Constraining uncertainty in interpretation of seismically imaged clinoforms in deltaic reservoirs, Troll field, Norwegian North Sea: Insights from forward seismic models of outcrop analogs

Abstract

Forward seismic modeling of outcrop analogs has been used to characterize the seismic expression of clinoforms in different deltaic depositional environments, and thus constrain uncertainty in interpretation of intra-reservoir clinoforms imaged in seismic data from the Troll field, Norwegian North Sea. Three outcrop analogs from the Cretaceous Western Interior seaway, United States, were studied to quantify the geometry, distribution, and lithologic character of clinoforms in fluvial-dominated and mixed-influence deltaic deposits. Outcrop-derived geometric data were calibrated to sedimentological and petrophysical data from the Krossfjord and Fensfjord Formations in the Troll field, and then used to create a suite of forward seismic models for comparison with real seismic reflection data from the Troll field. Clinoforms were imaged in the forward seismic models in which they were (1) spaced wider than the tuning thickness (>10 m [>33 ft]); (2) marked by pronounced interfingering of facies associations with different acoustic properties; and/or (3) lined by relatively thick (>50 cm [>20 in.]) carbonate-cemented layers. However, where clinothems are thinner than the vertical resolution limit of seismic data, destructive interference occurred creating misleading geometrical relationships. Furthermore, our ability to image clinoforms is dependent on (1) the frequency of the seismic wavelet; (2) the overburden velocity; and/or (3) the acoustic impedance contrast at the boundary between the overburden and the clinoform-bearing target. The established methodology has allowed characterization of deltaic clinoformal architectures in reservoir seismic data from the Troll field, and has facilitated a more robust interpretation by bridging the critical gap in resolution between well and seismic data.