The Origin, Accumulation and Escape of Natural Gas in the Songdong and Baodao Regions in the Eastern Qiongdongnan Basin, South China Sea

Abstract

The found gas pools in the eastern Qiongdongnan Basin, South China Sea have almost failed to realize expected commercial value due to either low reserve abundance or high CO2 content. To assist in further exploration, the genetic type, source and accumulation of natural gas have been investigated based on the analysis of the chemical compositions, carbon isotopes and light hydrocarbons in gases, rare gas isotopes, basin numerical modeling and fluid inclusions. The results indicate that there are three representative types of gases. The first type distributed in the Songdong region consists of biogenic gas derived from degradation of organic matters in the Miocene marine mudstones and oil-derived gas generated from the Eocene lacustrine mudstones in the Songdong sag. The second type distributed in the BD19-2 area in the Baodao region is composed of coal-derived gas generated from the coal-measure source rocks and oil-derived gas from the Eocene lacustrine source rocks in the Baodao sag. The third type distributed in the BD15-3, BD19-4 and BD19-2 areas consists of volcanic mantle-derived CO2 and a very small number of organic hydrocarbon gases. The presence of oil-derived gas directly confirmed that the Eocene lacustrine mudstones are present. The charge of oil-derived gas in the Songdong region occurred during the Pliocene and Quaternary. The biogenic gas was formed during the middle Miocene to Pliocene. The overlying thick mudstones began to deposit in the late Miocene. Therefore, only a small amount of oil-derived and biogenic gases were preserved. This may be a critical factor for current gas pools with low reserve abundance in the Songdong region. Intense activity of the No.2 fault and absence of overlying thick mudstones in the Huangliu and Yinggehai formations gave rise to the escape of a large quantity of oil-gas from the middle Oligocene to middle Miocene. During the late Miocene to Pliocene, the injection of hydrocarbon gases occurred continuously. During this period, the NO.2 fault activity became very weak, and the overlying mudstone caprocks were gradually formed. Therefore, some amounts of hydrocarbon gases may be preserved. Due to volcanic activity in the Quaternary, mantle-derived CO2 was injected into the reservoirs via No.2 fault. The previous accumulative hydrocarbon gases were driven off by CO2 in various degrees, which may be mainly responsible for current situation that most gas pools have high CO2 content in the Baodao region.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018