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Weimer Distinguished Lecture: Kevin Bohacs, ExxonMobil Upstream Research Company
November 29, 2018 @ 4:00 pm - 5:00 pm
Kevin Bohacs, KMBohacs Geoconsulting
241 Berthoud Hall, 4PM
Topic: The Vital Role of Mudstone/Shale Studies in Advancing Sequence Stratigraphy in General
Abstract: Studies of mudstone/shale have benefitted from using the sequence-stratigraphic approach but have also improved sequence stratigraphy as a discipline. Observations accumulated over the last 35 years, accelerated by the ‘shale-gas revolution’, indicate that mud is a quite active component of depositional systems at many scales and not just passive fill among coarser grains. It appears that various grain-size classes each have their own inherent geometry and the stratal record is the resultant of vigorous interaction among the grain sizes. The influences of mud range from being essential to many sediment-transport mechanisms at the bed scale to forming distinctive types of parasequences and complex stratal geometries at depositional-sequence to sequence-set scales.
What we learned using sequence stratigraphy includes: A wide variety of sedimentary structures occur in mudstones that indicate a comparably wide range of transport mechanisms, in many of which the presence of mud alters the fluid and transport properties of the flow. At the parasequence scale, most marine shelfal mudstone strata appear to have accumulated as one of three end-members that can be differentiated quantitatively. These end members can be related to depositional regimes dominated by storm waves, river floods, or tidal currents through characteristic modes of sediment transport and accumulation, as well as variations in benthic-energy and oxygen levels. At the depositional-sequence scale, most marine biogenic-rich mudstones tend to occur in one of three physiographic settings (constructional shelf margin, platform/ramp, continental slope—basin), each of which has a commonly recurring pattern of biogenic enrichment distinctive from the other settings. At the depositional-sequence-set scale, all major shale-gas/shale-oil plays can be grouped into four main families, based on repeated patterns of stratal stacking of depositional-sequence-scale biogenic-rich physiographic settings.
What we learned about sequence stratigraphy includes: Sequence-stratigraphic criteria (e.g., stratal terminations, geometric relations, stacking patterns) apply across the full range of composition and grain size (> 8 orders of length-scale magnitude). There are a variety of types of surfaces as well as of rocks— and surfaces are indeed much more important than the preserved strata because they record more of geological time (and are commonly easier to recognize in mudstone strata). The same types of surfaces, stratal units, and stacking patterns are seen across all grain sizes and compositions, but their expressions vary as a function of depositional environment.
Although these observations show that most mudstones accumulate discontinuously, they still preserve detailed records of paleoenvironmental conditions and depositional history, especially in microbially mediated authigenic products. The sequence-stratigraphic approach is particularly useful for organizing all these discontinuities and varying rock properties into a hierarchy of nested scales. Such a hierarchical sequence-stratigraphic framework is essential for integrating the wide range of physical, biogenic, and chemical attributes of mudstones into a comprehensive understanding of Earth history and hydrocarbon systems.