"Walking through fractured reservoirs and failed seals"
By: Alfred Lacazette, PhD, Consulting GeologistHGS Lunch Meeting: Wednesday, May 30, 2001
Figure One
Deformation band in Pismo Formation sandstones in a railroad cut next to the Arroyo Grande oil field in Southern California. The deformation band separates tar-filled sandstone (right) from clean sandstone (left). The detailed photo (inset at upper left) shows that the deformation band itself is oil-free. The Arroyo Grande field produces heavy oil from wells as shallow as 800 ft. Sedimentary layers can be traced across the deformation band showing that there is little offset across it. Deformation bands only form in sandstones and chalks with porosities >15%; in other words: they only form in excellent reservoir rocks. Deviated wells in this reservoir could link-up isolated compartments.
Figure Two
This fault zone in sandstones consists of the impermeable gouge zone that the person is on and a damage halo consisting largely of pinnate joints, which are permeable. Pinnate joints are an important type of fracture because they are one of the few slip-sense and slip-direction criteria that can be recognized in image logs.
Figure Three A
Extensional fault-bend folding above a flattening bend of a normal fault results in extensional strain and fracturing of the extended fold limb. The red rectangle shows the location of Figure 3B.
Figure Three B
Heavily fractured rock in the extended fold limb. Note that the extensional strain was accommodated by both normal faulting and jointing. Few fractures are present in the footwall block and to the left of the axial plane that divides the passive limb from the extended limb.
