Microtextural Properties of the Silt Fraction in Mudrocks

Abstract

Quantifying mudrock microtextural variations between basins with contrasting histories may help to differentiate the relative importance of transport, deposition, and diagenesis in mudrock evolution. To understand the history and physical properties of mudrocks requires data from the silt (2-63 μm) grain-size fraction. The volumetric content of silt grains in mudrocks reaches 40% or more and may partly control porosity. In order to examine specific microtextural qualities we have manually digitized the silt grains using scanning electron microscope (SEM) images with elemental X-ray maps, and quantified grain properties such as size, shape, and texture using Jmicrovision© software. We applied this approach to samples from: the Cretaceous Pearsall Formation of the Maverick Basin, the Mississippian Barnett Shale of the Fort Worth Basin, and the Devonian Marcellus Formation of the Appalachian Basin. All three formations are known hydrocarbon source rocks from predominantly siliciclastic systems that have been buried to depths that cover a range of diagenetic conditions. However, each sample set provides specific opportunities: (i) the Pearsall allows us to track changes from the up-dip to the down-dip section of the regional Gulf of Mexico strata, (ii) the Barnett allows us to contrast microtexture across differing porosities, and (iii) the Marcellus allows us to see how these mudrocks respond to substantial increases in temperatures. Moreover, in the Pearsall and Barnett formations, the preferred orientation of slightly elongate fine silt grains creates an anisotropy that dominates the fabric. In this 2D slice there are fewer observed grain contacts and more clay-size matrix. In contrast, preliminary results from the Marcellus appear to show more contacts between coarser silt particles, with only interstitial clay-size matrix. By employing the grain contact quantification methods previously applied to sandstones, we seek to present grain contacts in mudrocks as a scaling problem. Describing the frequency and nature of these contacts will further help to clarify the role of both mechanical and chemical diagenesis as they relate to external basin controls. We also attempt to establish a new 3 dimensional model that more accurately reflects the behavior of irregularly shaped silt grains rather than presenting them as smaller versions of closely packed spheres.

AAPG Datapages/Search and Discovery Article #90189 © 2014 AAPG Annual Convention and Exhibition, Houston, Texas, USA, April 6–9, 2014