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Applications of Fluorescence Microscopy to Study of Pores in Tight Rocks

Daniel J. Soeder

Observation of pore structures in thin section using the traditional method of impregnation with blue-dyed epoxy becomes difficult when the pores are smaller than about 1 micrometer and/or linear in shape. These types of pores are common in many gas-bearing "tight" formations, including coal seams, black shales, and lenticular sandstones.

Incident-light fluorescence microscopy can be utilized for observation of small, narrow pore structures in tight rocks. Adaptation of this common medical technique to petrography is accomplished by staining the epoxy with fluorescent Rhodamine-B dye instead of the usual blue material, impregnating under vacuum, and preparing a polished thin section in the normal manner. The thin section is observed using an incident-light microscope equipped for fluorescence, which usually involves only a simple lamp and filter change on an existing, universal-type microscope.

Under excitation of green light at a wavelength of 540 nm, the Rhodamine fluoresces a brilliant reddish orange, clearly showing impregnated pore space. A dichromatic interference filter in the light path allows only the reddish orange light to pass through, blacking out the mineral grains such that only the pore structure is visible. The incident-light configuration of the microscope has the advantage of inducing brighter fluorescence at higher magnifications due to the more intense concentration of the exciter beam through higher-power objectives. This technique also permits observation of impregnated pore space narrower than the wavelengths of visible light. Epoxy fluorescing in pores this small behaves like a point source and makes the pore visible. Fluorescence microscopy has much promise for studying pore structures in tight sedimentary rocks and can also be applied to crystalline rocks and other materials containing narrow, linear, or small pores.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.