The primary resources of Paraguay are its fertile soil and its forests. It has few mineral resources; limestone, copper, clay, and petroleum are the most important. Paraguay's rivers, well suited for hydroelectric projects, are another has been cut down.
When it comes to oil they do say that there is probably a large deposit of Gas and Oil in some parts of the Chaco. Currently, Paraguay is not getting any Gas or Oil from the Chaco or from anywhere else in Paraguay but some companies are interested in drilling.
But during the War Bolivia thought there could be oil there. Bolivia never really cared for the Chaco until that and until they lost there Sea Coast to Chile in 1873. Controlling all of the Chaco would have given them a long stretch of the Paraguay river.
Strange thing is Bolivia had complete control of both banks of the Paraguay river but much farther up North before 1867. They gave the river to Brazil in return for the right to navigate Brazilian rivers to the Atlantic Ocean.
Do know before the Chaco war Bolivia made sure if Brazil had any interests in the Chaco. The Brazilians said No.
Found out Bolivia even before becoming an independent nation in the Spanish era was considered a buffer State against Portuguese expansion.