What Lies Beneath? The Sub-Seismic Character of "The Perfect Fan"

Abstract

Classical models of submarine fans show a lobate geometry with simple channels radiating from the main feeder channel, however, such models are often challenged by production data. The Eocene Frigg Field is a giant gas field and interpreted as a submarine fan. Depositional topography enhanced by draped basinal mudstones and differential compaction provide the reservoir structure. Reservoir properties are excellent, but an active aquifer and intricate sweep pattern hinted at depositional complexity within the reservoir; after 27 years of production the field was shut down in 2004. In 2016 a new production license motivated a revisit to this giant reservoir. The field is penetrated by 17 exploration and 52 production wells; when including satellites there is over 1 km of core available. Excellent seismic imaging permits a unique opportunity to calibrate seismic geomorphology to core and log data. Facies associations were identified from core and the interpretation extended to uncored intervals using wireline log response. These interpretations were integrated with seismic geomorphology and biostratigraphy to develop a new depositional model. The results show that Frigg consists of four chronostratigraphic units, here termed fans, where the lowermost is dominated by large-scale remobilization and mass transport deposits (MTD) that steered deposition of the subsequent fans. Maximum cored thickness of the MTD is 80 m, but wireline logs suggest thicknesses up to 200 m. The directly overlying fan consists of semi-confined lobes complexes 25-75 m thick, that onlapped and filled topographic lows; these are erosionally overlain by several channel complexes that prograded far into the basin and set up the classical Frigg ‘outstretched hand’ geomorphology. Channel complexes are 1-2 km wide with lateral facies changes from amalgamated turbidites to heterogeneous deposits with slumps to successions dominated by bioturbated silty turbidites, reflecting channel axis to margin to overbank facies transects. These interpretations are directly supported by seismic geomorphology and have major implications for our understanding of reservoir connectivity, explaining the observed variability in gas contacts. This study demonstrates the importance of revisiting ‘mature’ datasets when new data and techniques become available, enabling efficient drainage strategies to be developed and the extension of field life

AAPG Datapages/Search and Discovery Article #90350 © 2019 AAPG Annual Convention and Exhibition, San Antonio, Texas, May 19-22, 2019