Outcrop Analogs for Fractured Reservoir Characterization: Rangely Field, Colorado

Orlando J. Ortega,1 Randall Marrett,2 Stephen E. Laubach,3 Kitty L. Milliken,4 Jon Olson,5 and Rob Reed3

ABSTRACT

Tertiary recovery benefits from a reservoir description of critical fluid-flow heterogeneities. Rangely Field is one of the largest oil fields in the Rocky Mountains region presently undergoing tertiary recovery. Natural fractures in the Weber Sandstone, the main reservoir in Rangely Field, play an important role as conduits of fluid flow. Careful consideration of natural fracture properties can help maximize oil recovery by locating injector/producer wells to take advantage of the natural fracture system in this reservoir.

The outcrop analog comprises length, aperture, orientation and spacing data, as well as mechanical stratigraphy and fracture cementation models. Length- and aperture-size distributions follow power-law scaling, provide scale-normalized fracture intensity values for comparisons, and they also show how fractures of different scales are quantitatively related. Outcrop-derived parameters are used to generate fracture system realizations, which resemble natural fracture systems in the area.

Microscopic fracture data from two cores in Rangely Field provide the necessary starting point for subsurface fracture system characterization by extrapolating fracture properties from the microscale to the reservoir-scale, as governed by analogous relationships observed in outcrop. Preliminary predictions highlight lateral and vertical variations in fracture abundance, quality, and connectivity in the reservoir. Also, significant differences between outcrop analogs and predicted subsurface fracture characteristics prevent surface analogs from being directly applied to the reservoir.

1Department of Geological Sciences, University of Texas at Austin, C1100, Austin, TX 78712, phone: 512-4078576, e-mail: o.ortega@mail.utexas.edu,

2Department of Geological Sciences, University of Texas at Austin, Austin, TX 78712,

3Fracture Research and Application Consortium, University of Texas at Austin, Bureau of Economic Geology, Austin, TX 78712,

4Department of Geological Sciences, University of Texas at Austin, C1100, Austin, TX 78712-1101,

5Department of Petroleum and Geosystems Engineering, University of Texas at Austin, Austin, TX 78712