A five-day investigation by a State Council team found the 23 December disaster in the southwestern municipality of Chongqing "was caused by numerous acts of workplace negligence", reports in state media said on Saturday.
The toxic cloud from the well spewed into the night sky, killing people as they slept or fled, poisoning thousands and turning a 25 sq km area into a death zone.
The cabinet-level probe into the accident, caused when the well burst while being drilled, would soon turn to meting out punishment, reports said.
"The next step...is to decide who was responsible for the disaster and have them pay the price," the official China Daily quoted Sun Huashan, vice minister of the State Administration of Work Safety as saying. He gave no specifics.
State-run China National Petroleum Corp, operator of the gasfield and parent of listed oil major Petrochina, has pledged to compensate victims, state media have reported.
More than 64,000 villagers had to be evacuated from the area for about a week until emergency crews capped the well and workers cleaned up and buried the corpses of cattle, pigs, chickens and other animals killed by the fumes.
"Workers on site did not follow operational regulations strictly and did not allow enough time for sufficient drilling mud to circulate to control the pressure"
The China Daily
The investigation found developers had underestimated the production capacity of the well and were not technically prepared for a well producing so much gas with such a high sulphur content.
"Moreover, workers on site did not follow operational regulations strictly and did not allow enough time for sufficient drilling mud to circulate to control the pressure," the China Daily said.
Investigators also found that a pressure control valve had been removed mistakenly from equipment "which was the direct cause of the blowout going out of control", the Xinhua news agency said.