Xinhua reported that up to 150,000 litres had leaked into the river from the pipeline.
In a statement on its website, PetroChina's parent company China National Petroleum Corporation (CNPC), said the leak was caused by a "third party" during construction work.
The pipeline is supposed to transport diesel from northwest China's Gansu province to central parts of the country.
Concern over reporting
Concern has been voiced about the amount of time between the contamination's discovery and its reporting by local media.
"There has been zero coverage of the accident on television or in newspapers in Chishui township or Hua county," the Hong-Kong based South China Morning Post quoted one local as saying.
Most of China's lakes and rivers are thought to be heavily polluted. Government data has found that more than 200 million Chinese do not have access to safe drinking water.
In November 2005, a major oil spill on the Songhua river in northeast China resulted in a cut off of water supplies to up to four million people. The pullution then flowed down river into Russia, sparking a diplomatic crisis between the two nations.