Microbial Carbonates from Core and Outcrop, Tertiary (Eocene) Green River Formation, Uinta Basin, Utah

Recent discoveries in Early Cretaceous microbialites in the deepwater offshore of Brazil (pre-salt Santos Basin reservoirs) as well as other large oil deposits in microbialites reveal the global scale and economic importance of these distinctive carbonates. Evaluation of the various microbial fabrics and facies, petrophysical properties, diagenesis, and bounding surfaces are critical to understanding these reservoirs. Utah is unique in that representative outcrop analogs of microbial reservoirs are present and cores from these areas are available for detailed study. The Eocene Green River Formation from the Uinta Basin of eastern Utah contains excellent examples of microbial carbonates.

The Uinta Basin is a major depositional and structural basin which subsided during the early Cenozoic along the southern flank of the Uinta Mountains. Freshwater lakes developed between the eroding Sevier highlands to the west and the rising Laramide-age uplifts to the north, east, and south. The Green River Formation, consisting of up to 6000 ft of sedimentary rocks, accumulated in and around Lake Uinta. Three major depositional facies are associated with lake sedimentation: alluvial, marginal lacustrine, and open lacustrine. The open lacustrine environment is represented by nearshore and offshore shales and mud-supported carbonates, including microbialites.

Analysis of newly acquired Green River cores reveals a variety of microbial fabrics and related features. The overall section consists of siltstones and mudstones to dolomitic mudstones with clay-rich and black organic-rich zones. Within the dolomitic mudstone are well-displayed, porous, microbial laminae and stromatolites with bulbous heads. Grainstones composed of ooids, coated grains, pisolites, and peloids often overlie the microbialites. Soft-sediment deformation, bioturbation, and rip-up clasts are also often associated with these microbialites. The grainstones and microbialites exhibit excellent storage consisting of microintercrystalline, interparticle, and moldic pore types.

Outcrops of the Green River Formation in the eastern part of the Uinta Basin also display many of the features observed in core, both vertically and horizontally. They offer a production-scale analog of the characteristics, geometry, distribution, and bounding surfaces of microbial and related lacustrine facies.

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