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The AAPG/Datapages Combined Publications Database

Utah Geological Association

Abstract


Hydrocarbon Systems and Productions in the Uinta Basin, Utah, 2008
Pages 81-100

Characterization of Fluvial Sand Bodies in the Neslen and Lower Farrer Formations (Upper Cretaceous), Lower Sego Canyon, Utah

Rex Cole

Abstract

Sand bodies in the Neslen Formation (260 ft thick) and lower Farrer Formation (290 ft thick) in lower Sego Canyon, Utah, correlate with gas-productive reservoir elements in the Uinta Basin. This paper summarizes the sedimentological characteristics of the outcrop analogs, with hope that the data will enhance subsurface resource recovery. At the study site, three measured sections (total of 1622 ft) were established and 55 sand bodies mapped within an area of about 1.4 mi2. Paleocurrent measurements (N = 374) were also collected to define sediment-transport directions.

The Neslen Formation, which consists mainly of carbonaceous mudrock, fine- to medium-grained sandstone, and coal, was deposited in a lower coastal-plain to marginal-marine setting. The lower Farrer, which has more sandstone and very little carbonaceous mudrock and coal, was deposited in an alluvial-plain setting. The average sandstone-mudrock ratio at the study site is 22:78 for the Neslen and 38:62 for the lower Farrer. Correspondingly, vertical and lateral sand-body connectivity is low in both units. Neslen sand bodies were mainly deposited by small, tidally influenced, high-sinuosity channels, with associated splays. Bay-head-delta facies and transgressive sand-flat facies also occur in the Neslen. Sand bodies in the Farrer were deposited by low- to moderate-sinuosity channels and associated splays.

Overall, the 55 mapped sand bodies range from 1.4 to 27.0 ft thick, and have apparent widths (linear distance between pinchouts) ranging from 52 to 2556 ft. For the Neslen sand bodies (N = 18), the average thickness values range from 1.4 to 12.9 ft and the apparent-width values from 52 to 1222 ft; for the lower Farrer (N = 37), these ranges are 1.5 to 27.0 ft and 92 to 2556 ft, respectively. In the Neslen Formation, 11% of the sand bodies consist of multi-story channel fills, 39% are single-story channel fills, and 50% are splays, whereas in the lower Farrer Formation, these distributions are 65%, 22%, and 13%, respectively. Paleocurrent values (N = 374) have an azimuthal vector mean of 41° and a circular standard deviation of 71°. The paleocurrent data suggest that the Neslen channel-form sand bodies have greater sinuosities that those in the lower Farrer.


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