Texas is large enough that the Railroad Commission regulates it through regional district offices, and most RRC records carry a district as well as a county. The two do not line up the way you might expect, and the county field holds more than counties.
Districts
The RRC has thirteen oil and gas districts: 1, 2, 3, 4, 5, 6, 6E, 7B, 7C, 8, 8A, 9, and 10. The lettered districts are historical subdivisions that were never renumbered. District 8 and 8A together cover most of the Permian Basin's Midland side, 7C covers the southern Midland Basin and the Eastern Shelf, 8 includes the Delaware Basin counties, 1 and 2 cover the Eagle Ford, 5 and 6 cover East Texas, and 10 is the Panhandle.
Districts matter for data because lease numbers are only unique within a district. A production record is identified by district and lease number together, and a lease number alone is ambiguous. Permit and well records carry the district as a number, which is how Petrora stores it.
Counties
Texas has 254 counties, each with a three-digit API county code. The code appears as the middle part of every API number, which makes it possible to tell a well's county from its number alone. See API numbers.
Permit records also carry the county name. Petrora's permit data lists 276 distinct county names, not 254. The extra names are state offshore tracts. Wells drilled in Texas state waters, which extend about ten miles into the Gulf, are assigned to named tracts rather than to a land county: High Island, Mustang Island, North and South Padre Island, Galveston, Brazos, and Matagorda tracts, each with directional suffixes like "High Island-E" or "South Padre Island-SB". These have their own county codes. Treat them as counties for filtering purposes, because that is how the RRC treats them.
A handful of land counties have only a few permits in the entire record. Hill Country counties like Blanco, Comal, Gillespie, and Llano appear with single-digit counts, as does El Paso.
Where to look
Recent permits can be filtered by county at /permits/tx, and the data page shows which counties are seeing the most permits over the last 90 days.