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ABSTRACT

The Middle Jurassic Brent Group of Northwest Hutton Field includes recently recognized incised valley systems in the Etive and Ness formations. These valleys were cut during relative sea-level falls and back-filled during subsequent rises. Valley-fills comprise mixed fluvial-tidal and purely fluvial rocks. Their grain size, orientation, and geometry contrast markedly with volumetrically greater highstand shoreface and delta mouth bar sheet sandstones. The valley-fills have significantly influenced production history of the field.

Published data from other Brent Group fields allows correlation of sequence boundaries semi-regionally. Incised valley-fills range from 1.5 to 15 km in width and appear to have been focused on the Northwest Hutton area through time. Similarly other Brent Group fields appear to have occupied interfluve positions at successive sequence boundary times. This observation reconciles historical conflicts over the presence of sequence boundaries in the Brent province, with some fields having only subtle expressions of these key regional surfaces. Comparison of Brent incised valleys with well exposed examples from the Book Cliffs, Utah increases confidence in valley-fill interpretation and allows prediction of likely ranges of geometry and fill type in undrilled areas. The Book Cliffs example also raises the issues of the nature and location of possible lowstand deposits in front of the Brent delta.

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