Sequence Stratigraphy

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Imagine the Mediterranean Sea drying out. Imagine the late Permian, as the Earth warmed and dried, and much of life faced extinction. Now put the two together, and you have the basis of an analog examined in the presentation “The Messinian Mediterranean Crisis: A Model for the Permian Delaware Basin?” at the upcoming AAPG International Conference and Exhibition in Istanbul, Turkey.

IODP Expedition 313 (New Jersey shallow shelf) cored a 3-hole transect across Miocene seismic clinothems (prograding sigmoidal sequences) in topset, foreset, and bottomset locations, providing an opportunity to integrate seismic, log, and core data into a sequence stratigraphic framework. Our interpretations of sequences and systems tracts are made independent of any preconceived relative sea-level curves. Rather, we use basic seismic, core, and stratigraphic principles to recognize sequence boundaries, Maximum Flooding Surface, transgressive surfaces, and facies successions within sequences.

Over the last two decades, numerical and physical experiments have repeatedly generated insights that contradict the sequence stratigraphic model that is near-universally used to interpret ancient strata in terms of relative changes in sea-level. This presentation will re-examine Upper Cretaceous strata (Blackhawk Formation, Castlegate Sandstone, Mancos Shale) exposed in the Book Cliffs, east-central Utah, USA, which are widely used as an archtype for the sequence stratigraphy of marginal-marine and shallow-marine strata. Stratigraphic architectures in these strata are classically interpreted to reflect forcing by relative sea level, but key aspects can instead be attributed to autogenic behaviors and variations in sediment flux.

This lecture presents a history of sea-level changes focusing on the last 100 Myr. Prior to the Oligocene (ca. 33.5 Ma), the Earth had been a warm, high CO₂ Greenhouse world that was largely ice-free back to 260 Ma, though recent evidence suggests that 15-25 m sea-level changes observed may have been caused by growth and decay of small, ephemeral ice sheets. The growth and decay of a continental scale ice sheet in Antarctica caused 50-60 m variations on the 10⁶ yr scale beginning ~33.5 million years ago (Ma).

Sequence stratigraphy is the study of genetically related facies within a framework of chronostratigraphically significant surfaces. Paleontologic data, integrated with seismic and well log data, are an integral part of sequence stratigraphic analysis.

"Breakthrough elegance": ExxonMobil geologists Jeff Ottmann and Kevin Bohacs shared their highly-coveted knowledge on sweet spots and producibility thresholds at a recent Geosciences Technology Workshop on Unconventional Reservoir Quality.

This article describes a 250-m (820-ft)-thick upper Eocene deep-water clastic succession. This succession is divided into two reservoir zones: the lower sandstone zone (LSZ) and the upper sandstone zone, separated by a package of pelitic rocks with variable thickness on the order of tens of meters. The application of sequence-stratigraphic methodology allowed the subdivision of this stratigraphic section into third-order systems tracts.

The LSZ is characterized by blocky and fining-upward beds on well logs, and includes interbedded shale layers of as much as 10 m (33 ft) thick. This zone reaches a maximum thickness of 150 m (492 ft) and fills a trough at least 4 km (2 mi) wide, underlain by an erosional surface. The lower part of this zone consists of coarse- to medium-grained sandstones with good vertical pressure communication. We interpret this unit as vertically and laterally amalgamated channel-fill deposits of high-density turbidity flows accumulated during late forced regression. The sandstones in the upper part of this trough are dominantly medium to fine grained and display an overall fining-upward trend. We interpret them as laterally amalgamated channel-fill deposits of lower density turbidity flows, relative to the ones in the lower part of the LSZ, accumulated during lowstand to early transgression.

The pelitic rocks that separate the two sandstone zones display variable thickness, from 35 to more than 100 m (115–>328 ft), indistinct seismic facies, and no internal markers on well logs, and consist of muddy diamictites with contorted shale rip-up clasts. This section is interpreted as cohesive debris flows and/or mass-transported slumps accumulated during late transgression.

The upper sandstone zone displays a weakly defined blocky well-log signature, where the proportion of sand is higher than 80%, and a jagged well-log signature, where the sand proportion is lower than 60%. The high proportions of sand are associated with a channelized geometry that is well delineated on seismic amplitude maps. Several depositional elements are identified within this zone, including leveed channels, crevasse channels, and splays associated with turbidity flows. This package is interpreted as the product of increased terrigenous sediment supply during highstand normal regression.

Numerous studies of sediment-dispersal systems have focused on the relative role of allogenic versus autogenic controls, and their stratigraphic imprint. Advancing our understanding of these vital issues depends heavily on geochronology.