Anisotropy and petrophysics of Floyd Shale, Alabama

Abstract

Floyd Shale has a measured anisotropy that is consistent with transverse isotropy but with a magnitude of nearly 40% in Thomsen parameters γ and ε. The measured anisotropy is very strong, with a relation between γ and ε but not δ. This requires incorporating strong anisotropy while locating any microseismic events induced during fracture stimulation. It makes velocities deduced from sonic logs in deviated wellbores considerably different than vertical velocities. Floyd Shale looks very similar to Barnett Shale in many of its petrophysical properties except mineralogy. Production to date has been disappointing. We review key petrophysical and rock-physics properties of 103 ft of Floyd Shale recovered from the McShan Timberlands well. Based on these analyses, Floyd looks very promising (i.e., total organic carbon is in excess of 4 wt%, and porosities average 6%). However, mineralogy is dominated by clays (typically greater than 50 wt%), and Barnett has generally less than 29% clay. High-pressure mercury injection capillary pressure and nuclear magnetic resonance indicate that pore throats and pore bodies are small (i.e., less than 15 nm). Physical inspection of the recovered core reveals fresh slickensides (i.e., evidence of hydraulic fracturing) and compromised calcite-filled fractures. If this core is representative of Floyd Shale, we suspect that any hydrocarbons generated during maturation likely migrated during later tectonic fracturing.