Nitrogen isotopes as indicators of depositional environments: Case of the Caney and Woodford shales in the Arkoma Basin, Oklahoma, USA

The nitrogen isotope composition of shales was investigated in order to determine the differences in the nitrogen isotopic composition of shales deposited in suboxic-anoxic settings versus anoxic settings. The analysis was carried out on samples from the Caney Shale, which was deposited under suboxic-anoxic conditions, and the Woodford Shale, which was deposited under anoxic conditions. The samples were taken from three cores collected in the Arkoma Basin, Oklahoma. The carbon isotopic compositions of acidified samples (delta13Corganic), the nitrogen isotopic compositions of bulk samples (delta15Nbulk), the total organic carbon (TOC) and the trace metal concentrations of the Caney and Woodford Shales were determined.;The results indicate that the average delta15N bulk values in the Caney Shale are approximately 7.2‰ heavier than the average delta15Nbulk values in the underlying Woodford Shale. The average delta13Corganic values in the Woodford Shale are approximately 1.5‰ heavier than the average delta13Corganic values in the Caney Shale. The TOC results indicate that the organic carbon concentrations of the Caney Shale (0.3 to 7.5 wt. %) and Woodford Shale (1.4 to 8.0 wt. %.) are similar. In addition, the trace metal enrichment factors relative to the Post-Archean Shale (PAAS) are similar for the Caney and Woodford Shales. The trace metal enrichment factors in the Caney Shale are 25 +/-16, 2.25 +/-2.33, 15 +/-17 and 2.86 +/-0.88 for V, U, Zn and Fe respectively, while in the Woodford Shale, the trace metal enrichment factors are 27 +/-10, 3.83 +/- 2.68, 13 +/-12 and 5.66 +/-1.69 for V, U, Zn and Fe respectively.;The overlap in the average TOC values for the Caney Shale (2.5 +/- 1.6 wt. %) and the Woodford Shale (5.4 +/- 1.8 wt. %), as well as in the average delta13Corganic values for the Caney Shale (-30.2 +/- 1.2‰) and for the Woodford Shale (-28.7 +/- 0.8‰) indicates that they have a common organic matter source. The similarities in the trace metal enrichment factors (EF) of V, U, Zn and Fe in both the Caney and Woodford Shales implies that, in the intervals sampled, the trace metal concentrations were not significantly altered by the change from the anoxic depositional environment of the Woodford Shale to the suboxic-anoxic depositional environment of the Caney Shale.;The differences in delta15Nbulk values between the Caney Shale and the Woodford Shale can be attributed to differences in their depositional environments, with the Caney Shale deposited under suboxic-anoxic water column conditions and the Woodford Shale deposited under anoxic water column conditions. Thus, incomplete denitrification was predominant in the depositional environment of the Caney Shale and resulted in enriched delta 15Nbulk values (6.2‰ to 11.2‰). In contrast, nitrogen fixation was predominant in the anoxic depositional environment of the Woodford Shale and resulted in relatively depleted delta15N bulk values (-0.8‰ to 3.9‰).