An Isotopic Study on the Origin of Carbon Dioxide and Nitrogen in Natural Gases of the Erawan Gas Field, the Gulf of Thailand

Abstract

In this paper, the origin of carbon dioxide and nitrogen in natural gases produced from the Erawan gas field is discussed on the basis of carbon isotopic data and several geological evidences. Maximum contents of carbon dioxide and nitrogen in natural gases recovered from three wells at the northwestern flank of this field, are 59.72% and 21.38%, respectively.As a result of carbon isotopic studies of methane, ethane and carbon dioxide in 11 gas samples of those wells, natural gases are divided into two groups, such as group A and group B.The group A is characterized by heavy methane (-30 to 33‰ PDB) on carbon isotopic compositon, and by high contents of carbon dioxide and nitrogen. Gases of this group are a mixture of magmatic gases and organic-origin gases. The magmatic gases consisting of mainly carbon dioxide, nitrogen and heavy methane have migrated into the present reservoirs from the pre-Tertiary basement through the east-dipping faults cutting the basement.The group B is characterized by normal contents of carbon dioxide and nitrogen, and by lighter methane (-38 to -41‰ PDB) on carbon isotopic composition. Gases of this group have been generated from the organic matters in the Tertiary sediments by thermal maturation.