Spatial and Stratigraphic Distribution of Microbialite Facies in the Eocene Green River Formation: Developing an Exploration Analog to the South Atlantic “pre-Salt”

Abstract

The discovery of large petroleum reserves in reservoirs composed of microbialites in lacustrine successions in the Lower Cretaceous “pre-salt” of offshore Brazil and Angola has motivated a quest for modern and ancient analogs. However, little is known about the spatial and stratigraphic distribution of microbialites in large lacustrine basins, primarily because there are few such microbialite-rich systems. The Green River Formation (GRF) of WY, UT, and CO is one of the few and probably provides the best analog. The GRF has outstanding exposures. Microbialites are abundant and varied, and include: bioherms, biostromes, oncolites, and microbialite associated tufas. Spring deposits forming mounds up to 30+ m thick also occur. The formation provides particular insights into the basinal distribution and temporal facies relationships of: (a) large, multimeter-size bioherms, (b) smaller, patch-reef-like bioherms, (c) inter-biohermal facies, (e) biostromes, (f) spring deposits (tufa) associated with microbialites, (g) shrubs, and (h) successions with stevensite, microbialites, and oolites to the overall lake system. The excellent outcrops and tuff marker beds allow microbialite-containing units and associated facies to be traced laterally over 100 kilometers from basin margin into source-rock, kerogen-rich facies at the basin center. The GRF also contains some of the largest lacustrine microbialite bioherms known. Large bioherms at Little Mesa, near LaBarge, WY, have been the focus of numerous studies as well as training for companies exploring South Atlantic pre-salt reservoirs. We are currently studying the spatial and stratigraphic relations of the GRF microbialites with an eye to developing models that explorationists can apply to pre-salt systems. For example, we have recognized three spatial patterns and vertical facies sequences that have a direct application. These are based on patterns that develop as the lake expanded and contracted in response to climate, precipitation/evaporation, basin subsidence (accommodation space), and rates of sedimentation. In addition these can be related directly to the under-filled, balance fill, and over-filled model of Carroll and Bohacs. The bottom-line question is, can these models predict the spatial and stratigraphic distribution of microbialite reservoirs? If the current interest in the Green River Formation is any gage, the answer is a definite yes.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016