Shale Reservoirs: Deposition in Active- Versus Passive-Margin Settings

Eoff, Jennifer D.

The U.S. Geological Survey (USGS Fact Sheet 2004-3022) previously reported that more than an estimated 98 percent of the undiscovered, technically-recoverable natural gas in the Bend Arch-Fort Worth Basin Province is expected to reside in the Mississippian Barnett Shale. This resource assessment, however, was conducted before horizontal drilling practices and corresponding development techniques were commonplace in shale reservoirs. The Barnett has served as the first analog for developing newer shale resource plays, such as the Devonian Marcellus Shale, the Jurassic Haynesville-Bossier Formations, and the Cretaceous Eagle Ford Shale.

Productive shale reservoirs, however, have proven to be more dissimilar than expected. Conceptual modeling of shared global-to-regional controls may at least help explain their stratigraphic distribution throughout geologic history. Comparisons are made between the Barnett and Marcellus Shales to understand how tectonics, eustasy, paleogeography, and climate affected the preservation of thick accumulations of marine organic matter in foreland-basin settings. Comparisons between the Barnett Shale and the Haynesville-Bossier Formations and the Eagle Ford Shale are used to contrast the deposition of organic-rich shale at active margins and along passive margins. The Haynesville-Bossier Formations and the latest Jurassic-earliest Cretaceous strata of the central North American Atlantic margin are also contrasted in order to understand controls on source-rock deposition in semi-restricted, passive-margin settings and open-marine settings, respectively.

Upwelling and anoxia are invoked frequently to explain generation and preservation, respectively, of marine organic carbon in these Paleozoic and Mesozoic shale source rocks. Alternatively, tectonism during deposition of shale reservoirs helped confine recycling of terrestrial and marine organic material to relatively closed, marginal-marine systems. This resulted in a positive feedback on marine primary productivity and the accumulation of marine organic matter. During eustatic flooding, thixotropic substrate that was poor in coarse siliciclastic sediment limited the diversity of marine benthos and effective feeding strategies, and it expedited the preservation of marine organic matter.

Integration of recent studies of tectonics, paleogeography, and climate, therefore, is providing a better-developed, more comprehensive model for future assessment of shale plays.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013