 Complete the following phrase:  Further seismic studies were undertaken in the swamps of Al-Sudd, but all of Chevrons exploration and development activities came to an abrupt end in February 1984, when rebels attacked the main Chevron base across the Bahr al-Ghazal from Bentiu, killing four Chevron employees. The company immediately terminated its development program and, despite repeated demands by successive Sudanese governments, refused to return to work its concession until the safety of its personnel could be guaranteed by a settlement of the 