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In recent years, large petroleum discoveries within the lacustrine microbialite facies of pre-salt systems of offshore Brazil, Angola, and Gabon have increased interest in the potential of microbial carbonates as reservoir rocks. Whereas a multitude of studies have been conducted on microbialites and the associated lacustrine facies separately, there is still little known regarding whether there are definitive dissimilarities between the geochemical footprints of microbialites and associated lithofacies within a system. To explore this unknown, organic geochemistry was evaluated with the aim of comparing the microbialite facies (stromatolites and thrombolites) with the associated lacustrine carbonates (dolomitic marlstone, ooid grainstone, peloidal packstone and wackestone) to test if there are dissimilarities in regard to reservoir and source potential. Specifically, the goal is to understand the petroleum significance of lacustrine microbialites deposited in various lake conditions. Total Organic Carbon (TOC) and extractable organic material were analyzed for microbialite and associated lacustrine lithofacies in the Green River Formation of Colorado and Utah. The samples were from three cores: two from Piceance Basin (Nielsen 17-1 and Colorado Core 01A) and one from Uinta Basin (1 Coyote Wash). Additional hand samples collected from an outcrop at Douglas Pass, Colorado, represent shallow lake-margin deposits. The lake of the Piceance Creek Basin was a hydrologically closed, hypersaline lake characterized by deposition of organic rich microbialites, and dolomitic marlstones. In contrast, the Uinta Basin lake was a hydrologically open lake characterized by accumulation of organically lean microbialites, siltstone, mudstone, ooid grainstone, and peloidal packstone and wackestone. Gas chromatography/mass spectroscopy (GC/MS) analysis yielded biomarker distributions distinct between the lacustrine microbialite facies and the associated lithofacies. The abundance of biomarkers associated with hydrocarbon potential is 10 to 100x greater in the microbialites than in the associated lacustrine carbonate lithofacies. Understanding the geochemical footprints of lacustrine microbialites and how they differ from associated carbonates as a function of depositional environment is crucial in identifying prospects in similar ancient reservoir systems.
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