Study On The Origins Of Natural Gas In The Xujiawezi Rift Depression, Songliao Basin

The Xujiaweizi Rift Depression is one of fault units in the Songliao Basin which consists of thirty-nine fault units. The Songliao Basin has been proven to be rich in gas and oil reservoirs. Due to the development of deep faults and high thermal evolution of organic matter, the characteristics and genetic types of the deep gas in the study area are complicated. Previous researches has made lots of works about the genetic characteristics and genetic types of natural gas in the Xujiaweizi Rift Depression.On the basis of previous researches, combined with a large amount of literature and sample analysis, the geochemical characteristics of gas source rocks and their development modes were reported. The geochemical characteristics, distribution and genetic types of natural gas were also carried on the comprehensive research. The paper illustrated the genetic characteristics of deep nature gas in the Songliao Basin, and the following results are achieved:(1) The distribution of gas source rocks in the Xujiaweizi Rift Depression is mainly controlled by faulting intensity and sedimentary environment. According to the distribution of hydrocarbon source rocks and regional stratigraphy, the sedimentary systems in the study area can be divided into steep slope belt, deep depression and gentle slope belt.Hydrocarbon source rock in the steep slope belt deposits thickly, distributes widely, and maturates highly. The sedimentary environment is a relatively reducing environment. In the gentle slope belt, hydrocarbon source rock is characterized by large thickness, well-developed coal-bearing measures, type III of organic matter and weak reduction environment. In a whole, the feature of source rock in the gentle slope belt is between the steep slope belt and deep depression.(2) The components of nature gas mainly include CO2, N2, He and H2. In different blocks, the contents of non-hydrocarbon gas differ with each other, with CO2 and N2 are dominated gases. However, the contents of H2.are higher with 1-2 orders of magnitude than that in the atmosphere. High purity of CO2 gas reservoirs are mainly distributed in the deep fault, which shows that these gases may be related to deep fracture or magmatic activity. The contents of hydrocarbon gas are lower than 5% in the majority of natural gas. It reflects the composition of natural gas is still given priority to with hydrocarbon gas.(3) The contents of hydrocarbon in natural gas account for more than 95%. Dry coefficient of nature gas is high, with an average of 96.83%; and it is higher than 96% in more than 83% samples. About 63% of samples, whose dry coefficients are higher than 97%, reach to the standard of dry gas. In the meanwhile, dry coefficient of nature gas in nearly 10% samples is less than 95%, and part of them is less than 90%, which shows a typical characteristic of moisture. The composition characteristics of nature gas also show that the components and genetic characteristics of natural gas in fault depression are complicated.(4) The results display that C2/C3 shows a good relationship with C2/iC4, and it also associated with the dry coefficient of natural gas. The dry coefficient is significantly increased with the increase of C2/C3 or C2/iC4, which illustrates that the C2/C3 and C2/iC4 are influenced by thermal maturity and can be served as maturity parameters. In addition, nature gas in the study area is characterized by high C2/C3 and C2/iC4. The C2/C3 values have a mean of 7.10; the C2/iC4 values have an average of 43.13. The experiments show that the C2/C3 values are around 2 with a Ro value of 1.5%, corresponding C2/iC4 value is around 10. The high C2/C3 and C2/iC4 values in the Xujiaweizi Depression exhibit that the hydrocarbon gas in natural gas is a product of relatively high evolution. The nature gas with high C2/C3 values also shows a low dry coefficient, suggesting that their genetic characteristics are particular.(5) Gas hydrocarbon carbon stable element analysis indicates that the δ13C values of methane distribute widely, but the distribution of the main frequency range is relatively narrow, mainly rang from--31‰ to -25%o. About 80% of natural gas are this type and characterized by the distribution characteristics of coal-type gas. However, the δ13C values of ethane, propane and butane are lighter than methane. On the whole, the values become generally lighter with the increase of carbon number, which suggests that the isotopic series appear within a wide range of reversal. It also explains the formation process of hydrocarbon gas is complex.(6) The distribution of carbon isotope in hydrocarbon series with different carbon numbers is diverse, showing certain regularity. In margin region with low thermal evolution, the δ13C values of hydrocarbon gas are characterized by δ13C1<δ13C2< δ13C3 < δ13C4. However, the natural gas in regions with a relatively high thermal evolution shows a hydrocarbon isotopic inversion phenomenon. Obviously, this feature is generally related to high values of C2/C3 and C2/iC4. Relatively high C2/C3 values, relatively low dry coefficient and high carbon number alkane gas with lowδ13C values should be related to cracking effect of liquid oil during the early generation process. The gas isotope reversal phenomenon should also be occurred under the condition of mixture of coal-derived gas and the cracking gas of crude oil in high temperature.(7) Hydrocarbon generation kinetics research shows that the nature gas generation phases are mainly distributed in 130-65 Ma (Denglouku Foramtion to Mingshui Foramtion), in which there are three gas-generating peaks. The first and second of gas-generating peaks mainly occur in the period of Quantou Formation, and the last peak occur in the period of Yaojia Foramtion. The gas-generating peak is basically in accordance with oil cracking period (110-70 Ma), which reflects the great quantities of deep natural gas are from cracking of crude oils. The conclusion can explain that the stable carbon isotopic reversal phenomenon which occurs in most of nature gas in the study area.