Porosimetry Measurement of Shale Fabric a N D Its Relationship to Illite/smectite Diagenesis

-The extent of illite/smectite (I/S) reactions is linked with quantitative measurements of shale fabric in a suite of samples from a lower Frio Formation well. Greater illitization of the I/S clays is found in laminated shales that possess a larger pore surface area/pore volume (SA/V) of 3.21 x 106 cm-~ than the adjacent massive shale lithofacies with a SA/V of 1.97 x 106 cm -~. Mean pore diameter in both shale lithologies is 0.0145 micrometers, though in the laminated shale distributions are skewed towards more smaller-sized pores. While no direct permeability measurements were made, estimates of permeability that are based on simple physical models using SA/V suggest that lower permeabilities are associated with laminated shales. The trend of greater illitization at higher SA/V values is contrary to expectations that reaction extent is enhanced by greater permeabilities, such as created by silt laminations in shale. The limitations of estimated permeabilities emphasize that porosimetry measurements of shale fabric are useful for estimating the access of material to all reaction sites, and do not just describe the effect of a few large pores that dominate permeability. Greater reaction extent in the laminated shales is associated with the accessibility of fluids to more pore space than in the massive shales. Key Words--Illite/Smectite, Shale fabric, Mercury porosimetry. I N T R O D U C T I O N A previous study demonstrated the relationship between the extent o f illite/smectite (I/S) diagenesis and shale fabric (Howard, 1987). Within a short interval of the lower Frio Formation of South Texas, I/S clays in laminated shales had a greater proportion of illite layers than I/S clays in the massive shale lithofacies. Classification o f shale fabric was subjective and based primarily on the presence o f fine-silt laminations. Siltsized quartz was dispersed throughout the massive shale lithology, while in the laminated shales the silt was found in thin 0.1-1.0 m m thick laminae. These silt laminations were separated vertically by several millimeters, and formed both laterally continuous layers and discontinuous lenses. For this interval in the Frio Formation it was proposed that the amount of available pore surface controlled the extent of the I/S reaction. In this context, pores are defined as the void space large enough to promote fluid flow, excluding the smaller interparticle and intraparticle voids less than 0.005 micrometers in diameter that do not contribute to fluid flow. It is important to distinguish between the available surface area of a pore and its total surface area. In this paper available pore surface area refers to the portion of the pore space that contributes to the fluid flow paths. It is expected that any measurement of available pore surface area would be less than the total surface area. It was suggested that fluid conduits formed by the silt lenses in the laminated shales served to create a greater pore surface area/volume (SA/V) than found in the massive shales. This concept o f silt laminations enhancing fluid flow in shales was illustrated by natural Copyright 9 1991, The Clay Minerals Society gas production figures for Devonian shales where a positive correlation between rate of production and number of silt laminations was observed (Nuhfer et aL, 1979). Therefore, the connection between fluid permeability and pore surface area/volume in laminated shales suggested by Howard (1987) needs to be reevaluated. For a normal distribution o f pore sizes in a porous medium, an increase in SA/V should result in a decrease in permeability (Dullien, 1975). The question remains as to why the laminated shales of the Frio Formation, which might be presumed to have higher permeabilities, also have larger values of SA/V than the adjacent massive shales. The purpose of this note is to assess further the relationship between shale fabric and degree o f illitization in the light o f quantitative measurements o f pore surface area and pore volume.