Sources of coal-derived gas in the marine strata of Central Sichuan, China

Abstract

There are gas reservoirs in the Triassic Leikoupo Formation, Jialingjiang Formation, Feixianguan Formation, and Permian Changxing Formation in the marine strata of Central Sichuan Basin. Over the years, the gas sources in these gas reservoirs, especially the gas source of the Leikoupo Formation, remain unclear. In this paper, the natural gas geochemistry, combined with the geological background, is utilized so as to compare the composition and isotopes of natural gas in the Permian and Triassic gas reservoirs in the central Sichuan Basin. It is believed that many samples of the 4th Member of the Leikoupo Formation are coal-derived gas and that the natural gas comes from the overburden Xujiahe Formation source rock. Also, there is some other natural gas that originates from the Lower Leikoupo Formation source rock. The gas of 1st Member of the Leikoupo and Jialingjiang formations is homologous to the natural gas of the Changxing and Feixianguan formations. In the Longgang gas field, the natural gas of the Changxing and Feixianguan formations mainly comes from the coal-bearing source rocks of the Longtan Formation. In some areas of the Moxi gas field, the Longtan Formation may be transformed into the Wujiaping Formation, which is mainly composed of sapropelic organic matter wherein the natural gas produced is an oil-type gas. The corresponding overlies the reservoirs of the Changxing, Feixianguan, and Jialingjiang formations, as well as the 1st Member of the Leikoupo Formation.