June 23 (Reuters) - Four foreign hostages employed by oil services giant Schlumberger were released unharmed on Saturday after more than three weeks in captivity, security sources said. Below is a chronology of some major attacks and kidnappings involving the Nigerian oil industry in the past three months. -- March 14 - Militants release two Italian workers they had been holding since Dec. 7. -- April 4 - A Dutch manager for German building contractor Bilfinger Berger, kidnapped in Port Harcourt on March 23, is released. A British worker, abducted from the Bulford Dolphin oil rig on March 31, is freed. Two Lebanese employed by Setraco, who were abducted in Bayelsa on April 2, are also released. -- April 7 - Gunmen kidnap two Turkish engineers from their car in Port Harcourt. One works for Merpa, a Turkish firm that maintains telecommunications on oil platforms. -- April 27 - Gunmen kill two policemen in an unsuccessful kidnap attempt in Port Harcourt as the officers are escorting a convoy of vehicles carrying expatriate staff to work. -- May 1 - Four Italians, an American and a Croat are kidnapped from an offshore oil facility operated by U.S.-based Chevron in Bayelsa state. They are freed on June 2. -- May 3 - Gunmen kidnap 20 foreign workers in three attacks in the Niger Delta, but eight are freed within hours. -- May 5 - Gunmen abduct a British oil worker from Trident 8 rig operated by U.S.-based Transocean off the coast of Bayelsa. Separately, gunmen abduct a Belarussian woman, a manager of Britain's Compass Group, from outside her home in Port Harcourt's exclusive GRA district. She is freed on May 16. -- May 8 - Three South Koreans and eight Filipinos are freed after being held for five days. -- Rebels blow up three oil pipelines in the Niger Delta, forcing Eni to halt production of 150,000 barrels per day feeding its Brass export terminal. The Movement for the Emancipation of the Niger Delta claims responsibility. -- May 19 - Two Indian staff of Eleme Petrochemical Company, majority-owned by Indonesia's Indorama, are kidnapped by gunmen in Port Harcourt. -- May 22 - Four gunmen kidnap a Lebanese man who is a financial controller at oil service company Nigercat in the city of Warri. -- May 24 - Gunmen kidnap a Polish engineer near Warri. He is released on May 28. -- May 25 - Nine oil workers are kidnapped from a ship off the Nigerian coast; those held are three Americans, four Britons. A South African and a Filipino are released on June 11. -- May 30 - Four U.S. oil workers, kidnapped on May 9 from a barge off the coast near Chevron's Escravos crude export terminal, are freed. -- June 1 - Gunmen kidnap at least three senior managers of Indonesian chemical firm Indorama from their residential estate near Port Harcourt. It later emerges 10 people were taken. -- June 3 - Gunmen kidnap six staff of United Company RUSAL, the Russian aluminium giant, in Ikot Abasi in the southeast. The men were working at the Aluminium Smelter Company of Nigeria, controlled by RUSAL. -- June 11 - Five Britons and three Americans are released in Bayelsa state. Two Indians are also freed, as well as a Filipino and a South African, who were kidnapped on May 25. -- June 15 - Gunmen kidnap two Lebanese men, working for Italian firm Stabilini, near Ogara in Delta state. Two Indian construction workers are kidnapped in Delta state. -- June 16 - Militants release 10 Indian hostages held since June 1. -- June 21 - Troops kill 12 militants and free some hostages in a dawn raid on an Italian-operated Ogbainbiri oil facility. -- June 23 - Four foreign hostages employed by oil services giant Schlumberger released unharmed after more than three weeks in captivity, security sources said. The men, from Britain, France, the Netherlands and Pakistan, were abducted on June 1 from the company's residential compound in Port Harcourt by kidnappers disguised as riot police.
