What is the relationship between 'Tullow Oil' and 'Petroleum' in the context of the input sentence.
Options: stock exchange, owned by, product/material produced, director/manager

Tullow Oil (LSE: TLW.L - news ) , Chevron (Swiss: CVX.SW - news ) , Anadarko and African Petroleum had been among companies carrying out exploration in Liberian waters.
product/material produced