Sedimentary facies in the fluvial-dominated Åre formation as seen in the Åre 1 member in the heidrun field

Abstract

The Åre 1 member in the Heidrun Field was deposited in a fluvial to deltaic setting where variations in accommodation space and sediment supply controlled the different sedimentary facies and reservoir properties. It shows an overall transgressive sequence from fluvial plain in the lower part to lower delta plain in the upper part. Fluvial channels and thick coal-bearing floodplain deposits dominate the lower part of the Åre 1. Stacked (multi-storey) channels were deposited as a response to relative fall in sea level. These sands have large lateral continuity compared to the meandering (single-storey) channel sands that are more a function of autocyclic processes and are not correctable between wells. The upper part of the Åre 1 member is dominated by stacked bay/lake fill sequences and thin, but laterally continuous coals. These represent deposition in a lower delta plain setting where the dominant depositional agents were crevasse channels and splays (crevasse deltas) with subsequent wave reworking in places. The reservoir properties of the Åre 1 member are strongly controlled by depositional processes and the sequence stratigraphic framework. Incised valley fill deposits (LST) have large lateral continuity and the internal reservoir properties are very good. The TST deposits are more dominated by thick, coal-bearing, floodplain deposits with no reservoir quality, thin crevasse splays with restricted lateral continuity and limited reservoir properties and single-storey channel sands with very good internal reservoir properties but limited lateral continuity. The individual parasequences in the stacked bay fill sequences (HST) have good lateral continuity but reservoir properties are limited, and the bases of individual bay fill sequences are vertical permeability barriers, at least on a local scale.