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Fracture detection in Manderson Field

A 3-D AVAZ case history

Veritas, Calgary, Alberta, Canada

Corresponding author: D. Gray, Dave_Gray@veritasdgc.com

The first 20% of the full text of this article appears below.

Oil from Wyoming's Manderson Field is produced from a complex fracture system with possible significant lateral connectivity. It is therefore important to obtain as much information as possible about the nature of the fractures within the reservoir, especially between the wells.

This paper describes how amplitude variation with offset and azimuth (AVAZ) analysis of 3-D seismic data was used to determine fracture strike and density at (and around) wells in the field. The results show that this technique compares favorably with other methods of determining fracture strike and density and also provides important information about fracturing between wells.

Manderson Field, discovered in 1951, is on a sharp, asymmetric, northwest-plunging anticline, and oil and gas are produced from Pennsylvanian, Permian, and Cretaceous intervals within it. The most productive zone, the Permian Phosphoria, consists of fractured carbonate at 6500–9000 ft, where production far in excess of original oil-in-place estimates, low matrix porosity, a rapid and then rapidly declining production rate, and strong pressure support in the wells suggest that oil is produced from a fracture system with significant lateral connectivity. A 3-D seismic survey was acquired in 1996 and reprocessed in 1998 to improve structural definition of the reservoir. This survey includes three wells for which oriented core analyses were acquired (between 1996 and 1998) and is near another core analysis and a well logged by using a formation microimager (FMI).

The method and preliminary results of the AVAZ analysis are described in Gray et al. (1999). It relies on fracture-induced variations in the rigidity of the rock. Fractured rock appears more rigid to seismic shear waves polarized parallel to open, fluid-filled fractures than to those perpendicular to them. Variations in rigidity are reflected in variations with azimuth in the rate of change of amplitude with offset, i.e., the AVO gradient (. . . [Full Text of this Article]