AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Expression of Sea Level Cycles in a Black Shale: Woodford Shale, Permian Basin
(1) Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB, Canada.
Sequence stratigraphic analysis of black shales is difficult, given the apparent monotonous repetition of organic-rich shale facies and, commonly, the lack of a clear connection between the deep basin and a marine shelf. Our investigation of the Woodford Shale in the Permian Basin has focused on deciphering a sea level signal though sedimentological, geochemical and log analysis, attempting to identify cyclicity at time scales that correspond to eustatic sea level curves. The data set is based on description and analysis of several long cores and correlation of cores to well logs.
In a core located near the carbonate platform that defined the western margin of the Permian Basin, intervals of continuous black shale alternate with intervals of interbedded carbonate turbidites and black shale, forming 10 meter-thick cycles. In a core from the basin center where the Woodford is thicker (Central Basin Platform), carbonate beds are largely absent, and cyclicity is demonstrated by repetition of intervals with numerous radiolarian-rich laminae alternating with intervals of continuous black shale. In both cases, the frequency of depositional cycles is consistent with 3rd order sea level cycles in global eustatic curves. Frequency analysis of total organic carbon (TOC) data, for which the gamma ray log is a proxy, indicates cyclicity at multiple scales, including repetitions consistent with 3rd order cycles. However, a simple model for TOC enrichment during transgressions probably cannot be applied; rather organic carbon is probably enriched during high stands and transgressions in the Middle and Upper Woodford but during low stands in the Lower Woodford.
A second order sea level fall during Woodford deposition is recognized by an upward increase in biogenic silica in distal parts of the basin, forming a brittle quartz-rich rock in the Upper Woodford. Though now largely recrystallized, the silica probably originated from accumulations of radiolarian. In northern parts of the basin closer to exposed clastic terrane, clay content increased in the Upper Woodford.
Shale lithologies and mineralogy are critical to the success of a shale gas play. In particular, the ratio of quartz to clay and the presence of biogenic silica dictate shale ductility and how the shale responds to hydraulic fracturing. This research demonstrates that regional sedimentological and sequence stratigraphic analysis can be applied to predictions of shale gas potential.