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Abstract

Detailed sedimentological analysis and subsurface paleoenvironmental mapping of Eocene sandstones in the upper Douglas Creek Member of the Green River Formation indicate that two major depositional systems, fluvial-deltaic and wave-dominated lacustrine shoreline, are represented along the northeastern margin of ancient Lake Uinta. Within correlative units, which include important oil-producing sandstones, depositional environments can be traced laterally from onshore fluvial-deltaic, to lacustrine barrier-beach, and to low-energy offshore lacustrine.

Sandstone-to-shale sequences that become finer grained upward above a sharp basal contact are interpreted to represent fluvial-channel deposition. Sandstone is quartzose in composition, commonly contains plant fragments, and is cross-bedded. Channel sandstone thickness is 1.5-3 m. Mapping of fluvial deposits indicates that elongate channel sandstone bodies trend southwest, which is consistent with a Uinta uplift source area to the north. General southwestward progradation of the fluvial-deltaic system was interrupted by lacustrine transgressions.

Sequences that gradually become coarser grained upward from shale and siltstone at the base to cross-bedded, fine- and medium-grained sandstone at the top are interpreted as prograding shoreface deposits. Sandstone is quartzose and commonly contains ooids. Thickness of shoreface sequences ranges from approximately 3 to 11 m. At the top of many coarsening-upward sequences, algal structures and probable mud cracks in thinly laminated dolomitic mudstone indicate very shallow water and possible subaerial exposure. Prograding shoreface deposits form thick, laterally continuous sandstone bodies interpreted as barrier-beach complexes. Paleoenvironmental mapping, based on data from 516 wells, shows that the barrier-beach facies occurs between offshore-lacustrine facies to the west-southwest and mud flat-lagoonal facies to the east-northeast. Maximum lateral dimensions of barrier-beach complexes are approximately 15 km parallel to paleoshoreline and 5 km across.

Comparison of these results with those with from previous studies of the Green River Formation in the Uinta basin indicates that the distribution of wave-produced sequences was restricted along the basin margin by local conditions related to shoreline configuration and lake bathymetry. Favoring development of shoreface sequences in certain areas were large wave fetch, shoreline stability, clastic sediment input, and deep water offshore (i.e., depth below wave base). During at least some of the time represented by deposition of the lower Green River Formation, those conditions persisted along the northeastern margin of Lake Uinta but not along other lake margin areas.

The description of lacustrine sandstones from this study can be used as a comparison with lacustrine deposits in other areas. As demonstrated for the lower Green River Formation, fluvial channel sandstones and shoreface sandstones form major hydrocarbon reservoirs in proximity to one another along margins of ancient lakes.

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