This image from the United States Geological Society (USGS) dovetails the Berman (Supetra etal.; 2021) chart discussion elsewhere in Forum Stuff. It is a structure map on the top of the Wolfcamp formation in the Permian Basin. The grey area essentially outlines the entire Permian Basin which was included in the 2016 and 2018 USGS assessments of technically recoverable tight oil. The pink cores outlined in each sub-basin (Mike), represent 10-11 partial counties by areal extent and from these cores roughly 85% of ALL Permian tight oil production has been produced. There are an estimated 48,000 HZ wells in these cores, some of them drilled as close to together as 500 feet, lateral to lateral.

In the Midland Basin three benches produce roughly 75% of shale oil extracted in the sub-basin; in the Delware there are five productive benches that produced 90% of all production.

The pink cores do not represent the productive limits of each sub-basin, however, and some 6-8,000 additional wells have been drilled outside the cores, in less productive acreage that some people like to refer to as Tier 3/4 acreage. I sometimes refer to this flank acreage outside the cores as goat pasture.

The USGS recoverable resource assessments covered the entire Permian Basin outlined in the two images and placed enourmous technically recoverable tight oil resources everywhere, even as far north as Lubbock, and has messed with people's heads, badly. There are lots of people in the U.S. who want you to believe if its technically recoverable, its all going to come out of the ground, all it takes is more money.

The actual cores of the HZ shale oil play in the Permian represent approximately 20-25% of the assessment areas and productive limits, where wells have been drilled and found bascially no commerical shale oil, about 40% of the assessment area. The rest, where theoretical technically recoverable, as yet unfound resources lie, would not support a goat for 1 day. Lets call that Never-Never Land.

In their 2021 paper, Saputra etal predicts there can be 100,000 more HZ wells drilled in flank, Tier 3/4 acreage, outside the cores, and 21 G BO +/- more oil can be produced over and above the 16 G BO that has been produced from the cores. The authors even believe wells can be drilled in Never-Never Land and more shale oil produced. Yikes. No word on what the price of natgas would have to be to make THAT happen.

Again, thus far less than 15% of Permian shale oil has come from flank areas.

Core areas and sweet spots within those cores are "sweet" for a reason. They represent the best rock and the oil and gas business always drills its best stuff, first. As you can see in the structure map the lowest top of the Wolfcamp lies pretty much in the cores. A net isopach map would show the Wolfcamp correlative interval to be the thickest in the core, a function of deposition and how sediments washed into the basin and stacked, bottom up. The interval in the core has the best oil saturations, the highest initial GOR, better nanopermability, higher carbonate content, the best thermal gradients, total organic carbon and the lowest sulphur, nitrogen, etc. contents...everything in the core kitchens are perfect to cook and produce the most oil when hydraulically frac'ed.

Cores are where they make the highest volumes of oil and the most money. The mudstone (shale) down structure, in higher structural areas, is thinner, generally has higher water saturations and is much gassier. Nobody is anxious to drill Tier 3/4, non-core goat pasture at oil prices of less than $100 and $5 natgas, sustained. Given declining liquids productivity in the cores, rising GOR, rising WOR and obvious well inteference, if it was good in the goat pasture they'd be out there right now drilling the snot out of it.

About the two cores in the Permian....the shale sector doesn't call it low-grading, they call it high-grading. There is a reason for that.