Faults with limited vertical displacement are often difficult to identify conclusively in well correllaions, seismic images, or reservoir performance data, but they may still form fluid flow barriers as a consequence of grain shearing during strike-slip motion. Such minor faults are common as subsidiary features in a strike-slip tectonic regime. Hydrocarbon production anomalies in the upper Miocene reservoirs of the supergiant Matzen field in the Vienna Basin, a pull-apart basin in Austria, were investigated using 3-D seismic, high-frequency genetic stratigraphy, and reservoir engineering.

A complex pattern of faults in units below a lower Miocene unconformity, relating to a pulse of basin subsidence, was identified in 3-D seismic. A series of NE-SW-oriented horsts are separated and offset by a series of N-S normal faults. Later normal and strike slip motion along the NE-SW-oriented faults created the main structural trap of the field. The N-S-oriented faults were reactivated as translational faults to accommodate block rotation in the strike-slip regime. Throw on these subsidiary faults decreases from less than 20 m at the unconformity to less than 2 m in the 8th Badenian reservoir located 400 m above the unconformity. Displacement in the 8th Badenian reservoir is only apparent through integrated correlation of high-frequency (fifth-order) flooding surfaces in fortuitous well pairs located less than 100 m apart. Faults appears as subtle characteristic lineations in 3-D seismic mapping that offset shelf-slope break trends and disrupt attribute patterns in delta topsets. Patterns of water encroachment seen in time-series watercut maps and anomalous high cumulative production in a well on the downthrown side of one such feature document the importance these faults play in reservoir compartmentalization.

Recognition of this subtle fault compartmentalization resulted in a review of completion and water-injection patterns, and a significant increase in recovery efficiency is anticipated from future injection adjustments. This integrated approach can be applied to other reservoirs in strike-slip settings such as the Los Angeles, Ventura, and San Joaquin basins of California to increase production from mature reservoirs in danger of premature abandonment.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah