Pore Characterization by Scanning Electron Microscope of Low-Permeability Gas Sandstones, Travis Peak Formation, East Texas
Shirley P. Dutton
The scanning electron microscope (SEM), used in conjunction with thin-section petrography and mercury-injection analysis, is a valuable tool to evaluate authigenic cements, pore size, and pore geometry in low-permeability sandstones of the Lower Cretaceous Travis Peak Formation, east Texas. Information derived from the SEM can help determine appropriate completion techniques for these sandstones, in which permeabilities measured under in-situ conditions of overburden pressure are commonly less than 0.1 md. Abundant cements, which include quartz, ankerite, illite, and chlorite, are the main cause of low permeability in these quartzarenite and subarkose sandstones. In addition, reservoir bitumen occurs in zones near the top of the formation, and where abundant, it further r duces porosity and permeability.
SEM and mercury-injection analysis distinguished two types of pore systems in Travis Peak samples. Matrix-rich sandstones have less authigenic cement and higher porosity and permeability than do matrix-free sandstones. Values of pore-aperture radii have a unimodal distribution, most radii being between 0.1 and 5 µm. Under SEM, the larger pores were identified as intergranular and solution pores; micropores (pore-aperture radii less than 0.5 µm) occur among detrital clay minerals. In contrast, sandstones that lack detrital clay matrix have abundant authigenic cement and reservoir bitumen, and lower values of porosity and permeability. These sandstones have a bimodal distribution of pore-aperture size, the major mode being 0.1-0.5 µm and the minor mode being less than 0.0 5 µm in radius. Most of the larger pore apertures in these samples (0.1-0.5 µm) seem to be associated with micropores in clay minerals, and solution pores, and the smallest pore apertures (less than 0.055 µm) seem to occur within the reservoir bitumen. In a sample with 18% reservoir bitumen, one-third of the total pore-aperture radii is less than 0.055 µm in diameter.
AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.