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Lower Green River Formation Depositional Environments in the Uinta Basin, Utah

Darrin Burton, Kurtus Woolf, and Bobby Sullivan
Newfield Exploration

The petroleum systems of the Eocene Lower Green River Formation are tied to lacustrine, marginal lacustrine, and fluvial-alluvial depositional settings. These depositional settings can be best identified and mapped spatially and temporally by integrating outcrop, core, and wireline logs. Maps of depositional environments are essential to de-risking exploration and field expansion and development activities. The five principle depositional environments of the Lower Green River Formation are: 1) deep lake, 2) shallow lake, 3) lacustrine delta, 4) lacustrine coastal plain, and 5) alluvial plain. Deep lake environments are characterized by laminated oil shales and fine-grained carbonates. These facies are typified by anomalously high neutron porosity, and low bulk density. Shallow lake environments are dominated by weakly-laminated to massive grey mudstones, and limestones, with occasional thin, high bulk density sandstones. Lacustrine deltas (both sand-prone, and mud-prone) grade from shallow lake muds to ripple-laminated sandstones to cross-bedded sandstones. The upward decrease in mudstone can be seen in the gamma-ray, neutron porosity, and bulk density profiles of deltaic intervals. Coastal plain mudstones have a greenish hue, and frequently contain organic matter. Channels in coastal plain settings are typically thin, isolated and heterolithic. Alluvial plain channels tend to be sandier, thicker, and less isolated than coastal plain channels. Alluvial mudstones are reddish with more abundant pedogenic features. The highest gamma-ray readings are most common in the alluvial plain setting, and are believed to correlate to well-developed paleosols. The vertical association of depositional environments in the Lower Green River indicates both high amplitude and high frequency lake level fluctuations. However, the macroscale trend shows a rapid deepening of the lake lower in the section, followed by a gradual filling of the lake, and a gradual flooding near the top. The Lower Green River depositional environments form key petroleum systems components. Oil shales in the deep lake settings are the major source rock, and coastal plain muds are a potential minor source. Deep and shallow lacustrine shales and tight carbonates form regional seals. Delta, coastal plain, and alluvial plain sands form the principal reservoirs. Deep lake mudstones and carbonates are also potential unconventional reservoirs.

AAPG Search and Discovery Article #90169©2013 AAPG Rocky Mountain Section 62nd Annual Meeting, Salt Lake City, Utah, September 22-24, 2013