Microbialites of the Eocene Green River Formation as Analogs to the South Atlantic Pre-Salt Carbonate Hydrocarbon Reservoirs
Stanley M. Awramik and H. Paul Buchheim
Department of Earth Science, University of California
The 2000-meter-thick Eocene Green River Formation stretches across 350 km of Wyoming, Colorado, and Utah, and contains what may be the richest record of lacustrine microbialites. Most microbialites occur in laterally extensive biostromes that have the facies association of ooids/flat-pebble conglomerate, microbialite, and kerogen-rich laminated mudstone. The facies association forms pronounced stacking patterns or parasequences with microbialite biostromes that were deposited on a low- gradient lake bottom. Multi-meter-size bioherms occur in the northwest corner of Green River Basin and are found over an aerial extent of over 2000 km². The bioherms are composed of clusters of meter-scale stromatolites and tufa. The 10-30 m thick bioherms are composed of stacked, 1-3 m-thick successions of oolite and grainstone, microbialite, wackestone, and carbonate mudstone. The various microbialite facies associations and even spring-associated facies occur in specific stratigraphic relationships and their basinal and stratigraphic occurrence can be predicted. They are related to lake phase (under-filled, balance-filled or over-filled), water depth and energy, basinal location (margin to lake center), lake-bottom gradient (low to high gradient), and lake chemistry (calcium saturation, salinity). The Green River Formation is probably the best analog for the large lake systems that formed with the opening of the South Atlantic. The Green River lakes were saline-alkaline (bicarbonate lakes) as indicated by trona evaporates and the stevensite clays (both only occur in saline-alkaline lakes). Volcaniclastics are locally abundant. And now that we have found abundant 'arborescent shrubs' associated with Green River microbialites, we are confident that the Green River Formation will continue to provide many additional and significant insights into better understanding the pre-salt lacustrine systems.
AAPG Search and Discovery Article #90169©2013 AAPG Rocky Mountain Section 62nd Annual Meeting, Salt Lake City, Utah, September 22-24, 2013