Characteristics Of Fluid Inclusions In Volcanic Yingcheng Formation In Xujiaweizi Depression And Its Geological Significance

There are both hydrocarbon gas and carbon dioxide gas reservoirs in Yingcheng Formation of Xujiaweizi Faulted-basin, Songliao Basin. Fluid inclusions may preserve the geochemic information of the gas when it was charging. This approach avoids the multiple impacts of gas passing post-dating, and also has great advantages in determining the genesis of gas. Homogenization temperatures are generallly used in understanding the gas charging time. In order to discuss the gas genesis and charging mode in Xujiaweizi Faulted-basin, petrographic characterization, composition, homogenization temperatures and carbon isotope of fluid inclusions in volcanic rocks in 19 wells of the Yingcheng Formation are studied in this paper.Based on Fluid-inclusion petrography, CH4-rich and CO2-rich gas-liquid secondary fluid inclusions are discovered in volcanic rocks of the Yingcheng Formation. The homogenization temperatures of CH4-rich fluid inclusions in different samples range from 40.0℃to 298.0℃, and its main-frequency are 146.0℃-173.0℃, 180.0℃-225.0℃and 230.0℃-248.0℃. The CO2-rich ones range from 213.8℃to 325.0℃, while its main-frequency is 230.0℃-270.0℃.The homogenization temperature and the highest buried temperature of source rocks (Shahezi Formation) have been inferred as inclusion-breaking temperatures. Heating the samples to these temperatures breaks the inclusions and subsequently natural gas in fluid inclusions can be extracted for carbon isotope analysis. Theδ13Cco2 values of fluid-inclusion-gas range from 1.4%o to -66.8‰. Distribution sequence of carbon isotopes display normal, reverse and partially reverse distribution. This indicates that both organic and inorganic natural gas charged in Xujiaweizi Fault-basin according to comprehensive analysis of various geochemical conditions, and organic hydrocarbon takes dominance.The comparison between main components of fluid inclusions and natural gas indicates there are four charging modes in this area:(1) former gas replaced by the later; (2) early gas preserved and the later not charged; (3) gas charged in later stage; (4) gas charged by multiple stages.Wangjiatun experienced three gas charging events, but early natural gas was replaced by later natural gas charging.CH4-rich fluid inclusions of 146.0℃～173.0℃and 180.0℃～225.0℃and CO2-rich fluid inclusions are detected in Changde region. It experienced organic and inorganic gas charging. Gas of Fangshen701 Well was charged by multiple times, and Fangshen9 Well was charged by early inorganic gas.CH4-rich fluid inclusions of 180.0℃～225.0℃and 230.0℃～248.0℃are detected in Xingcheng region. It experienced organic and inorganic gas charging, with a charging model of the early natural gas was replaced by the later.CH4-rich fluid inclusions of 180.0℃～225.0℃and 230.0℃～248.0℃and CO2-rich fluid inclusions are detected in Changwu Town region. It experienced organic and inorganic gas charging. The gas charging mode of Xushen23 and Xushen302 Well is that the former gas charging was replaced by the later. Gas of Xushen22 Well was charged by multiple times, and Xushen28 Well was charged by early inorganic gas.CH4-rich fluid inclusions of 180.0℃～225.0℃and 230.0℃～248.0℃and CO2-rich fluid inclusions are detected in Fengle region. It experienced organic and inorganic gas charging. The gas charging mode of Xushen15 is that gas charged by late time. Gas of Xushen19 Well was charged by early inorganic gas.The results of this study confirmed that the inorganic gas charged in Xujiaweizi Fault-basin, but the organic gas took dominance. The natural gas charging in different regions have differente modes, resulting to the variation of gas components in respective reserviors. Different gas charging modes have been revealed in Xujiaweizi Fault-basin is understood, which will provide guidance for further exploration and development of natural gas.