STABLE ISOTOPE AND ELEMENTAL GEOCHEMISTRY FROM 1-D PROFILES ARE A USEFUL TOOL FOR REVEALING 3-D PHASING ARCHITECTURE OF UPPER CAMBRIAN MICROBIAL REEF COMPLEXES, WILBERNS FORMATION, MASON COUNTY, TX

Upper Cambrian microbial reef complexes exposed in Mason, TX serve as useful analogues for microbial carbonate reservoirs. The microbial reefs consist of 3 geometrical phases consisting of: phase 1 with sharp walled microbial mounds and intermound skeletal grainstones that are onlapped by siltstone, phase 2 that prograded over and intertongued with intermound carbonate and siliciclastic facies, and phase 3with sharp walled microbial mounds onlapped by siltstone followed by termination and drape by thin bedded carbonates. Pphasing is interpreted to result from either sea-level fluctuation or pulses of siliciclastic input. Boundaries between phases 1-2 and phases 2-3 are projected to occur at 1.3 and 5.15 meters within the interior of the reef. This study tests whether geometric phasing can be recognized within the reef, and evaluates sea-level vs. siliciclastic flux as causes for the phasing using geochemistry.

δ18O shows a negative shift and δ13C remains stable at 1.3 m, both δ13C and δ18O show a statistically significant negative shift at 3 m, and a positive shift at 5.15 m. The shifts correspond to phase boundaries, although the negative shift at 3 m does not correspond to recognized phases. This indicates presence of an additional subphase or a 4-phase architecture not previously recognized. Lesser δ13C and δ18O suggest that phase 2 was deposited in shallower water. This is consistent with the horizontal growth pattern seen in outcrop as opposed to the vertical growth of phases 1 and 3. However, δ13C values are not negative enough to represent vadose diagenesis. Alternatively, the negative δ13C shift may represent the Sunwaptian return to normal δ13C following the global Steptoean positive excursion. This correlation indicates an age of ~ 494.5 MA.

Using Al, K, Ti, and Si as proxies for siliciclastic flux, we observe an increase in terrestrial input at elevations 1.3, 3.8, 5.15, and 5.6 meters as well as a decrease in terrestrial input at 6.4 meters. The fluctuations can be attributed to changing sea level.

A significant challenge in petroleum exploration is the recognition of 3-d geometrical phasing of reservoirs from vertical record in wellbore. Vertical facies from a standard core through the reef would not reveal the phasing of the 3-d geometry. However, the vertical patterns in geochemistry would identify the phases.