Geologic Constraints on Oil Recovery 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 recovery factor). Commonly, resource estimates of future discoveries and production implicitly assume a recovery 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 drive. Classifications of recoverability must include considerations of at least these four elements for successful prediction of recovery.

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 recovery. 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 drive. 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 drive by combined gas-cap expansion and water drive. 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 drive mechanisms are not the critical factors controlling recovery, as economic and technical recovery 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 drive 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)