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Updated August 05, 2003

November 2001

AVO Analysis in the Middle Miocene, Central Gulf of Mexico:  
Exploration Risk Reduction for Petroleum and Reservoir Presence

Marc L. Sbar, Phillips Petroleum Company, Bellaire, Texas

Abstract Summary:This paper presents a case study of 2D, AVO modeling and analysis. AVO was used to predict reservoir presence and fluid content pre-drill and reservoir quality post-drill for a deep-water wildcat well drilled in the offshore Gulf of Mexico. The key risk for this prospect was reservoir presence.

The target zone was predicted to be Middle Miocene and deeper than 15,000 feet below the water bottom. Stratigraphic analysis in the target zone identified mounded facies, interpreted as a mid-slope fan or channel/levee complex, which may contain sand. An analog well from elsewhere in the Gulf of Mexico indicated that strong trough/peak amplitudes on seismic may not result for petroleum-filled sands in this environment. AVO analysis also predicted sand as well as moderate GOR (gas-oil ratio) oil predrill, using four 2D lines and rock properties from an analog well.

BP Exploration, the operator, drilled the well about 800 feet from the nearest 2D line and encountered thin, 20% porosity, oil sands in the target zone. Models generated using rock properties from this well demonstrated that 1) the AVO anomalies observed on the 2D lines may result from thicker, more porous sand that is possibly charged with gas or high GOR oil and that 2) the well may have encountered the edge of a more porous reservoir. The modeling also showed that 3) sands with porosities 20% would be difficult to detect using AVO regardless of their thickness. Thus, AVO could be used in an appraisal mode to map the variability in sand porosity and high-grade those areas of higher porosity. This AVO analysis should be applied in 3D.

Biographical Sketch: Marc L. Sbar received his Ph. D. in geophysics at the Lamont-Doherty Earth Observatory of Columbia University where he did research in earthquake seismology and tectonics. Later he taught and continued academic research at the University of Arizona. In 1983 he joined Sohio, which later merged with BP. During this time he worked in variety of geologic settings from hard rocks to soft muds and environments including exploration, development and research. He left BP in March, 1999 and worked as an independent consultant and director of interpretation at Geophysical Development Corporation. In December, 2000, he joined the Applied Geoscience Technology group of Phillip Petroleum's Worldwide Deepwater Exploration team.