I read some years ago that Pemex is basically a quasi-private corporation who is taxed by the Mexican government more than they actually earn. The result is that there is no money available to survice the oil fields. So you have wells that start producing brine and end up being shut-in where a little maintenance would have extended production for years longer.
No. It is 100% owned by the Mexican Government.
who is taxed by the Mexican government more than they actually earn
That would be the case for essentially all private oil companies around the world. Governments typically take 2~4 times what company keeps as profit.
PEMEX is used to fund the government. As a result they do not put enough back into the development of new and existing fields.