Multicomponent Vertical Seismic Profiles For Reservoir Characterization, South Casper Creek Field, Natrona County, Wyoming

TO further evaluate the role of multicomponent seismology in reservoir characterization, the Colorado School of Mines Reservoir Characterization Project, Phase III, carried out an integrated study in the South Casper Creek Field, Natrona County, Wyoming. The purpose of the study was to image zones of heterogeneity and identify anisotropy in a structurally complex reservoir. As part of this project, multicomponent vertical seismic profiles (VSPs) were recorded in the UNOCAL 10-6-3 well. Shear-wave birefringence measurements determined from the multicomponent VSP data along with wire-line logs, threedimensional (3-D) surface seismic data and geological information were incorporated in the study. Faults and associated open fractures were determined to be the dominant cause of velocity anisotropy for vertically propagating shear-waves in the South Casper Creek field. Polarization of the fast shear-wave from the near-offset VSP is parallel to the strike of several of the faults interpreted from the surface 3-D seismic data. Three isolated zones of velocity anisotropy between the surface and 2700 ft were determined from shear-wave bimfringence measurements. These zones correspond with a decrease in tube wave amplitude (indication of open fracturing) determined from a full waveform sonic and are associated with brittle rock units such as limestones and sandstones. Cores taken from the producing interval (Tensleep Sandstone) and a Formation Microscannert survey indicate open fractures with a dominant strike direction nearly parallel to the fast shear-wave polarization direction. Far-offset VSP shear-wave birefringence measurements indicate a variability in polarization direction. Further studies are being pursued to determine if South Casper Creek has a more complicated symmetry system generated by multiple fracture directions or if non-vertical propagation of shear-waves caused a change in polarization direction of the fast shear-wave. Introduction: Integrating geological, geophysical, geochemical and petroleum engineering data can reduce economic risk in petroleum exploration and enhance recovery efficiency in reservoir development. Petroleum reservoirs are heterogeneous, and often contain lithologic, stratigraphic and structural barriers that impede the flow of hydrocarbons. Previous studies of the Reservoir Characterization Project at the Colorado School of Mines have focused on characterizing a deep flat lying fractured chalk reservoir in the Silo Field, Wyoming (Martin and Davis, 1987; Davis and Lewis, 1990). The purpose of this study was to develop processing and interpretation techniques for multioffset, multicomponent VSPs and incorporate wire-line logs, surface seismic data and geological information with the VSPs in order to image zones of reservoir heterogeneity and identify anisotropy in a structurally complex, folded and faulted sandstone reservoir, the South Casper Creek Field, Wyoming. Processing procedures 1 Trademark of Schlumberger for the VSP data will be discussed in a separate paper entitled: Multicomponent, Multioffset VSP Processing. Shear-waves propagating through an azimuthally anisotropic medium (generated by parallel vertical cracks) will have components of displacement parallel and perpendicular to the fractures. The shear-wave component parallel to the fracture direction will propagate faster causing the polarized shear waves to split and to be recorded at different times. The resulting separation of shear-waves with different polarizations is known as shear-wave birefringence or shear-wave splitting. A multicomponent VSP provides the opportunity to record downward propagating shear-waves at different levels so that zones of anisotropy can be identified in situ. Geologic Background: The South Casper Creek Oil Field is located on the Casper Arch in Natrona County approximately 24 miles west of Casper, Wyoming (Figure 1). The field was discovered in 1918 when gas was found in the Jurassic Sundance Sandstone. Oil was discovered in the underlying Pennsylvanian Tensleep Sandstone Formation in 1922. The South Casper Creek field is a structurabstratigraphic trap with two doubly plunging anticlines (domes). Dips on the limbs of the fold structure vary up to 30 degrees. Locally, the Tensleep is an eolian dune and sheet sand, folded, faulted and fractured through several episodes of tectonic activity (Ray and Keefer, 1985; Skeen and Ray, 1983). Porosity in the Tensleep Formation at South Casper Creek ranges between 18 and 338, and permeability varies from 10 to 3200 md (Tanean, 1990). The producing Tensleep interval has 580 ft of closure and is at a depth of 2400 to 2600 ft. Although oil has been produced for 67 years, only 10 MMbbls of an estimated 70 MMbbls in place have been recovered. The cause of such low recovery is reservoir heterogeneity and the heavy oil (1.5 API) in the reservoir. Late in the Laramide Orogeny the principle stress direction was southwest-northeast (Gries, 1983). “Two leftlateral shear zones divided the Casper Arch into three distinct styles of deformation” (Skeen and Ray, 1983). To the south release of this stress generated left stepping enechelon folds, two of which are identified as the Poison Spider and South Casper Creek Oil fields (Figure 2a). Parallel vertical faults, wrench faults and multiple fracture sets observed locally are caused by brittle failure of the rocks and support he conceptual diagram showing the formation of enechelon folds presented inFigure 2b. Data Acquisition: The UNOCAL USA 10-6-3 well, located in the southwest portion of the northeast comer of Section 3, Township 33 North, Range 83 West, (SWME-SEC3-T33N-R83W) was the site for acquisition of wire-line logs and a multioffset multicomponent VSP survey. A three-dimensional (3-D) multicomponent SUrfaCe seismic study was also conducted over a portion of the southern dome centered about the 10-6-3 well. Figure 3 shows the location of the 10-6-3 well, the VSP source positions and areal