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

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