Shell and Grain Layers in the Barnett Shale; Event Deposition or In Situ Accumulations

Authors: R. G. Loucks and S. C. Ruppel

Venue: 2008 American Association of Petroleum Geologists (AAPG) Annual Convention and Exhibition, San Antonio, TX, April 19-24, 2008 “The Geology of Mudrocks”, poster session chaired by S. C. Ruppel and R. G. Loucks (http://www.aapg.org)

Abstract: The Barnett Shale in the Fort Worth Basin (FWB) was deposited in a deeper water depositional setting under predominately anoxic conditions. The siliceous mudstone that composes the majority of the stratigraphic section contains thin shell and grain layers. These layers have been attributed to two contrasting depositional processes: (1) event beds deposited by gravity flows and (2) in situ accumulations. Basinal setting, biota composition, grain types, textures and fabrics all suggest that these shell and grain layers were deposited by gravity-flow processes. Evidence for event deposition include (1) grain composition that suggests mechanical mixture of grain types (phosphatic coated grains and filibranch mollusk), (2) sorting of grains attributed to turbidity and debris flow processes, (3) absence of biota in living position, and (4) basal scours. Although sediments have been highly compacted during burial distorting some of the evidence, carbonate concretions, which formed within a few meters of the sediment surface, reveal the uncompacted state of the shell and grain layers. Shell-layer textures and fabrics demonstrate that shells are generally disarticulated and suspended within a peloidal matrix. In the northern FWB no bioturbation is associated with these shell accumulations, whereas in the southern FWB bioturbation is locally apparent at the base of shell accumulations. Burrows may be the product of “doomed pioneers”, organisms that briefly populated bottom sediment following short-lived oxygenation of the sediment-water interface during event deposition. The interpretation of these deposits as gravity-flow events is consistent with the depositional setting of Barnett mudstones in the FWB, which has been documented to have been deeper water, anoxic, and hostile to biota.

Related NETL Project
This presentation is related to the NETL project DE-FC26-04NT15509, “Integrated Synthesis of the Permian Basin: Data and Models for Recovering Existing and Undiscovered Oil Resources from the Largest Oil-Bearing Basin in the United States.” The objectives of the project are twofold: (1) to produce a detailed, comprehensive analysis and history of Paleozoic depositional and reservoir systems in the Permian Basin, and (2) to create spatially integrated databases of depositional, stratigraphic, lithologic, and petrophysical properties for selected reservoir plays and stratigraphic horizons. These objectives will be undertaken and completed sequentially during the 3 years of the project. The overall objective is to provide Permian Basin operators with (a) outcrop and subsurface reservoir specific data, data syntheses, and models to be applied to geological-, engineering-, and completion-based redevelopment of existing reservoirs, and (b) a detailed regional stratigraphic framework for applying such models to new exploration targets.

Project Contacts
NETL – Virginia (Ginny) Weyland (Virginia.WEYLAND@netl.doe.gov or 918-699-2041)
University of Texas at Austin – Stephen Ruppel (stephen.ruppel@beg.utexas.edu or 512-471-1534)