China has no interest in unduly disrupting North Korea and what sanctions they have agreed to impose to date have been largely symbolic. Actually cutting off oil or isolating Pyongyang would have serious repercussions for Beijing including a refugee crisis and a reunified peninsula. The former implies internal upheaval in China as millions stream north. The latter would mean US troops on China's border, a national security threat they do not want to contemplate (note their response over the deployment of THAAD for example). Their preference is for the status quo to prevail. That said China is no fan of Kim's insistence on provocative nuclear tests that may upset this arrangement. Nor are they pleased that Kim disregarded Chinese protection of his half-brother and choose to assassinate him. Nonetheless, China's main objective is concerned with preserving the existing balance, not encouraging the US to do its dirty work.