John C. Lorenz1,
Lee F. Krystinik2,
Thomas W. Griffith2,
Thomas H. Mroz3
(1) Sandia National Labs, Alberquerque, NM
(2) Union Pacific Resources, Fort Worth, TX
(3) Federal Energy Technology Center, Morgantown, WV
Abstract: Production from natural fractures reactivated by faulting: The Rock Island Unit 4-H Horizontal Well, Green River Basin, WY
Data from 78 ft of horizontal core and a wellbore-image log, from a 1700 ft lateral drilled into the Frontier Formation in the Rock Island Unit 4-H well, Sweetwater Co., Wyoming, document a compound natural fracture system. This system currently supports production of 12-20 MMCFG/D from a depth of 15,000 ft, despite the average matrix permeability of only 25 md. 76 natural fractures occur in the cores, suggesting an average fracture spacing of one ft. However, spacing ranges from less than an inch to nearly 17 ft. Moreover, spacing is facies-dependent: fracture intensity in the first two cores is only a third of that in the third core, 250 ft downhole and in a finer-grained facies.
There are two near-vertical fracture sets: an east-west set striking about 100°, and a younger, north-south set striking about 5°. The older, east-west trending fracture set is mineralized with an early, quartz druze, a younger carbon film, and a youngest patchy to covering kaolinite. Geometries indicate that this set opened in extension, but 30-50% of the individual fractures have been reactivated in right-lateral, oblique shear. Shear reactivation occurred during activity on an underlying, basement-involved thrust and associated tear faults. The younger, north-south fracture set is also mineralized with quartz druze, although the larger crystal habit suggests a different phase of precipitation. Later mineralization of patchy to covering calcite occurs in these fractures; they do not contain carbon and have not been sheared.
AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana