Abstract: Fault Detection and Analysis using Borehole Image Logs at Pescado Field, Santa Ynez Unit, Offshore California

PARKIN, E. L., University of California Los Angeles, CA;D. F. LOCKMAN, and J. R. SCHWALBACH, Exxon Company USA, Thousand Oaks, CA

Conventional well logs, dipmeters and seismic data have been the major tools used to map faults in the Santa Ynez unit of offshore California for the past two decades. Today, however, borehole image logs are used to interpret fault characteristics to a level of detail not achievable with the older methods.

The formation evaluation program at Pescado field emphasizes fault and fracture characterization and quantification by integrating borehole images to core. We have calibrated approximately 1000 feet of continuous cores from the Monterey formation. This enables us to quantify the levels of faulting and fracturing in wells without core. We also examined image logs from wells interpreted to contain missing or repeat section on conventional logs to confirm the fault interpretation. These data allow us to develop strategies for mapping faults based on their style, character, and distribution. Image logs and integrated interpretation confirm the existence of thrust and normal faults throughout the stratigraphic section, and that low angle faults appear restricted to the less competent mudstones and intervals known to contain volcanic ash beds.

Faults with less than 15 feet of vertical displacement are nearly impossible to detect and characterize in the Monterey using conventional logs. For example, commonly occurring low angle faults dipping less than 30 degrees in the Middle Shale subzone were previously not mapped.

Creating a detailed fault character data base provides input for reservoir evaluation in geologic models and direct confirmation of faults for seismic interpretation.

AAPG Search and Discovery Article #90935©1998 AAPG Pacific Section Meeting, Ventura, California