More than a fifth of the illegal taps occurred in Tamaulipas, the Gulf state neighboring Texas that is a cornerstone for Mexico's future oil plans. It has Mexico's largest fields of recoverable shale gas, the natural gas extracted by fracturing rock layers, or fracking.

Mexico, overall, is believed to have the world's sixth-largest reserves of shale gas — equivalent to 60 billion barrels of crude oil. That's more than twice the total amount of oil that Mexico has produced by conventional means over the last century.

The energy reform passed in December loosened Mexico's protectionist policies, opening the way for Pemex to seek foreign investors and expertise to help it exploit its shale fields. It hopes to draw $10 billion to $15 billion in private investment each year.

The attractiveness of the venture may hinge on bringing Tamaulipas under control.

"The energy reform won't be viable if we aren't successful ... in solving the problem of crime and impunity," said Sen. David Penchyna, who heads the Senate Energy Commission. "The biggest challenge we Mexicans have, and I say it without shame, is Tamaulipas."

One foreign oil company that had a brush with violence appears undeterred.

In early April, gunmen opened fire at a hotel in Ciudad Mier, in Tamaulipas' rough Rio Grande Valley, where eight employees of Weatherford International Ltd., a Swiss-based oil services company, were staying.

They were not injured, and Weatherford said in an email message that "Mexico continues to be a focused market for us with growing potential in 2014 and 2015."

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