Targeting Remaining Barrels in Brown Field Environments: Assessment of Statfjord Formation Using OBN Seismic in Alwyn

Stock-tank oil-in-place (STOIIP) estimates are calculated for two development opportunities in the Alwyn North field in the Northern North Sea. They are from the lower unit within the Statfjord Formation (the Statfjord D), which have been discovered to be light oil-bearing by two wells, N50 and N54Z. Both wells are currently producing gas from the upper Statfjord Unit A. This study presents a methodology workflow with the final aim of calculating reliable STOIIP estimates for the Lower Statfjord.

Using newly acquired and processed (2016) OBN seismic, the existing interpretation of the Top Statfjord horizon is refined with the use of seismic attributes. Extracted wavelets are used to generate synthetic seismograms at three well locations to identify the Statfjord D reflector in the seismic data. An iso-thickness of 100 m is added to the refined Top Statfjord horizon to create a pseudo Statfjord D horizon, which is further corrected for use in volumetric calculations. STOIIP volumes are estimated probabilistically using Monte Carlo stochastic simulation to incorporate uncertainty ranges and distributions for each input parameter. Sensitivity analysis to find out which parameter most influences these estimates revealed that errors in the velocity model contribute the most to the STOIIP uncertainty range.