--> Abstract: Geologic Constraints on Oil Recovery and Uncertainties in Undiscovered Resources, by D. L. Gautier, C. J. Shenk, and R. F. Mast; #91007 (1991)
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Geologic Constraints on Oil Previous HitRecoveryNext Hit and Uncertainties in Undiscovered Resources

GAUTIER, DONALD L., CHRISTOPHER J. SHENK, and RICH F. MAST, U.S. Geological Survey, Denver, CO

Estimated amounts of undiscovered world resources of crude oil range from about 250 billion to almost 1000 billion bbl. These estimates commonly depend upon (1) geologic estimates of total undiscovered in-place resources, and (2) predictions of the recoverable fraction of the resource (the Previous HitrecoveryNext Hit factor). Commonly, resource estimates of future discoveries and production implicitly assume a Previous HitrecoveryNext Hit factor similar to the historical average of 20-40%. However, ultimate recoveries in individual reservoirs may range from less than 10% to as much as 60% of original in-place resource, thus imparting a large degree of uncertainty in evaluating potential petroleum resources. For the most part these uncertainties derive from four geologic factors: the degree of compartmentalization of reserv ir rocks (a function of depositional environment, structure, fracturing), diagenesis and mineralogy of the reservoir rock, properties and chemistry of the contained fluids, and the type of reservoir Previous HitdriveNext Hit. Classifications of recoverability must include considerations of at least these four elements for successful prediction of Previous HitrecoveryNext Hit.

For example, well-sorted eolian sandstones with relatively simple diagenetic histories and degree of compartmentalization are commonly cited as ideal reservoir rocks in terms of oil Previous HitrecoveryNext Hit. Reservoirs of the Permian upper part of the Minnelusa Formation in the Powder River basin, Wyoming, consist mainly of eolian-dune sandstones with good to excellent reservoir properties, with a low degree of reservoir compartmentalization and a simple diagenetic history. However, the eolian reservoirs display primary recoveries generally less than 15%, mainly because of a weak fluid expansion Previous HitdriveNext Hit. Recoveries are commonly brought up to 25-30% with water flooding and EOR technologies. In contrast, eolian reservoirs of the Jurassic Nugget Sandstone in the Utah Overthrust belt have much higher recov ries because the reservoirs have a strong Previous HitdriveNext Hit by combined gas-cap expansion and water Previous HitdriveNext Hit. However, the eolian Nugget Sandstone is highly compartmentalized by interbedding of dune and interdune facies, fracturing, and diagenesis, in contrast to reservoirs in the upper part of the Minnelusa Formation. Compartmentalization of Nugget reservoirs makes high recoveries difficult to sustain. Conversely, in many cases, reservoir properties and Previous HitdriveNext Hit Previous HitmechanismsNext Hit are not the critical factors controlling Previous HitrecoveryNext Hit, as economic and technical Previous HitrecoveryNext Hit in some reservoirs depend upon fluid viscosity and oil composition, an extreme example being heavy oil deposits of Venezuela.

Systematic evaluation of the influences of compartmentalization, diagenesis, fluid properties, and Previous HitdriveTop mechanism will be necessary for improved prediction of future petroleum production from both today's proved reserves and remaining undiscovered resources.

 

AAPG Search and Discovery Article #91007© 1991 AAPG International Conference, London, England, September 29-October 2, 1991 (2009)