Joint Meeting Pacific Section, AAPG & Cordilleran Section GSA April 29–May 1, 2005, San José, California
Relationship Between Deformation Bands and Petroleum Migration in an Exhumed Reservoir Rock, Los Angeles Basin, California, USA
James C. Sample
Geology, Northern Arizona Univ, Box 4099, Flagstaff, AZ 86011-4099, [email protected]
Structures in a petroleum-bearing and sand-rich unit of the Monterey Formation were examined to determine the relationship between fluid flow and damage-zone structures. The sandstone outcrop, at the mouth of Newport Harbor, contains abundant deformation bands (DB) that most likely developed during deformation along the active Newport-Inglewood fault zone that transects the area. The DB crop out as resistant ribs and fins within a coarse- to very-coarse-grained sandstone comprising mainly quartz and feldspar detritus. DB strike 323 and dip variably (N = 113). The average strike of the NIFZ in the area is similar. DB exhibit both normal and right-slip separations, but slickenlines are lacking and net slip directions were rarely possible to determine. Where observed, crosscutting relations suggest normal slip was an important part of the kinematic evolution. DB are commonly oil-free, while the undeformed matrix preserves abundant oil in pore space. The DB acted as baffles to flow, but continuing deformation likely breached permeability barriers over time. The DB reported here have similarities to dilation bands in that the margins preserve porosity higher than the surrounding matrix. Yet these bands are cemented uniformly by calcite and enclose a central core with lower porosity, no cement, and locally a thin seam of petroleum. This type of DB formed by early oil migration along fractures and consequent carbonate cementation along fracture margins. In the sandstone there is evidence for at least three distinct phases of oil migration: 1) an early phase along discrete fractures extending meters to the entire height or width of the outcrop (tens of meters); 2) a middle phase of migration into spore space in the undeformed sandstone matrix; and 3) a late phase of movement along open fractures lining veins and sandstone dikes.
Posted with permission of The Geological Society of America; abstract also online (http://gsa.confex.com/gsa/2005CD/finalprogram/abstract_85620.htm). © Copyright 2005 The Geological Society of America (GSA).