PALEOENVIRONMENTAL FACTORS CONTROLLING MICROBIALITE BIOHERM DEPOSITION AND DISTRIBUTION IN THE GREEN RIVER FORMATION

The Eocene Green River Formation of Wyoming, Utah, and Colorado contains the most diverse and abundant occurrences of lacustrine microbialites known. One of the largest oil fields in the Uinta Basin, Utah, is sourced from a 30 m thick, 4 km long microbialite/oolite bioherm that has produced close to a million barrels of oil. Outcrop exposures of analogous bioherms occur in the Piceance Creek Basin (Colorado), Fossil Basin (Wyoming), and Green River Basin (Wyoming). These occurrences provide analogs for exploration of petroleum in other lake basins.

Recently described microbialite bioherms up to 9 m thick from the La Barge area of the Green River Basin of Wyoming may provide the best outcrop analogs. The bioherms are composed of domical and columnar stromatolites, some 2 to 3 meters in diameter, as well as ooids, oncoids, calcified caddisfly larval cases, and other carbonate components. Laterally, bioherms are discontinuous (each up to 150 m in diameter); however, the lake-margin bioherm system is found in a large arc (over 250 km) across the western, northern, and eastern greater Green River Basin. Bioherms grade laterally into adjacent calcitic lake facies over a distance of 100 meters and into dolomitic oil shales of the Wilkins Peak Member over 15 km.

Since thick microbialites are reservoir rocks in some oil fields, it is important to understand the paleoenvironmental conditions that favor the formation of bioherms over biostromes. In the southeastern part of the Green River Basin, biostromes are the exclusive megastructure of microbialites, whereas in the northwest to northeast part of the basin bioherms occur in addition to biostromes. The biostromes are found in facies sequences interpreted to be balanced-filled lake deposits where regressions and transgressions over very low gradients were frequent. Bioherms appear to have been favored by under-filled lake basin conditions, where localized fresh-water deposition was restricted to the lake margins.

The Green River Formation bioherms provides valuable insights into the exploration of microbialites as petroleum reservoirs in ancient lake basins. Further study of the bioherms of the Green River Formation should provide data that will help to better understand their origin, stratigraphic, and spatial location in ancient lake basins.