gouging, or other mechanical damage. Argon-ion-beam milling produces flat surfaces on mudrock samples that are free of damage related to differential hardness and sample friability at micro- to nanoscale. When observed using a scanning electron microscope (SEM), micro- and nanopores seen on such surfaces can reliably be interpreted as naturally occurring pores and not artifacts of sample preparation.

Mudrock samples from a variety of depths, thermal maturities, and lithologies have been milled and examined. An impressive array of different pore sizes, shapes, locales, and formation mechanisms have been observed. Primary intergranular pores are most common in samples that have undergone only shallow burial and limited diagenesis. Intergranular pores are rare to absent in more thermally mature samples, which generally have had more extensive diagenetic reactions. Carbonaceous grains can show abundant intragranular pores, but development of these pores appears to be controlled by both grain composition and thermal maturity. Intragranular pores are also noted in some noncarbonaceous grains, and show a range of morphologies, suggesting that more than one formation mechanism exists. Because difficulty in determining whether particular components are detrital or diagenetic complicates pore nomenclature, some pores are best described as intercomponent or intracomponent. Microfractures containing open pores, which have commonly been postulated as an explanation for measured permeabilities of mudrocks, have only rarely been observed, despite extensive searching.


AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009