Tight Shale Heterogeneity and Pore Structure at the Nanometer to Centimeter Scale

Schieber, Juergen; Newhart, Richard; Green, Sidney; Suarez-Rivera, Roberto; Gathogo, Patrick; Petriello, John; Huster, William

Rock heterogeneity of tight shale reservoirs exists at all scales (regional/basin, wellbore, core, petrographic/pore). Understanding this heterogeneity at the petrographic scale (micron to sub-micron scale) is particularly critical for understanding pore structure, pore connectivity, and subsequently hydrocarbon movement within the rock. Because of the high magnification required to visualize particles and pores in tight shales, the field-of-view for each image is very small (50 to 100 microns wide), and little material can be inspected with each observation. Thus inferences are necessarily limited.

Visualizing pore structure and pore connectivity over a large cross-section (mm's to cm's) greatly enhances the understanding of heterogeneity, in particular with regard to interpretation of laboratory porosity and pressure decay or pulse decay permeability measurements. Examination of large cross-sections also provides insight into coring induced and/or natural micro-cracks.

To combine high magnification with large cross-section observations, we assembled 40 nanometer resolution scanning electron microscope images into 0.5 centimeter wide mosaics. Once compiled, these 16 giga-pixel mosaics allow visualizing petrographic and textural relationships at a previously not attainable breadth and resolution. Several tight shale mosaic images from Jurassic, Devonian and Silurian organic rich mudstones have been assembled; and results for the Jurassic Haynesville shale are discussed in detail.

At ultra-high magnification, pores and pore connections in the 50-100 nanometer size range and larger are clearly visible. At 'zoomed-out' lower magnification, component elements, such as fossil fragments, fecal pellets, burrows, winnowed laminae, matrix, and diagenetic minerals, are visible. Although shales show a limited range of pore types, such as framework pores, dissolution pores, and organic matter pores, each component element has unique proportions, sizes, and distributions of these pore types. From the mosaic images, the relative porosity contribution of the various components, as well as connections between regions with different pore types and porosities can be examined and evaluated. Combining the component specific pore characteristics with the cross-sectional contribution and the geometric relationships between low- and high-porosity domains, allows for a much better understanding of pore functionality at the representative volume scale.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013