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Can Fecal Pellets in Unconventional Resource Shales Generate Hydrocarbons?


The petroleum precursors are potentially chemically diverse and can partly explain the variety of hydrocarbon compounds found in crude oils. Other organic components found within source rocks rather than kerogen are fecal pellets, the excrement of marine organisms, that are houses of organic matter and pores, and may provide a significant source of gas during burial. Pellets are common in shale rocks and typically are enriched in undigested organic matter, and are porous. The fecal pellets are compacted in the gut of animals and are protected by a mucilaginous membrane, which also helps preserve the contents from biological, geochemical, or physical disruption and dissolution. The preservation of pellets and banding in sediments occur in anoxic and/or hypersaline environments which reduce or exclude benthic forms and prevent microbial activity. Such environments are required to be of permanently low-energy in order to limit and minimize biological activity and weathering. Since single fecal pellets in the rock matrix cannot effectively be isolated from shales, to test their potential as a hydrocarbon 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. 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 of fecal pellet aggregates. The primary cracking resulted in the release of significant C1-C5 compounds and also C6-C25 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 oil and gas generation in unconventional resource shales.