1999 Conference Abstracts

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1999 Abstract: Kjærefjord

Bayfill Successions in the Lower Jurassic Åre Formation, Offshore Norway: Sedimentology and Heterogeneity Based on Subsurface Data from the Heidrun Field and Analog Data from the Upper Cretaceous Neslen Formation, Eastern Book Cliffs, Utah

J.M. Kjærefjord
Statoil—Den norske stats oljeselskap a.s
N-5020 Bergen


Interdistributary bays form a major sedimentary subenvironment within many delta systems. These interdistributary bays occur as open bodies of water that are typically open to the sea. They may cover large areas of the lower delta plain and are normally surrounded by marsh and distributary-channel features. The interdistributary bay deposits in the Åre Formation of Heidrun field, offshore Norway, and in the Neslen Formation, Utah, occur as shallow, 2- to 7-km-wide features that were deposited in brackish to marine waters. Within these bays, low-energy wave action reworked a large proportion of the sedimentary features, such as crevasse splays, bayhead deltas, and distributary-mouth bars. The resulting wave-influenced bayfill lithofacies association (WLA) shows a relatively similar vertical development of lithofacies in both the Åre and the Neslen Formations. Normally the WLA is 2 to 8 m thick and contains a basal mudstone overlain by a wave-influenced lenticular and/or wavy-bedded heterolithic unit. The upper few meters consists of wave-rippled and small-scale hummocky cross-laminated sandstone with good reservoir qualities. Roots or coal or both are present at the top. Within the sandstone, mud drapes or heterolithic layers are common. In the Neslen Formation these drapes seem to be concentrated along gently dipping clinoform surfaces. Laterally, along the depositional strike and landward, the reservoir quality changes abruptly as the WLA passes into the bayhead delta, marsh, sub-bay/lake, and distributary-channel-fill lithofacies associations. Although individual bayfill successions are relatively thin, continuing subsidence and repetition of similar processes resulted in a stacking of one bayfill succession on top of another, building an 80-m-thick package of lower delta plain deposits.