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Previous HitStratigraphicNext Hit Architecture and Bayfill Classification in the Upper Cretaceous Neslen Formation, Eastern Previous HitBookNext Hit Cliffs, Utah


The Upper Cretaceous Neslen Fm. of the Mesaverde Gp., eastern Previous HitBookNext Hit Cliffs, Utah, USA, records a complex interaction of coastal plain, estuarine and interdistributary bay environments. A dataset based on 56 sedimentary logs along 22 km of outcrops in combination with 2 cored wells and log data from 42 wells enables detailed mapping of facies distribution and construction of a Previous HitstratigraphicNext Hit framework. Based on basinwards and landwards shifts in palaeoenvironment across Previous HitstratigraphicTop surfaces of regional significance, a three-fold subdivision of the Neslen Fm. into a lower, middle and upper member has been applied.

Significant variations in accommodation is recorded in the Neslen Fm. There are observed three episodes of regional incision and subsequent transgressive fill and one period of low accommodation with no local incision. The Lower and Upper Neslen Mbs. is composed of fluvial channels, fines, crevasses and coals deposited in a coastal plain. The transition into Middle Neslen is characterized by a landward shift in facies resulting in deposition in a marginal marine interdistributary bay environment. Six upward-cleaning heterolithic to sand-prone bayfill units are identified, bounded by high frequency flooding surfaces. Most units show large lateral variations in thickness, facies associations, and lithology. The size of the interdistributary bays varies due to changes in accommodation. Basinwards, the bayfills pinch-out into a brackish water lagoon within the study area.

This study also proposes a new work flow for quantitative classification of bayfill facies associations based on the work carried out in the Middle Neslen Mb. The classification is based on the influence of waves and currents as wells as the mud content. Bayfill deposits constitute reservoirs in many fields on the Norwegian continental shelf. The proposed classification has been tested on the Oseberg South Field. The Middle Neslen Mb. represents an important outcrop analogue which has contributed to increased conceptual understanding of bayfill reservoir type and improved the predictability of spatial distribution of reservoir properties.