Fecal Pellets and their Significance in Unconventional Resource Shales: Part I: Physical and Petrophysical Properties
Slatt, Roger M.; O'Brien, Neal; Torres, Emilio J.; Philp, R. Paul
Fecal pellets---the excrement of marine organisms---are a constituent of many unconventional resource shales. They contain significant quantities of undigested organic matter (OM) and their open structure gives rise to porosity often in the range of 10%.
We have examined a number of fossil pellets in different shales under a scanning electron microscope (SEM/FESEM) and find them to be easily recognizable by their oval to rounded shape, which is similar to the shape of modern marine fecal pellets. They can also be identified by compaction-induced, tangentially-oriented clay flakes around the pellets.
Intra-pellet porosity is made up of micrometer (>1um length)- and nanometer (<1um length)- size pores, with about equal amounts of each, thus making the pores likely storage sites for gas/oil molecules.
Energy Dispersive X-ray (EDX) analysis indicates fossil fecal pellets contain 40-80 wt. % carbon, compared with modern pellets from Daphnia shrimp which contain 40+ wt. % carbon. Using modern day OM waste (such as occurs in landfill or barnyard manure) as an analog, we suggest that the pellets are a source of methane gas. Details of geochemistry of pellets are presented as a companion presentation (Torres et al., 2013; Part II, this convention). Pellets are often concentrated in laminae or thin beds, which may be an indicator that they were sourced during algal blooms. When present in sufficient quantities in such strata, fossil fecal pellets could provide zones with significant gas potential.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013