Fecal Pellets and their Significance in Unconventional Resource Shales: Part II: Generation of Hydrocarbons
Torres, Emilio J.; Philp, R. Paul; Wang, Ting; Slatt, Roger M.; O'Brien, Neil
Fecal pellets, the excrement of marine organisms containing both undigested organic matter and pores, are common in many shales and may provide a significant source of gas during burial. Such pellets are protected in the gut of animals by a membrane, which also helps preserve the contents from biological, geochemical, or physical disruption and dissolution (Porter and Robbins, 1981). The preservation of pellets in sediments occurs in low energy, anoxic or hypersaline environments which reduce or exclude benthic forms and reduce microbial activity.
Since single pellets cannot currently be isolated from shales, to test their potential as a gas source, experiments were conducted on fecal material from modern freshwater fish and shrimp using Micro-Scale Sealed Vessel analysis (MSSV), an artificial maturation technique that simulates petroleum generation from a source rock in a closed system (Horsfield et al., 1989). This method allows the oil and gas generation from the primary cracking of kerogen and the generation of gas from the secondary cracking of oil and bitumen. Primary products may be recombined by aromatization/condensation reactions into a gas-prone residue and the cracking at high maturity of pyrobitumen extends gas generation beyond the conventional secondary cracking window and provides significant quantities of methane.
For this specific case, simulations were run within the oil and gas generation windows. The primary cracking resulted in the release of significant C1-C5 compounds and also C6-C20 (liquid hydrocarbon traces). The secondary cracking process produced more methane and C2+ gases than liquids. The liquids that were produced suggest these modern animals don't provide a complete digestion process of proteins, carbohydrates, and lipids that constitute their primary food, thus these molecular constituents are incorporated into their excrement. If this analogy is correct, fecal pellets might be an important contributor to gas in unconventional resource shales.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013