Texas would find itself a majority brown-skinned country with oil. You know, the kind of country the United States loves to “spread democracy” to.
Texas already owns all its oil, including ocean mineral rights. This was part of the terms of statehood.
They were allowed to keep their unallocated lands and mineral rights in order to pay off their debts and become a state, rather than the United States assuming the debt.
This turned out to be a huge financial boon for the state and continues to be.