Print this pageAAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
The Pony Discovery, Green Canyon Block 468, U.S. Gulf of Mexico: Structural Uncertainty Analysis
(1) E&P, Hess Corporation, Houston, TX.
The Pony Discovery is a large subsalt 4-way dip closure with Middle and Lower Miocene reservoirs at depths near 30,000 feet. Five wells have penetrated the reservoir all clustered near the crest of the 
structure
. The structure is relatively low relief therefore small changes in the migration velocity cause differences in the depth structure that translate to large changes in both the area of closure and the calculated reserves. A methodology is presented that integrates the well information and the surface seismic to estimate this structural uncertainty. Subsalt GOM seismic data yield a low frequency velocity model even with the latest wide azimuth acquisition methods and anisotropic imaging algorithms. This poor velocity resolution is due to the low signal to noise and small angle of incidence range subsalt. Velocity information from the five wells at Pony were analyzed utilizing sonic logs, seismic-to-well ties and VSPs. This analysis of the well data indicated substantial subsalt velocity variations that are not detectable from the surface seismic analysis. The velocity variations observed in the wells were used to create end member velocity estimates. These end member velocities were used to extrapolate the velocities away from the well control. Three different velocity structure scenarios were used in the extrapolation of the end member velocities: flat model; base of salt structure model and reservoir structure model. The most pessimistic structural scenario (low case) is the fastest velocities and flat structural extrapolation away from the well control. The most optimistic structural scenario (high case) is the slowest velocities and reservoir structure model. These high case and low case structural scenarios define the best estimate of the range of structural uncertainty of the reservoir. An analysis of the well derived velocities indicate that the reservoir structure model matches observed well velocities better than the flat velocity model or the base of salt structure model. Consequently, the best technical estimate (BTE) of the true subsalt structure is derived from an extrapolation of the average of the well derived velocities following the reservoir structure model
away from the well control.