It’s not only BP…
Nigeria holds 11,700 square kilometres of mangrove forest: the third largest in the world and the largest in Africa. Most of this mangrove is found in the Niger Delta.
Nigeria is also a major oil producer and most oil extraction takes place in the Niger Delta. There, petroleum or crude oil abounds in rock formations. The complex mixture of hydrocarbons and other organic compounds that make up the flammable liquid fossil fuel is extracted from oil wells found in those oil fields.
When crude oil is pumped out it also drags associated gas with it. Such natural gas could be separated from the oil and be used but oil companies prefer to burn it off. Shell-BP was the first one to start with this practice in the 1960s.
Flaring of natural gas associated to oil extraction has been internationally acknowledged as a significant source of greenhouse gas emissions and a major contributor to climate change. In combustion, gaseous hydrocarbons react with atmospheric oxygen to form carbon dioxide (CO2).
Shell in Nigeria: What are the issues? - Essential Action | Boycott Shell/Free Nigeria Campaign