A role for organic petrology in integrated studies of mudrocks: examples from Devonian black shales of the eastern US

Abstract

Over the years, the study of mudrocks has lagged far behind that of other lithologies, a circumstance that is in part due to their fine-grained nature, and in part due to economic realities. Yet, within petroleum systems mudstones are usually the main source of hydrocarbons and typically are also important as hydrocarbon seals. Recent work on Late Devonian mudstones from the eastern US shows that much progress can be made through an integration of outcrop study, macro- and micropaleontology, ichnofossils, gamma ray spectroscopy, microscopic examination of thin sections in transmitted and reflected light, electron microscopy (SEM, BSE), electron microprobe, carbon and sulfur isotopes, and organic geochemistry. Erosion surfaces within this black shale succession have been traced over large distances and provide the foundation for a sequence stratigraphic re-interpretation of these rocks. Evidence of storm wave reworking of the seabed, as well as the realization that benthic colonization was much more widespread than previously believed, suggest that anoxic conditions in the water column were not a controlling factor in the accumulation of the large quantities of organic matter found in these shales. These distal Devonian shales accumulated slowly and allowed accumulation of large proportions of organic matter. In modern settings of abundant organic matter accumulation, the original material is broken down by bacteria within a matter of months into a mass of largely unidentifiable organic particles and extracellular bacterial slime. Although one can still find identifiable material within this mass, slime and amorphous material strongly dominate. After burial and maturation this material turns into the various organic macerals that organic petrologists are accustomed to describe. One might wonder in this context, how much information about the origin of a given mudstone unit can we hope to extract through organic petrology? In order to illustrate how organic petrology of mudrocks can contribute to their better understanding, Late Devonian black shales of the eastern US will be examined from a sedimentological perspective. Combining sedimentologic and geochemical data with basic observations on organic petrology, illustrates how the latter can contribute to more realistic scenarios of black shale genesis.