REGIONAL OR LOCAL DIAGENESIS? INTEGRATING MICROANALYTICAL TECHNIQUES TO EXTRACT BASIN-SCALE DIAGENETIC HISTORIES FROM MICRON-SCALE CEMENTS IN THE BAKKEN FORMATION

Cementing minerals are known to preserve chemical records of the timing and causes of diagenesis in sedimentary rocks, but whether this information reflects local or regional processes is often hotly debated. In order to investigate the extent to which regional burial processes will affect pore-scale diagenesis, a microanalytical survey was conducted of the Mississippian-Devonian aged Bakken Formation. Samples were gathered from nine drill holes covering a ~250km transect from basin margin to basin center. After thorough SEM mapping and petrographic characterization of those samples, calcites and iron-zoned dolomite-ankerite series cements were analyzed by SIMS and EPMA to obtain data on the extent and magnitude of δ¹⁸O, δ¹³C and cation zonation (Denny et al. 2020, Chem Geol).

Observed core to rim isotopic variability frequently exceeds 10‰ for both δ¹⁸O and δ¹³C in individual ~50 µm calcite and dolomite-ankerite series cements. The centers of dolomite grains are isotopically similar basinwide (~2‰ δ¹³C VPDB, ~25‰ δ¹⁸O VSMOW), and are interpreted to have formed early during initial sediment burial. Subsequent growth bands in these grains may be correlated with chemically similar growth bands in drill holes 10s of km away, indicating that ankerite-series carbonates preserve records of prolonged thermal and chemical processes that operated basinwide. The most abrupt shifts in δ¹⁸O and δ¹³C (changes as large as 12‰ for δ¹⁸O and 6.5‰ for δ¹³C within distances <15 μm) are observed along the basin margin, and several lines of evidence indicate that these chemical shifts are correlated first with a period of hydrocarbon expulsion that occurred basinwide, followed by a period of infiltration of meteoric fluid along basin margins during Pleistocene glaciations. Applying microanalytical techniques to samples 10s of km apart can reveal subtle changes in µm-scale geochemical data that provide a more complete view of pore-scale processes than could ever be obtained if one were to look only at a single pore’s cements. These results provide important constraints on how carbonate isotope records may be altered during burial in organic-rich sedimentary rocks and emphasize the need for caution in using bulk powder samples for geochemical analysis in carbonate systems.