Mud deposition often has been attributed to slow fallout from suspension into low-energy, often anoxic, settings. In reality, sediment delivery and basinal oxygenation, plus the resultant organic content, grain size, and lithology, are quite variable. Understanding sedimentologic processes help us better describe mudrock cores and interpret stratigraphic and basinal variations in reservoir quality and mechanical properties.
Compositional variability reflects the complex interaction of biologic productivity, detrital input, organic preservation vs. destruction, and diagenesis. Intrabasinal biogenic material includes calcareous and siliceous hard parts of zooplankton and phytoplankton; cellular matter from algae, bacteria, spores, and pollen; fecal pellets; and feeding nets. Much of this matter sinks rapidly as aggregates, not individual particles, removing it from shallow, oxidizing waters. At the sea floor, suspension and deposit feeders (e.g., polychaetes and nematodes) may ingest the organics, reducing carbon content.
Biogenic influx is not constant. Extrabasinal carbonate and/or siliceous detritus vies with, and often overwhelms, intrabasinal input. Much extrabasinal material is delivered under the influence of storm waves or density flows. Storms suspend and mold bottom sediment, variably producing wave-enhanced sediment-gravity flows, graded tempestites, and hummocky and wave-rippled bedforms. Hyperpycnal deposits form as rivers transit through flood cycles. Ignitive events yield slides, slumps, debrites, and turbidites. Sedimentary structures produced by these various processes may be difficult to recognize or interpret. They typically are subtle, due to small variations in grain size and/or post-depositional bioturbation, or very thin.
Thus, the paradigm of grain-by-grain settling of mud onto a deep, quiet, stagnant sea floor is being revised. The concomitant slow, continuous rain of organic matter is unlikely. Instead, mud accumulates under dynamic conditions. Active bottom currents are frequent. Persistent bottom-water anoxia is overestimated; diminutive, often " cryptic", bioturbation is common. These interacting processes produce deposits of varying reservoir quality and cycles of various frequencies - seasonal, climatic, tectonic, and eustatic - that can be interpreted in core, outcrop, and well logs. Consequently, the study of mudrock sedimentology is generating new concepts that can be applied to rock description and appraisal and mapping of drilling targets.