There are numerous shallow fields where as I don't think people have bothered to look deeper.
The example was the Carlsbad Field (Tom Green county) in the early 80's. It was producing at 8K ft since the 50's until a independent came in 30 years latter and found another zone below at 10K ft of the same size and reserve.
About 60 completions were achieved and drilling moved elsewhere with out anyone looking for what was below that second zone.
The old Ranger Field has been worked to death at the 4-5K ft depth with little information of what's below that zone. Every few years one might see a lone thumper truck working a small lease but nothing on a larger scale.
From the article;
[Operators have coined the term “Wolfberry” to denote the combined Spraberry and Wolfcamp target intervals of interbedded shales and carbonates encountered at depths ranging from 7,000 to 10,500 ft, Fig. 2.]
Is this close to the Ranger field you describe?