Sequence Stratigraphic Architecture for the Late Devonian Woodford Shale, Southern Permian Basin, West Texas
The Woodford Shale in the Permian Basin, west Texas and southeast New Mexico is an example of the Upper Devonian black shales that are widespread across North America. The Woodford is highly enriched in organic carbon and considered an important source rock in the basin. We are carrying out detailed sedimentological and stratigraphic studies of the Woodford, in order to define lithofacies, characterize depositional environments and establish a sequence stratigraphic framework for a classic black shale deposit. Previous stratigraphic studies of the Woodford Shale, limited to northern portions of the Permian Basin, divided the formation into three units (lower, middle, and upper) based on radioactivity, log response, and lithology. This study extends our knowledge of the Woodford Shale to southern portions of the basin that appear to have been significantly influenced by carbonate production. Detailed core descriptions were conducted on several long cores through the Woodford Shale in Reeves and Pecos counties, Texas. Five facies were identified, including a massive (structureless, homogeneous, bioturbated) carbonate facies, black (structureless, laminated) mudstone facies, grey (structureless) mudstone facies, siltstone facies, and a laminated (planar-parallel, ripple) carbonate facies. These are organized into consistent vertical stacking patterns that can be utilized to develop a sequence stratigraphic framework for the Woodford Shale, typically: (1) a massive carbonate facies, overlain by; (2) a black mudstone facies, overlain by;
3 a combination of grey mudstone, siltstone, and/or laminated carbonate facies. The massive carbonate was deposited by sediment gravity flows and is interpreted as a lowstand deposit. The black mudstone is interpreted to have been deposited during a transgression, and the grey mudstone, siltstone and laminated carbonate are interpreted as being deposited during a highstand. Based on this stacking pattern, the Woodford Shale can be subdivided into ten sequences. The Woodford Shale can be interpreted as a mixed siliciclastic / carbonate sedimentary system along a passive margin, with relative sea-level changes providing the primary control on carbonate production and sedimentation, and sediment transport into deep parts of the basin. Future work will test this sequence stratigraphic model for the Woodford Shale to the entire Permian Basin, West Texas.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009