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Technical Paper: Recipe for Success in Ultradeep Water
Paper 77625
Abstract
Generally, for sandstone reservoirs, shallow deposits tend to be unconsolidated containing heavy oil, and deep deposits tend to be hard rock of low permeability containing gas. The result is an overall leveling of mobility, which relates directly to well potential. Carbonate reservoirs offer significant departures from sandstone trends, and this explains why 8 of the 10 largest and most prolific reservoirs are carbonate formations.
Typical trends are altered when a significant portion of the depth to the formation is water depth. More or less, the formation age to depth correlation follows depth below mud line rather than below the water surface. However, the fluids may have experienced pressures and temperatures that would accelerate the maturation process from heavy oil to light oil and, even, gas. The result would be much lower than typical fluid viscosity in very large- permeability sediments, leading to high mobility and extraordinary well productivity.
The paper makes the case that under ultra deep waters, in sedimentary environments such as the Gulf of Mexico, offshore Brazil and West Africa, reservoir quality and productivity can be readily inferred and even more to the point, the depth at which specific grade of hydrocarbons can be found. In particular, very large natural gas reservoirs are likely to lie beneath oil reservoirs, the latter containing also large volumes of gas in solution.
Information from some of the largest ultra-deep-water accumulations of hydrocarbons shows correlation of depth versus mobility, which is contrasted with data from terrestrial reservoirs. This phenomenon is explained through basic principles.
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