Characterization of Tensleep Reservoir Fracture Systems Using Outcrop Analog, Fracture Image logs and 3D Seismic

Tom H. Wilson, Alan Brown, and Valerie Smith
West Virginia University

We develop a model of the discrete fracture network in the Tensleep Sandstone reservoir, Teapot Dome, Wyoming, using design approaches and criteria that may have general applicability to characterization of naturally fractured unconventional reservoirs. Seismic discontinuities related to fracture zones and small faults are extracted from 3D seismic over the dome. Fracture image logs (Formation MicroImager (FMI) logs) provide information about open fracture trends, aperture and present-day orientation of SHmax. We also incorporate field observations of fracture height, length and spacing from Tensleep exposure in the Fremont Canyon area of southern Wyoming. The field analog was compiled from outcrop measured fracture trends and analysis of outcrop photos and WorldView satellite imagery. Fracture height distributions observed in Fremont Canyon on the NE flank of Granite Mountain anticline generally exhibit power law behavior. Decreased power in more intensely fractured zones suggests increased probability of greater fracture height. The power within fracture zones was about -1.6, while that in less intensely fractured zones was about -2.2. Spacing distributions exhibit power law response over a limited range of scales. The power is increasingly negative for larger spacings. Fracture intensity ranges from 0.02/m to 0.075/m. Fracture intensity within fracture zones varies from about 0.05/m to 0.06/m while that in less intensely deformed zones varies from about 0.025/m to 0.03/m. Geometric mean spacing varies from about 11 to 14 meters in fracture zones and from about 19 to 26 meters in less deformed zones. Geometric mean spacing, layer-by-layer, varies from about 10 meters to 27 meters. Length distribution determined from satellite imagery follows a power law (power -1.85), as does the distribution of discontinuity lengths measured in 3D seismic data over the dome (power -2.29). Aperture distributions obtained from FMI logs exhibit limited power law behavior for apertures larger than 0.05 mm, but, overall their distribution is more accurately characterized as log-normal. SHmax (present-day maximum horizontal compressive stress) determined from drilling induced fractures observed in fracture image logs trends ~N76W. The dominant open fracture trend in the Tensleep reservoir, inferred from image log interpretations, parallels SHmax. These fracture parameters combined with output from 3D seismic workflows are used to build a reservoir fracture model.

AAPG Search and Discovery Article #90169©2013 AAPG Rocky Mountain Section 62^nd Annual Meeting, Salt Lake City, Utah, September 22-24, 2013