AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Sequence Stratigraphy as Expressed by Shale Source Rock and Reservoir Characteristics – Examples from the Devonian Succession, Appalachian Basin
(1) Geosciences, SUNY Fredonia, Fredonia, NY.
(2) EQT Production, Pittsburgh, PA.
Most shale gas reservoir properties reflect a history of base level fluctuations that can be cast in terms of a predictive sequence stratigraphic famework. Our approach to the sequence stratigraphy of the Devonian gas shale succession of the Appalachian Basin is grounded in the transgressive-regressive (T-R) sequence concept. A single T-R sequence comprises transgressive systems tract (TST) deposits overlain by a regressive systems tract (RST) succession, the contact being a maximum flooding surface (MFS); the sequence is bounded on top and bottom by maximum regressive surfaces (MRS) or equivalent ravinement surface. Early results of a multi-faceted investigation of the Devonian shale succession of the Appalachian basin reveal that such parameters as mineralogy, microfabric, TOC, and source rock quality vary predictably within the T-R sequence stratigraphic framework. A general increase in silica, much of it diagenetic, and reduction of clay upward through the TST reflects the rapid landward migration of the shoreline. Further, spectral gamma-ray analysis reveals generally increasing levels of authigenic uranium through the TST. TST deposits are commonly pyritiferous and organic-rich, both parameters attaining maximum values close to the MFS. Increasing thermal maturity of these deposits is accompanied by increasing porosity, principally nanoporosity. Bacterial reworking of transgressive organic -rich sediment, especially proximal to maximum base level, appears to have resulted in some degree of vitrinite suppression. Accumulation of RST deposits is recorded by increasing clay and detrital quartz and concomitant dilution of the organic flux. The result is higher bound water contents and a pervasive planar clay-grain microfabric disrupted only by occasional discrete detrital quartz lamina or isolated grains. Base level minimum is defined by minimal TOC and local carbonate horizons. The predictive capabilities inherent to sequence stratigraphy make it especially applicable to exploration programs of seemingly homogenous shale successions.