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.
Refer to the input text as context and select the correct relationship between 'Tullow Oil' and 'Petroleum' from the available options.
Options: director/manager, owned by, product/material produced, stock exchange