Reservoir Characterization of Eastern Shallow Oil Zone, Elk Hills Field, California

Detailed reservoir characterization and surveillance techniques are being integrated to redevelop the Eastern Shallow Oil Zone (ESOZ) of Elk Hills Field, California. Initial production of Pliocene ESOZ sands on the east plunging anticline began in 1919, under solution gas drive. As pressures declined, gravity drainage became the dominant drainage mechanism, combined with water influx on the north and east flanks. The current production method is pressure maintenance of a low-pressure gas cap via gas injection.

Upon acquisition of Elk Hills in 1998, Oxy undertook a reservoir characterization effort focused on comprehensive correlation, 3D seismic, detailed mapping, and petrophysical modeling. The interpretation includes more than 200,000 correlation tops and 3,800 fault cuts in a 3,500 well dataset over an area of 20,000 acres. This characterization is the foundation for reservoir surveillance, 3D modeling and reservoir simulation.

The six major ESOZ reservoirs are highly stratified, near-shore, tide-dominated marine sands separated by laterally continuous shale intervals. Within the reservoirs, widespread thin shales have been mapped to define 26 correlative sub-zone layers. Sand distribution patterns are a result of accommodation space and offset stacking. The trend of sand development is N30E with E-W sand termination by pinch-out or shale-out. Numerous N45E trending normal faults that post-date deposition cut the reservoir. Currently, both the faults and thin sub-zone defining shales are barriers to fluid flow. This results in several hundred dynamic, current gas/oil and oil/water contacts that have been mapped for the sub-zones. The relationships of reservoir dip, sealing faults, high stratification, gravity drainage, and localized water influx result in location dependent drainage mechanisms.

This new reservoir characterization improves understanding of reservoir quality sand distribution, drainage mechanisms, and remaining potential of the ESOZ. It revealed development inefficiencies and is key to generation and implementation of a new development strategy by sub-zone and fault block.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California