Lacustrine depositional environments in the Green River Formation, Uinta Basin: Expression in outcrop and wireline logs

Abstract

Lacustrine basins are key oil-productive areas of the world. Because fewer lacustrine exist than marine basins, lacustrine systems are relatively less well studied. This paper investigates fluvial-lacustrine depositional environments and their representation in wireline logs in the lower part of the Green River Formation, Uinta Basin, Utah. The five principal depositional environments of the lower Green River Formation are (1) deep lacustrine, (2) shallow lacustrine, (3) lacustrine delta, (4) coastal plain, and (5) alluvial plain. Deep-lake environments are characterized by laminated oil shales and fine-grained carbonates. These facies exhibit anomalously high neutron porosity, and low bulk density relative to other settings. Shallow-lake environments are dominated by weakly laminated to massive gray 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 to cross-bedded sandstones. The upward decrease in clay 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 abundant pedogenic features. The vertical association of depositional environments in the lower Green River indicates both high-amplitude and high-frequency lake-level fluctuations. However, the macro-scale trend shows a rapid deepening of the lake lower in the section, followed by a gradual filling of the accommodation, and a gradual flooding near the top of the studied interval. The lower Green River depositional environments form key petroleum system components. Oil shales in the deep lacustrine settings are the major source rock, and coastal plain muds are a potential minor source. Regional seals are formed by deposits of tight lacustrine shale and carbonate deposited in both marginal and deep lacustrine settings. Delta, coastal plain, and alluvial plain sands form the principal reservoirs. Some deep lacustrine mudstones and carbonates are also potential unconventional reservoirs. Correlation of outcrop observations from well log expressions allows depositional environments to be interpreted in the Uinta Basin and other lacustrine basins.