The SPE Style Guide has several rules for capitalizing names of locations, both regional and geological.
Titles of major bodies of land and water are capitalized:
Gulf of Mexico, North Sea, Indian Ocean, Death Valley, South Padre Island, Middle East
Real properties and legal entities are also capitalized:
Platform C, South Swan Hills Unit
However, general directions and general locations are not capitalized:
east Texas, western Alberta, offshore Africa, midcontinent region
Capitalize geologic ages, including leading adjectives:
Mesozoic, Lower Permian, Upper Jurassic
But do not capitalize geologic formations, such as:
basin – Campos basin
belt – Orinoco belt
delta – Nile delta
field – Woolybutt field in Australia, the funniest field name ever!
formation – Kareem formation
play – Bakken play
pool – Keg River F pool
reservoir – Gharif reservoir
sand – Milk River sand
shale – Marcellus shale
trend – Spraberry trend
zone – Lower Ratawi zone
There are two exceptions to the above rule: Permian Basin and Overthrust Belt.