Abstract: Loma Vieja Field: Structural Geology and Related Velocity Fault Shadow in the Upper Wilcox (Fandango) in South Texas

MEYERHOFF, JAMES C., and RION BRADDOCK
Samedan Oil Corporation, Houston, TX

A new Upper Wilcox (Fandango) field discovery was made at Loma Vieja Field in 1989 in Zapata County, Texas. Production from the field to date is 44.1 BCF from numerous Fandango sands that trend from Bob West Field northward to E. Seven Sisters Field.

The principal trapping mechanism for Loma Vieja Field is a high side closure against a very large down-to-the-coast fault. This fault trends across Zapata County through Escobas Field, which is immediately updip to Fandango Field, northward to NE Thompsonville Field in northwest Jim Hogg and southeast Webb Counties.

The large down-to-the-coast fault that creates the trap for the field has a definitive velocity shadow resulting in a major time sag on conventional migrated seismic data. The expansion of Weches, Queen City, and Reklaw shales on the downthrown side of the fault cause major changes in the average and interval velocities across the trapping fault at Loma Vieja Field. Seismic data and time structure maps over the field show southeast dip toward the fault where actual dip as measured from well log correlations and dipmeter data reveals northwest dip away from the fault. While the field was discovered on 2-D seismic data, 3-D seismic data have confirmed the presence of a velocity gradient across the field and have reduced structural risk during continued drilling in the field.

AAPG Search and Discovery Article #90932©1998 GCAGS/GCS-SEPM Meeting, Corpus Christi, Texas