THE USE OF CHEMOSTRATIGRAPHY TO REFINE AMBIGUOUS SEQUENCE STRATIGRAPHIC CORRELATIONS IN MARINE MUDROCKS. AN EXAMPLE FROM THE DEVONIAN WOODFORD SHALE, OKLAHOMA

The Woodford Shale provides an opportunity to test recent advances in handheld XRF (HHXRF) technology to develop and refine sequence stratigraphic frameworks by comparing chemostratigraphic profiles directly to gamma ray logs obtained from the same locations. This study examines three cores and two outcrops from proximal and distal environments of the Woodford Shale in the Arkoma Basin. Clean surfaces at each area are scanned at no greater than one foot intervals using HHXRF to determine the elemental profiles. At the same resolution, a gamma ray profile is scanned using a GR scintillator or core spectral gamma ray. The lithologic description, gamma ray profile, and elemental profiles are then used to develop the sequence stratigraphic interpretation.

Chemostratigraphy is capable of highlighting facies shifts within apparently homogenous mudrock successions with greater confidence than is possible with visual inspection. The principal elements for this approach in this study are: titanium (Ti), zirconium (Zr), silicon (Si), Calcium (Ca), strontium (Sr), phosphorous (P), aluminum (Al), potassium (K), molybdenum (Mo), and vanadium (V). Ti and Zr are associated with continentally derived sediment. Ca and Sr are associated with carbonate accumulation. Al and K are associated with feldspars and clays. Mo and V can be used as an indication of restriction. Si is found in biogenic quartz, detrital quartz, feldspars, and clays. As such, it is useful to evaluate Si as a ratio between Si/Al. Interpreting these changing chemostratigraphic trends in a sequence stratigraphic context allows workers to resolve high frequency cyclicity and refine sequence stratigraphic frameworks used as a means of correlation across a basin.

Within this basin, the chemostratigraphic profile of the Woodford Shale can be interpreted within a sequence stratigraphic framework using the following general criteria. The Lowstand Systems Tract (LST) is defined by increasing concentrations of Ti, Zr, Al, and K as well as high levels of Mo and V. The Transgressive Systems Tract (TST) is defined by declining concentrations of Ti and Zr, though Al and K may remain elevated, Mo and V also show a general decline. The Highstand Systems Tract (HST) is defined by increasing concentrations of Ti, Zr, Al, and K, but is distinguished from the LST by low levels of Mo and V.