Utilize the input text as a context reference, choose the right relationship between energy company and natural gas from the options. Options: product/material produced, manufacturer, distributed by, industry, position held, original broadcaster, owned by, founded by, distribution format, headquarters location, stock exchange, currency, parent organization, chief executive officer, director/manager, owner of, operator, member of, employer, chairperson, platform, subsidiary, legal form, publisher, developer, brand, business division, location of formation, creator.

Warren Resources, Inc. is an independent energy company engaged in the acquisition, exploration, development and production of domestic oil and natural gas reserves.

Answer: product_or_material_produced