AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Very Large Oligocene Age Frio Reservoirs at Great White Field in the Ultra-Deepwater of the Gulf of Mexico Offer Significant Development Challenges and Long-Term Growth Potential
(1) Shell Exploration & Production Co., Houston, TX.
(2) Shell International Exploration & Production Co., Houston, TX.
The Great White Field is located in the Perdido Foldbelt in the western Gulf of Mexico, 200 miles south of Freeport, Texas. Water depth in the field is 8000ft. Great White was discovered in 2002 by exploratory well AC857-1. A total of three oil-bearing formations have been penetrated in the field: M. Oligocene age M. Frio Fm. Sands, L. Eocene U. Wilcox Fm. Sands, and L. Paleocene L. Wilcox Fm. Sands.
The Great White Field is one of a number of northeast-southwest trending, compressional structures in the Perdido foldbelt and is just south of the prominent Sigsbee Escarpment. The structure is a complexly faulted, four-way closure. The main structure is a fold-thrust that is fully offsetting at depth, but dies out into a fold at the Oligocene level.
The oil bearing M. Frio Sands at Great White have been buried to a depth of about 1500 to 3500ft below the mudline. Low overburden pressure and reservoir temperature result in the M. Frio sands remaining highly unconsolidated and uncompacted. These six, low pressure reservoirs contain biodegraded, low GOR, 18 degree API oil.
M. Frio sands are composed of fine grained, feldspathic litharenites. There is an abundance of unaltered, volcanic glass fragments. The presence of glass and the lack of mechanical compaction or chemical alteration of the original sediment results in very porous, compressible reservoir sands. In addition, these reservoirs are highly faulted. The unusual characteristics of these reservoirs mean that significant uncertainties remain in predicting Frio development well performance; including: the strength of reservoir drive, the risk of compaction-related well failure, uncertain initial permeability distribution, and the degree of permeability degradation with depletion.
M. Frio reservoir quality deteriorates north of the crest of the structure. Well AC813-1 penetrated poor quality, wet M. Frio. The few thin sands that were present had little volcanic glass and abundant clinoptilolite, a zeolite that is an alteration product of volcanic glass.
The M. Frio Sand reservoirs at Great White contain approximately two billion stb of OIP, making these reservoirs a key long-term target for development. However, development presents significant challenges. Thus, we plan a staged development of the Frio to better assess the upside potential and the risks of drilling and producing these reservoirs.