“All You Wanted to Know About the Origins of Mudstones in Hydrocarbon Systems,

but were afraid to ask” Mudstones (shales) are significant, but poorly understood rocks, that form important elements of hydrocarbon systems. They are challenging to characterize, using conventional geological techniques, because they are very fine grained (average grain size < 0.0625 mm) and composed of materials that readily react to the conditions that exist at the Earth’s surface (i.e., high concentrations of water and oxygen in the atmosphere). General observations, coupled with careful analysis of their fossil assemblages, allow geologists to broadly predict their environments of deposition. These studies are limited, however, as field-based techniques neither have the resolution to determine their grain/pore sizes, nor do they provide reliable data on bed- and laminae-scale variability. Recent application of high-resolution imaging techniques (combined optical and electron optical microscopy) reveals that the assumptions about grain origins in mudstones have seriously underestimated the impact that pre-compaction diagenetic processes have on unconventional reservoir, and source rock attributes in these materials. In this presentation I will review how pre-compaction diagenesis and particularly the processes associated with microbial respiration impacts the fabric and final composition of mudstones.