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Study On Deep Natural Gas Origins And Conservation Conditions In Xujiaweizi Fault Depression Of Songliao Basin

Posted on:2011-02-03Degree:DoctorType:Dissertation
Country:ChinaCandidate:J K LiFull Text:PDF
GTID:1220360302993022Subject:Mineral prospecting and exploration
Abstract/Summary:PDF Full Text Request
The main source rock in the deep of Songliao Basin is buried deeply with rare wells reaching it and it was hard to predict the thickness and distribution. To deal with this problem, sequence stratigraphy, well data and seismic data were integrated to depict the distribution of deep source rock in the Xujiaweizi fault depression. In the study area, the primary gas-prone source rock is in the formation Shahezi where 75%of source rocks bear TOC greater than 1.0%and kerogen type is type III with high-post mature.Most of natural gas in the Xujiaweizi fault depression contains more than 95%of methane withδ13CH4 values between-26%o--30%o. The stable carbon isotopes of gaseous alkanes of most gases in the deep Xujiaweizi fault depression are either partly-inverse or totally-inverse in the order of ascending carbon number. The trends of stable carbon isotopes of gaseous alkanes vary in different areas in the fault depression. Most of stable carbon isotopes of gaseous alkanes in the areas of Xuzhong, Xudong and Xunan (the south of Xujiaweizi fault depression) are totally-inverse byδ13C1-δ13C2, while in the areas of Shengping-Anda, most have normal trends. The stable carbon isotopes of natural gases in the Xujiaweizi fault depression is heavier and part of them have inverse trends. As a result, the origin of gaseous alkanes in this area is still in debate by now.The Fishcher-Tropsch (F-T) synthesis reaction was conducted to study the origin of natural gases in the Xujiaweizi fault depression. The reactant CO2 of mantle-origin was sampled from well Fangshen-9. The stable carbon isotopes of methane synthesized by F-T reaction range—17.91‰~-46.36%o, most of which are lighter than-30‰. It indicates that the stable carbon isotope of inorganic-origin methane is not necessarily heavier than -30‰. In the 33 groups of tested generation products, only one group of them has inverse stable carbon isotope trends byδ13C1-δ13C213C3 and others have partly-inverse or normal trends. It is concluded that gaseous alkanes synthesized by F-T reaction do not necessarily have inverse stable carbon isotope trends.To determine stable carbon isotope trends of mixed gases, samples of both shale and coal with low maturity from the deep Songliao Basin were then selected for pyrolysis experiments to study the mechanics of inverse stable carbon isotope trends. Gas generated by low-mature shale and that by coal were mixed by continuous ratios to check the trends of stable carbon isotopes. The results showed that gas mixed by two end-member gases with normal stable carbon isotope trend could result in inverse stable isotope trend. It indicates again that the rule of "inverse stable carbon isotope trends" can not be applied to identify the inorganic origin of gases.The maturity period when the gas in a specified gas reservoir had been generated is important. In this study, the generation time of gas in a specified gas reservoir and the preservation conditions after trap were accomplished based on the fractionation of stable carbon isotopes. The result shows that gases from gas reservoirs in Xuzhong and Xudong areas were generated when Ro was above 1.0%and the gas lost mass was about 5%of total generated gas. In the border of the fault depression, the gas preservation conditions were poor and in the area of Xuxi, about 25%of gases were lost in the early age.
Keywords/Search Tags:Xujiaweizi fault depression, natural gas, Fishcher-Tropsch, stable carbon isotope, inverse trends
PDF Full Text Request
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