Research On Source Rock And Preservation Of Natural Gas In Wu-xia Area, Junggar Basin

The hydrocarbon exploration in Wu-xia area has achieved fruitful results, but the ratio of natural gas was relatively low, discovered reserves less than 4.5 percents. About the gas source rock, preservation conditions, and other problems hampered the further exploration. The northwest of the Junggar basin sedimented serveral source rocks in Permian and Jurassic, lacustrine shales, coals and mudstones all can be gas source, and the effective source rocks recognition was an unsolved problem exsiting all the time. Foreland fold-thrust belt thrust structure movement is intense, faults were developed. oil and gas easy to escape, so the preservation condition is the key to oil and gas accumulation. Through the gas source rock comparisons and preservation analysis, pointed out the effective gas source rocks and beneficial preservation units, designated the beneficial area for the further natural gas exploration.Natural gas component in Wu-xia erea mainly is methane, most of is rich gas, major configuration are normal and intermediate types, high degree of evolution, enriched in light carbon isotopes. By the analysis of light hydrocarbon and carbon isotope charts, It is considered as mainly associated gas, a small amount of condensate gas and mixture gas. Through the analysis of the features of components, genetic types and maturity, judged the main gas source rocks is the Fengcheng group of lower Permian, Jiamuhe group and lower Wuerhe source rocks are also had little contributions.Through the analysis of the lithologic juxtaposition, the shale-smearing, stress mechanical properties of the main faults witch controlled natural gas accumulation, and applied fuzzy mathematics method to evaluated fault sealing capacity. The results show that the different regions, different layers fault sealing performance is different: the deep fault sealing capacity better than the shallow; Wuerhe area is better than the eastern of Xiazijie area and worse than the western of Xiazijie area.Wu-xia area deposited several effective set of regional and local caprocks, through the analysis of replacement pressure, hydrocarbon density sealing ability, the sedimentary environment and deposition continuity, lithology, diagenetic stage, and other macro-and micro-analysis of the sealing ability, and further used cluster analysis method made comprehensive evaluation. Wu-xia area sealing ability basically between moderate and good, Slope belt is the best, Wuerhe fault fold zone and eastern of Xiazijie fault fold zone take second place, Frontland thrust belt and Wu-xia thrust zone are bad.Wu-xia area had complicated hydrogeological conditions, palaeohydrogeologic can be divided into 4 cycles and 8 steps. Permian wate type is mainly CaCl2, high salinity, showing a closed environment, beneficial for hydrocarbon preservation; NaHCO3 is the main wate type in Triassic, CaCl2 relatively decreased, the high-value areas of formation water salinity and the present position of Oil and Gas reservoirs are match, showing the relatively closed geological environment, it's conducive to the hydrocarbon accumulation and preservation; Jurassic is showing NaHCO₃ water type, low salinity, showing poor preservation conditions. such as rCa²⁺/rMg²⁺ ratios, rCl-/rMg²⁺ ratios, rNa+/rCl- ratios and desulfate coefficient geochemical parameters reflecting Wu-xia area had good oil and gas preservation conditions. Cretaceous and Jurassic be the free exchanging zone, Triassic and upper Permian be the exchanging-retarded zone, the lower Permian and Carboniferous be the exchanging-stagnant zone.According to the comprehensive analysis of faults sealing capacity and caprock sealing ability, hydrological conditions and conditions of regional geology, divided the beneficial preservation units in Wu-xia area, and definited the direction of the further exploration.