The firm has been hired by Nexen Petroleum UK, which has been a subsidiary of CNOOC since the $15 billion (£9.1bn) acquisition of its Canadian parent in February.
An AMEC spokesman said the value of the contract was "multi-million pound", but confidentiality restrictions in the deal prevented him from giving more details.
The reimbursable contract, which ensures AMEC is recompensed for its expenditure, will run for six years, with the option of two extensions, each for a further two years.
The company estimates 150 jobs will be created onshore and offshore to support the contract's operations.
The work will cover operational modifications and brownfield projects.
Alan Johnstone, managing director for AMEC's brownfield and asset management business in Europe, said: "This is a great opportunity to demonstrate our strength in delivering engineering, procurement and construction services, which complements our leading brownfield capability in project management, operations and maintenance.
"We look forward to building on our long-term relationship with Nexen and working with them to achieve safe and reliable operations in the North Sea."
Nexen says it is the second largest oil producer in the UK North Sea.