High-Resolution Depositional and Sequence Stratigraphic Analysis of the Woodford Shale Combining Sedimentary Facies, Elemental Chemostratigraphy and Organic Contents
In marine organic-rich shales, the interplay between intra-basinal (pelagic, biogenic) and extra-basinal (terrigenous) supply has proved to be insightful for interpretations of depositional systems tracts related to fluctuations of the sea level. In this work, combining results from sedimentary facies, elemental chemostratigraphy and organic richness, we propose a sequence stratigraphic framework of the prolific unconventional Woodford Shale in south-central Oklahoma.
Lithologically, the Woodford Shale is described as high frequency alternations of radiolarian-bearing chert beds and organic-rich shale beds. Elemental signatures reveal higher concentrations of terrigenous proxies (Al, Ti, Zr, K) in the clay-rich shale facies, which also coexist with elevated concentrations of anoxic proxies (Mo, U, V) and very high TOC content (avg. 14%), thus implying that in shaly beds the organic matter was efficiently preserved. Adversely, although chert facies suggest vigorous organic productivity evidenced by the abundant biogenic material (radiolarian), TOC content of cherts is significantly lower (avg. 3.1%) and coexists with low concentrations of anoxic proxies (Mo, U, V), suggesting biogenic dilution and/or weaker reducing conditions.
The complete Woodford Shale section of this study (350 ft. thick) records a single second-order depositional sequence. During early transgression, maximum water restriction is interpreted along with moderate terrigenous input, which led the highly efficient preservation of organic matter (largest TOC’s at the base of section). As transgression continued, the mixing of nutrient- and oxygen-rich deep waters with surficial waters (upwelling) begins to promote the primary productivity and coeval pelagic supply, resulting in the radiolarian cherty beds. Then, when sea level attained its maximum flooding surface (MFS), the pelagic (biogenic) supply overpassed the terrigenous input, partially leading to biogenic dilution of organic matter. Consequently, our interpretation of the MFS within the Woodford Shale does not coincide with the highest organic content. Above the MFS, the regressive phase or highstand system tract (HST) is dominated by pelagic supply that increases upward to its maximum near the top of the HST. Just at the very late stages of the HST, the terrigenous pulses were more intense and gradually outpaced the biogenic supply to its minimum as well as the organic contents.
AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018