Applications of Scanning Electron Microscopy to Characterization and Evaluation of Reservoir Rocks
John W. Neasham
Scanning electron microscopy (SEM) is a versatile and powerful tool for the evaluation of reservoir rock. Its magnification (10X-20,000X +) and sample imaging capabilities, coupled with the ease of sample preparation and instrument operation, have made the SEM a routine tool for the analysis of conventional core, sidewalls, and cuttings samples. Over the past decade the petroleum industry's knowledge and understanding of reservoir rock mineral, texture, and pore space properties, particularly those affecting log response, fluid flow, and rock/fluid interaction, have significantly increased utilizing SEM data.
Sample preparation for SEM analysis is relatively simple and fast, although samples must be cleansed of fluids and properly dried. Recent data suggest critical point drying may be best for certain shaly sandstone rock types.
SEM characterization and photodocumentation (Polaroid or 35 mm) of a sample can take less than 1 hour, and both the large depth of focus and image resolution produce high-quality visual data illustrating reservoir rock matrix properties. Most SEM's are equipped with energy dispersive x-ray detectors (EDS). EDS provides semi-quantitative "elemental spectrums" of mineral surfaces through detection of x-ray's emitted by rock (mineral) elements at "energies" reflecting atomic number. Such data are routinely utilized, for example, to identify rock mineral "unknowns" or to map the distribution of a given element. Back-scattered electron imaging (BSE) can also be employed to document the areal distribution of different rock mineral components.
Use of SEM/EDS/BSE for the evaluation of reservoir rocks is multifaceted. Principal applications include: (1) determining the type and mode of occurrence of diagenetic clay crystals, (2) visual characterization of pore systems, particularly microporosity, and (3) evaluation of finely crystalline rocks (e.g., chalks) which cannot be properly characterized under the petrographic microscope.
AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.