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Abstract: Open Natural Fractures in Sandstone at 18,300 Ft: Do They Help or Hinder Production of Gas?

John C. Lorenz, Randall L. Billingsley, Lesley W. Evans

Vertical core, from relatively undeformed Cretaceous (Frontier Fm) sandstones at a depth of 18,300 ft in the Green River Basin, contains three sets of mineralized natural fractures. The earliest fractures opened in extension as the strata passed through the hydrocarbon window. Continued subsidence and the maturation of organic: material created overpressured conditions, causing oil to be injected into the fractures. Only a carbon residue of the original oil remains in these fractures, suggesting continued burial and maturation. The residue seriously inhibits permeability along and across fractures. Oil residue is also present in oblique, unmineralized mare's-tails at the ends of fractures, suggesting that the in-situ stress orientations had changed slightly. Quartz crystals mineralize the fracture walls, growing over the oil residue or pushing it aside. A second set of extension fractures strikes 20-30 degrees oblique to the first set. These fractures are mineralized with calcite, which was also deposited over the quartz in the first fracture set. Continued tectonism reoriented the horizontal stresses by nearly 90 degrees, forming a third set of extension fractures and further degrading permeability by narrowing apertures along earlier fractures. Significant porosity remains along many of the fractures at this depth, yet the in situ stresses and oil residue have combined to degrade fracture permeability to uneconomic matrix values.

AAPG Search and Discovery Article #90959©1995 AAPG Rocky Mountain Section Meeting, Reno, Nevada