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Abstract: Combining Subsurface Natural Fracture Predictions with Oil Generation Estimates: An Example from Uinta Basin, Utah

HORN, MYRON K., Consultant, Tulsa, Oklahoma

Summary

The Uinta basin of Utah contains oil and gas in fractured reservoirs (Lucas and Drexler, 1976). The Altamont Bluebell field of this basin is chosen in order to test a concept that could be used to predict hydrocarbon-rich fractured reservoirs in, not only other parts of the Uinta, but in basins in other parts of the world.

The premise of the concept is to quantitatively predict the simultaneous occurrence of fracture formation and hydrocarbon generation. The advantage of such a prediction is that early emplacement of hydrocarbons in fractures enhances the probability of the said fractures remaining open through time; therefore enhancing reservoir potential. The early emplacement of hydrocarbons in fractures inhibits post-fracture diagenetic healing. Also, fluid pressures associated with the hydrocarbons would assist in maintaining fracture permeability.

Two well-established models are used to a) predict fracture formation (Narr and Currie, 1982) as a function of burial history, and b) predict hydrocarbon generation (Hunt et al, 1991) also as a function of burial history. The two models are combined to produce a unique indicator, called the Fracture Oil Index or FOI. An FOI of less than -1 is an indicator of hydrocarbon-rich fractures. Using Microsoft EXCEL spreadsheet formats, the calculated minimum FOI for the Eocene Green River formation in the Altamont Bluebell is -58, which occurred -13 million yearn ago at a burial depth of 5.73 km (18,800 ft). The -58 FOI ranks 62 out of 614 in technique-related global studies carried out in 174 basins (61 studies yielded FOI's less than -58).

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah