Abstract: Bob West and Lopeno Fields: Structure and Stratigraphy of Two Significant Upper Wilcox Gas Fields in South Texas

Richard W. Debus

Bob West Field (Starr and Zapata Counties, Texas) and Lopeno Field (Zapata County, Texas) are both large Upper Wilcox fields in South Texas. Located six miles apart and producing from the same "Lopeno" sand sequence, Bob West Field may ultimately produce up to 1 TCF and Lopeno Field 200 BCF.

Although the principal trapping mechanism for both fields is high side closure against down-to-the-coast faults, there are significant structural differences. Bob West Field which produces primarily from one west dipping fault block, also has downthrown production on east dip. The field is bounded by large depositional growth faults to the west and east.

Lopeno Field is highly faulted and produces from closures against Lower Lopeno aged faults. The field is transected by a very large growth fault system. This fault traps some, but not all of the pay sands at Lopeno Field.

Both fields exhibit post-depositional rotation. Bob West Field has been rotated basinward to the east, while Lopeno Field was rotated to the southeast. The greater structural complexity of Lopeno Field may explain why it is the smaller of the two fields.

Bob West Field produces from 29 Lopeno Sands and Lopeno Field from 18. Based on regional mapping these sands are interpreted to be shallow water strike oriented deposits with excellent sand correlations extending 40 miles on strike.

Bob West Field which was discovered in 1990, being 38 years after Lopeno Field, implies that the Upper Wilcox Trend of South Texas has yet to be fully explored.

AAPG Search and Discovery Article #90950©1996 AAPG GCAGS 46th Annual Meeting, San Antonio, Texas