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Telling Time in Microbial Carbonates: A Case Study from the Latest Precambrian, Sultanate of Oman

Bergmann, Kristin D.*1; Grotzinger, John 1; Osburn, Magdalena 1
(1) Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA.

The Huqf Supergroup, Sultanate of Oman, preserves a succession of rocks documenting the transition from late Proterozoic into early Cambrian time. In the latest Proterozoic, localized uplift and subsidence dissected the broad carbonate ramp of the Buah Formation, a unit in the Huqf Supergroup. The microbially-influenced carbonates deposited on this uneven topography are spatially variable. In the South Oman salt basin these carbonates are interbedded with and capped by thick evaporites, which provide seals for proven petroleum reserves. Thick carbonates overlying the Buah Formation along the Eastern Flank and on the Huqf-Haushi high have also been targeted as potential reservoirs because in most cases they are also capped by evaporites. Facies heterogeneities, sparse age constraints, a limited fossil record and complex topography have hampered lateral correlation and spatial predictions of reservoir properties. Previous work on these carbonates and a potential outcrop analog, the Sirab Formation, found significant stratigraphic variation in their δ13C and δ34S isotopic composition.

We employ an integrated approach to test whether these geochemical proxies can be used to tell time in the outcrop exposures of the Sirab Formation with the ultimate goal of testing findings in subsurface wells. This approach aims to develop a sequence stratigraphic model based on lithofacies stacking patterns, key surfaces and gamma ray logs. We combine this model with high-resolution chemostratigraphy (δ13C and δ34S) to assess the ability to correlate geochemical proxies. Work to date includes multiple stratigraphic logs and high-resolution isotopic analysis from seven outcrop locations. From the outcrop exposures, potential reservoir facies have been identified and characterized for spatial heterogeneity. These facies include subtidal microbial buildups of large (>1m) conical stromatolites and thick oncolite shoals.

Assessing whether the geochemical proxies are stratigraphically meaningful in these microbial carbonates and can substitute for time could improve subsurface correlation and reservoir characterization. In younger rocks, environmental restriction in marine or lacustrine settings allows microbial deposits to form over large areas, sometimes forming prolific hydrocarbon reservoirs. The techniques employed here could potentially be useful for correlation and interpretation of restricted rocks where index fossils may be limited or absent.


AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California