Please what requirements are needed for a z visa to China.

You should prepare following documents: Passport, Application Form, Recent Photo and any of the following Work Permits:
a. Foreigners Work Permit - issued by the Ministry of Human Resources and Social Security of the PRC.
b. Registration Certificate of Resident Representative Offices of enterprises of foreign countries (regions) - issued by Chinese authorities of industrial and commercial administration.
c. An approval document for commercial performances - issued by the Chinese government authorities for cultural affairs.
d. Invitation Letter to Foreigners for Offshore Petroleum Operations - issued by National Offshore Oil Corporation