Font Size: a A A

Geochemical Characteristics And Origin Of Natural Gases From Yuanba Area Of The Northern Sichuan Basin

Posted on:2015-08-09Degree:MasterType:Thesis
Country:ChinaCandidate:W HuFull Text:PDF
GTID:2180330431479648Subject:Institute of Geochemistry
Abstract/Summary:PDF Full Text Request
Based on the analysis of organic matter abundance, kerogen type and maturity of J1q, J1z, T3X group terrestrial source rocks and P2W, P1m, P1q group marine source rocks in Yuanba region, this research focuses on the analysis of organic geochemistry and their sources of J1q, J1z, T3X group continental natural gases and T2l, T1f, P2ch, P2w, P1q group marine natural gases.The component of continental and marine natural gases is very different in Yuanba region. Continental natural gases’chemical component is dominant in hydrocarbon gases. The N2, CO2content of continental natural gases is not high, and the gas samples are almost free of H2S. And the content of C2+is comparatively high. Thereinto, dryness coefficient of natural gases from J2q group is below0.85, belonging to typical wet gases, while natural gases’dryness coefficient from J1z and T3X group is above0.95, indicating dry gases of high-temperature pyrolysis. However, non-hydrocarbon content of marine natural gases is relatively high, and marine natural gases don’t have so many gaseous hydrocarbon. The hydrocarbon’s content of natural gases from T2l, T1f and P2ch group is substantially above90%and their dryness coefficient value is greater than0.98. The total hydrocarbon content of natural gas samples from P2w and P1q group is more than97%and the content of CH4is very high. Their dryness coefficient value is close to1. Besides, C1/(C2+C3) value of T1f, P2ch, P2w and P1q natural gases is mainly greater than400, while that ratio of J1z and T3X natural gases is less than200. Therefore, C1/(C2+C3) value can be taken advantage of to distinguish continental and marine natural gases in Yuanba region.The carbon and hydrogen isotopic composition of continental and marine natural gases differ significantly.δ13C1value of continental natural gases is less than that of marine natural gases, and so is δD1value. On the contrary,δ13C2value of continental natural gases is generally larger than that of marine natural gases. Besides,δ13C2value of natural gases from J2q group ranges from-32.3‰to-31.4‰, namely typical mature oil-related gases. Nevertheless,δ13C2value of natural gases from J1z and T3X group is heavier, which means they are typical high-over mature coal-type gases. The carbon isotopic value of ethane from T3X1~2group is significantly lighter and the majority of the natural gases’ carbon isotopic value is lighter than-28%o, indicating that there are oil-related gases mixing with. Carbon isotopic composition of hydrocarbon series of natural gases from T3X1~2group partially appears turnover, namely δ13C1>δ13C2. However,513C2value of the T2l gases ranges between-29.6~36.6%o, less than-28%o, meaning oil-related gases. For the T1f and P2ch natural gases, the sequence of alkane carbon isotopic distribution is δ13C1<δ13C2, but also appears reverse distribution,δ13C1>δ13C2, which is due to mixing and dissipation. Further, the pertinence of△(δ13C2-δ13C1) value and δ13C1value can be uesed to differentiate the continental and marine natural gases.δD1value of continental natural gases is less than-140%o, while that of marine natural gases is larger than-140%o. So,δD1value can also be applied to distinguish the continental and marine natural gases. Furthermore,δ13C1value and8D1value become heavier with their maturity increasing, and the variation tendency of continental and marine gases is very different.In addition, for T1f, P2ch marine gases having a high content of H2S, CO2, the relationship between the content of CO2, H2S and carbon isotope of CO2can be uesd for further analysis. The content of H2S form P2ch layers increases with CO2content increasing, and813Cco2become lighter. However, the content of H2S and CO2from T1f layer shows a negative correlation, and813CCO2become heavier with the increase of the CO2content. In view of the content of SO42-and Mg2+from T1f and P2ch group, the role of TSR in Changxing group is more significant. Based on the thorough analysis, we can get the consolution:H2S and CO2of P2ch group are generated from TSR; CO2of T1f comes from natural gases of P2ch and the reaction between the surrounding carbonate and H2S which is from TSR occuring in the P2ch strata during migration. But we can not rule out the role of CO2which is produced from TSR in the T1f strata.In consideration of geochemistry characteristic of continental and marine gases and gas-source correlation, some consolutions can be drawn. Jurassic and T3X3~5gases are mainly self-sourced. And natural gases in T3X1~2strata are from local source rocks, maybe mixed with the marine gases. T2l natural gases may are derived from Silurian and Xujiahe source rocks. The natural gases from T1f, P2ch, P2W and P1q are all derived from Permian marine source rocks. Besides, natural gases of Tif layers are mainly oil-cracking gases from P2ch group, and there is also a small amount of kerogen cracking gas migrating into T1f layers. However, the natural gases from P2ch group are mixed gases, which show the characteristic of mixture between the secondary cracking gases from oil with TSR corroding and the primary cracking gases from kerogen.
Keywords/Search Tags:Yuanba area of the northern Sichuan Basin, terrestrial natural gases, marine natural gases, carbon isotope, chemical composition, thermal sulfate reduction, the primary cracking gases from kerogen, the secondary cracking gases fromhydrocarbon, gas origin
PDF Full Text Request
Related items