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

Abstract

ABSTRACT Bob West field (Starr and Zapata Counties, Texas) and 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 (trillion cubic feet) and field 200 Bcf (billion cubic feet). 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. field is highly faulted and produces from closures against lower 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 field. Both fields exhibit post-depositional rotation. Bob West field has been rotated basinward to the east, while field was rotated to the southeast. The greater structural complexity of field may explain why it is the smaller of the two fields. Bob West field produces from 29 sands and 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 field, implies that the Upper Wilcox Trend of South Texas has yet to be fully explored.