October 2018: Jeffrey May

Jeff May | Colorado School of Mines EOG (retired)

The Sedimentology of Mudrocks: Organics, Organisms, and Occasional Occurrences



Mud deposition often has been attributed to slow fallout from suspension into low-energy, frequently anoxic, settings. In reality, sediment delivery and resulting organic content, grain size, and lithology are quite variable. Understanding sedimentologic processes helps 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.

BIOGRAPHY: Jeffrey A. May

Jeff received his B.A. in Geology from Earlham College, M.S. in Geology from Duke University, and Ph.D. in Geology from Rice University. He has worked in the oil and gas industry for over 35 years: as a research geologist with Marathon Oil Company (1981- 1994); as a geological and geophysical consultant with Enron Oil & Gas (1994-1996) and GeoQuest Reservoir Technologies (1996-1998); as an exploration geoscientist with DDD Energy (1998-2001); and with EOG Resources beginning in 2001, first as Chief Stratigrapher and then as Chief Geologist, until his retirement in 2011. In addition, he recently became an Affiliate Faculty member in the Department of Geology and Geological Engineering at Colorado School of Mines.

Jeff has conducted sedimentologic, sequence stratigraphic, and seismic stratigraphic projects on basins and fields worldwide. Areas of expertise include onshore and offshore Gulf of Mexico; onshore and offshore California; Uinta, Green River, Washakie, Denver, Powder River, and Williston Basins; northern and eastern Egypt; and Natuna Sea, Indonesia. At EOG, he provided regional to prospect-scale stratigraphic interpretation and evaluation plus training in support of all divisions. Jeff also conducts a variety of classroom and field seminars on clastic facies, deep-water sandstones, sequence stratigraphy, and mudrock deposition and stratigraphy, most notably for the American Association of Petroleum Geologists, the Petroleum Technology Transfer Council, Nautilus Worldwide, many oil and gas companies, and universities. His publications encompass numerous papers and abstracts on deep-water sandstones, sequence stratigraphy, geophysical interpretation, and mudrock deposition. Jeff has twice been presented the best luncheon speaker award by RMAG, completed an AAPG Distinguished Lecture tour, and received the Outstanding Scientist Award from RMAG (shared with Donna Anderson) in 2017.