ORIGIN AND SOURCE OF NATURAL GAS IN CENTRAL ANDAMAN SEA,MYANMAR

Located in an active continental margin, the central Andaman Sea is a gas-rich depression in Myanmar. The origin and the source of gases are critical factors to determine exploration targets. In this paper, according to the chemical composition and carbon isotope of the natural gases in the central Andaman Sea back-arc depression of Myanmar, the generation types and the sources of hydrocarbon gas and carbon dioxide were identified. The result suggests that the gases in the Pliocene Formation with light carbon isotopic methane was biogas,while heavier carbon isotopic methane was thermogenic. The gases in the Miocene and Oligocene Formations with heavier carbon isotope were all thermogenic gas. Carbon isotope of carbon dioxide shows that there are different origins, organic and inorganic. Besides, there is mixed gas composed of biogas and thermogenic or inorganic gas.Gas of different origin came from different sources. Inorganic gas, carbon dioxide with methane, came from the basement or the upper mantle through basement faults. Biogas mainly generated from the source rocks in the Pliocene Formation itself. The thermogenic gas both in Pliocene Formation and in Miocene and Oligocene Formation migrated from the highly matured source rocks in the deeper Formations such as Upper Pliocene, Miocene or Oligocene.