Characteristiscs of Natural Gases and the Correlation of Gas/Rock in the Eastern Fault Basisns of Songliao Region

Abstract: With the further exploration work,more and more natural gases have been found m eastern fault basins of Songliao region,such as Shiwu fault basin,Dehui fault basin,Fulongquan fault basin and Ying一Xing fault basin etc. Based on the abundant geochemical data of the gases,the genetic types of the natural gas and the correlation of gas/rock in these basins have been studied.The main conclusions are drawn as follows; 1)The natural gases are generally hydrocarbon gases. Methane ranges from 75~96%,and heavier gases are totally about 0. 73~10. 6500,containing various amount of nitxogen (N₂ 1~ 2000)and a little carbon dioxide (C0₂). These gases are chemically dry with C₁/C₂-C₅ generally larger than 0. 95. 2)Four kinds of genetic types of natural gas have been subdivided by using the data of carbon isotopes. ①Sapropelic一type gas;The isotopes of C₁ and C₂ are relatively lighter,i,e.,δ¹³C₁一48. 08‰ and δ¹³C₂一35. 95‰. The gas associated with oil,indicating that this kind of gas was derived from hydrogen一rich type kerogens during the mature stage of hydrocarbon generation. ② The lower mature humic一type gas:δ¹³C₁一46一一99‰,δ¹³C₂一26. 58一一29. 73‰. The carbon isotope of ethane is heavier than一29‰ witch is usually an indicator of humic一type gas.The gas it is with lighter isotope of methane indicates that it is generated from oxygen一rich type Hf kerogen at the lower mature stage. ③ The post一mature humic一type gas:with δ¹³C₁>一30‰ and δ¹³C₂>一27‰,was found predominantly in the relatively deep levels (Ka一basement). Besides methane,ethane,propane etc,there are also a little hydrogen ( H₂ ) and helium ( He).The gas with heavier isotope of methane commonly interpreted as post一mature humic一type gas. .Mixed一type gas with δ¹³C₁一32~一42‰ and δ¹³C₂一26~一33‰,is the mast aboundant gas type in these fault basins. Some of the sampled gases contain mercury which is usually an indicator of humic一type gas. Considering the geological setting, the author believe that this kind of gas is possibly either the product of the type I kerogen or a mixture of sapropelic一type gas and humic一type gas. 3)The correlations of gas/gas and gas/rock have been done by studying the data of the light hydrocarbon (C₁~C₂),hydrocarbons absorbed by source rocks, the relation between carbon isotope and vitrinite reflectance and the other geological data,in orger to determine the source rocks. It is suggested that these gases,whether they occur in the Quantou depressed Formation or in the Denglouku Formation and underlying farlted Formation,are all derived from lower Cretaceous coal一bearing measure of the fault basin fill(K₁d一K₁sh),not from Qingshankou depressed formation.