Robert, have a look at this "Diccionario técnico del petróleo y del gas" from Argentina, I've used it and it's very handy and uses a somewhat neutral/standard Spanish. Unfortunately it is out of print right now.
PennWell glossaries are nice too (specially useful for the Mexican market, PEMEX related issues). Besides, you have an E-book version where the English-Spanish is sold separately from the Spa-Eng volume at USD 39.
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