Saturation Mapping From 4-D Seismic Data in the Statfjord Field
Authors: Doyen P.M. , Psaila D.E. , Astratti D. , Kvamme L.B. , Al-Najjar N.F. 
Published: October 2008

Abstract
A primary objective of the Statfjord 4-D program is to improve recovery by identifying unswept oil pockets in the Brent reservoirs as targets for infill wells. To assist the mapping of bypassed pay from time-lapse seismic data, a detailed 4-D earth model was constructed over the central portion of the Statfjord Field. The earth model integrates seismic data from a pre-production survey acquired in 1979 and repeat surveys from 1991 and 1997. Log data from 180 wells and flow simulation saturation and pressure predictions, at time steps corresponding to the three survey times, have also been incorporated in the earth model. A Statfjord-specific rock model was applied to link the flow simulator response to acoustic impedance and enable the calculation of timedependent synthetics for comparison with the real seismic amplitude data. The rock model predicts significant increase in impedance when water replaces oil in the Brent sands. Seismic difference analysis with region growing techniques reveals the existence of strong production-related anomalies consistent with the rock model predictions. Impedance difference maps were calculated by vertical averaging over the Tarbert Formation (Upper Brent) from inverted volumes for the three surveys. The seismic impedance differences were calibrated against average saturation changes at the well locations, and 91-79 and 97-91 saturation difference maps were calculated via cokriging, using seismic impedance as a guiding attribute. Comparison of the seismic-derived saturation maps with flow simulator predictions reveals several areas that may contain bypassed oil. A 3-D model of 97-91 saturation changes in the Tarbert Formation was also constructed using a Bayesian stochastic simulation technique constrained with seismic impedance difference data. In particular, a probability volume was derived, allowing quantification of uncertainty in the classification of unswept areas. The entire study was conducted using the EarthGM? software which provides 3-D as well as 4-D earth modelling capabilities.

Introduction
Discovered in 1974, Statfjord[1] is a giant, mature oilfield straddling the Norwegian and UK sectors of the North Sea, with recoverable oil reserves in excess of 4 billion barrels. Located 220 km northwest of Bergen in the East Shetland Basin, the field comprises two parts, as shown in Figure 1. The Main Field consists of a major fault block tilted in a westnorthwest direction. To the East of its crest, lies a structurally complex area described as the East Flank, which comprises many reservoir compartments bounded by listric faults.
Oil production originates from 3 separate reservoirs in the Brent, Dunlin and Statfjord Formations. The deltaic sandstone reservoirs of the Middle Jurassic Brent Group, which are the focus of this work, contain approximately 80% of the total original oil in place. The Brent Group has good to excellent reservoir properties, with average porosities between 18 and 29%, high net-to-gross ratios (0.75) and high initial oil saturations (80%). The sediments of the Tarbert, Etive and Rannoch formations in the Brent Group were deposited in higher energy depositional environments and tend to have the best reservoir properties.