Print this pageAAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Damage Zones Associated with Second Order Faults at Depth: Observations from SAFOD and a Gas Field in Southeast Asia
(1) Geophysics, Stanford University, Stanford, CA.
(2) Subsurface Technology, ConocoPhillips, Houston, TX.
We report a study on fault damage zones associated with second-order faults in two different regions—one adjacent to the San Andreas Fault in central California, and the other adjacent to a major fault in a gas field in Southeast Asia. The importance of characterizing damage zones arises from the pivotal role that fractures play in governing fluid flow through fractured, low permeability reservoirs. Damage zones studied adjacent to the San Andreas Fault are encountered in well-cemented arkosic sandstones immediately southwest of the main fault at the SAFOD site. Fifteen second-order faults have been identified in electric image logs on the basis of changes in lithology, orientation of bedding planes and anomalous physical properties such as decreased seismic-wave velocities. Most second-order faults have identifiable damage zones in which the density of smaller-scale faults and fractures (third order features) within the damage zones is anomalously high. Damage zone widths associated with second-order faults are typically on the order of 50-100 meters. The damage zone associated with the San Andreas Fault is about 250 meters wide. Within the damage zones of second-order faults, there are approximately two to three identifiable third-order features per meter. The density of these third-order features decreases rapidly with distance. The faults in the arkosic section have a variety of orientations, but many appear to be southwest-dipping reverse faults. The conjugate set of these is missing. However, this may be due to a sampling bias.
The second region of study is a fault zone in a gas reservoir in southeast Asia. Twenty-seven seismically-resolvable second-order faults are observed. The peak fracture density is approximately two to three fractures per meter. While most of the wells do not intersect the second-order faults, several peaks of increased fracture density are observed, leading us to suspect the presence of sub-seismic second-order faults. Production data indicates poor correlation of production with the reservoir-borehole contact length, but a strong correlation with the number of critically stressed fractures that the borehole intersects. Production data also indicates significantly larger production from a non-vertical well as compared to vertical wells. Majority of the fractures are steeply dipping and fail to be sampled by vertical wells. A correction to remove the sampling bias is applied to characterize the fractures correctly.