Abstract

The 2.5 million acre Lobo gravity slide of South Texas contains several extensive, intensely faulted sands that have produced over 1 trillion cubic feet of gas and 4.5 million barrels of condensate from over 891 wells since discovery in 1973. Seven Lobo sands are generally recognized throughout the Webb and Zapata County area. Two of the “sands” represent large-scale sequences of multiple influxes of sand. These very fine-grained lower Wilcox sands were deposited in areally extensive sheets on a broad, flat shelf underlain by thick Midway shales.

Gravity sliding and intense faulting of the entire Lobo section into numerous fault blocks over the unlithified Midway shales occurred soon after deposition of the Lobo sands. This structural activity was followed by a period of erosion that removed or reworked upper Lobo sands in many of the higher fault blocks. The faulted and erosionally limited Lobo sands were finally covered by thick middle Wilcox shales which trapped the hydrocarbons that are targets of the present drilling activity.