Resolution of Carbonate Reservoirs at Depth—An Integrated Approach to Lower Development Risk, Smackover Trend, Alabama
Timothy J. Petta, J. Gregory Bryant, 1991. "Resolution of Carbonate Reservoirs at Depth—An Integrated Approach to Lower Development Risk, Smackover Trend, Alabama", The Integration of Geology, Geophysics, Petrophysics and Petroleum Engineering in Reservoir Delineation, Description and Management, Robert Sneider, Wulf Massell, Rob Mathis, Dennis Loren, Paul Wichmann
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Porous Smackover algal mounds associated with granite knobs produce oil throughout southwestern Alabama. A detailed gravity survey over Vocation Field demonstrates lateral density contrasts between reservoir and non-reservoir rocks can be measured even when the reservoir depth exceeds 14,000 feet (4270 m). A similar gravity survey combined with standard seismic and subsurface interpretation methods could have drastically increased the success rate and lowered development costs.
Vocation Field will produce nearly 5 million barrels of oil from high energy algal mound and ooid facies deposited on the flanks of the basement knob. Porosity is dolomite intercrystalline, moldic and interparticle. Most of the porosity was inherited from the original high energy rock fabric. Porosity values as high as 18 percent occur within the pay section. Effective porosity exceeds 250 feet on the flanks of the structure. The trap was not filled to the spill point. Water provides the reservoir drive and recovery is expected to exceed 40 percent of the oil in place. Eight of the sixteen wells have produced.
The basement high is a slight anomaly on regional gravity and aeromagnetic surveys. Seismic lines over the feature indicate structural closure but near the crest the Smackover and basement events are indistinguishable. The first wells were drilled near the crest and were successful discovery and confirmation wells. Only six of the remaining fourteen wells drilled were successful, a disappointing rate of 42 percent. Some of these wells were structurally low and wet, others structurally high and tight. Seismic data provided no clue