AVO ANALYSIS IN THE MIDDLE MIOCENE, EASTERN GULF OF MEXICO: EXPLORATION RISK REDUCTION FOR PETROLEUM AND RESERVOIR PRESENCE
MARC L. SBAR, 1996. "AVO ANALYSIS IN THE MIDDLE MIOCENE, EASTERN GULF OF MEXICO: EXPLORATION RISK REDUCTION FOR PETROLEUM AND RESERVOIR PRESENCE", Stratigraphic Analysis Utilizing Advanced Geophysical, Wireline and Borehole Technology for Petroleum Exploration and Production, Jory A. Pacht, Robert E. Sheriff, Bob F. Perkins
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AVO analysis was used to high-grade a prospect defined by a four-way structural closure in the eastern Gulf of Mexico. Petroleum presence was indicated by nearby shallow wells which encountered oil and gas. Seal was suggested by low reflectivity seismic data above the target zone indicating shales. The key risk in this prospect was reservoir presence, since strong trough/peak amplitudes characteristic of many Gulf of Mexico petroleum sands were not observed.
The target zone was predicted to be Middle Miocene and deeper than 15,000 feet below mud line. Analogs from elsewhere in the Gulf of Mexico indicated that strong trough/peak amplitudes on seismic may not result for petroleum sands in this environment. Stratigraphic analysis in the target zone identified mounded facies, interpreted as a mid-slope fan or channel/levee complex, which may contain sand. AVO was analyzed to see if the risk for reservoir and petroleum presence could be reduced. Sand with moderate GOR oil was predicted predrill using four 2D lines and rock properties from an analog well.
The well was drilled about 800 feet from the nearest 2D line and encountered thin oil sands in the target zone. Models were generated using rock properties from this well to demonstrate that these anomalies may result from thicker, more porous sand that is possibly gas-charged. The modelling also showed that sands with porosities less than 20% would be difficult to detect using AVO regardless of their thickness. Thus, AVO could be used for appraisal to map the variability in sand porosity and high-grade those areas of higher porosity. This would best be applied in 3D.