摘要
Carbon isotopes of natural gases are controlled not only by source and mature effect, but also by accumulating (leakage and mixing) effect. The gases, distributed in the three paleo\|uplifts, Tazhong, Tabei and Bachu in the Tarim Basin, are generated mainly from Cambrian and Lower Ordovician hydrocarbon\|source rocks. Being under good preservation conditions, gas pools in the Tazhong region experienced a long accumulation period, resulting in a successive gas accumulation region in this region. The gas carbon isotope values are more negative than those in other regions because of the strong accumulating effect. The Tabei and Mazhatage regions have poor preservation conditions, the gas pools are formed in a short accumulation period, and the gas carbon isotope values are more positive. This is called the transition\|stage gas accumulation region. Because of involvement of low mature gas, even biogenic gas from Upper\|Middle Ordovician, some gas in Ordovician reservoirs on the northern slope of the Tazhong uplift is characterized by much more negative \{δ\{\}\+\{13\}C\-1\}. The \{δ\{\}\+\{13\}C\-2\}\|\{δ\{\}\+\{13\}C\-1\} value is an effective index to measure the gas maturity of the main body in a gas pool. Combining \{δ\{\}\+\{13\}C\-2\}\|\{δ\{\}\+\{13\}C\-1\} with \{δ\{\}\+\{13\}C\-1\} and \{δ\{\}\+\{13\}C\-2\}, the effects described above can be explained very well. Results of our study show that the gas maturity for the Mazhatage region is the highest in the craton basin, followed by Tabei. Gases in the Tazhong area are high\|over mature gases though the gas carbon isotope values are highly negative.
Carbon isotopes of natural gases are controlled not only by source and mature effect, but also by accumulating (leakage and mixing) effect. The gases, distributed in the three paleo\|uplifts, Tazhong, Tabei and Bachu in the Tarim Basin, are generated mainly from Cambrian and Lower Ordovician hydrocarbon\|source rocks. Being under good preservation conditions, gas pools in the Tazhong region experienced a long accumulation period, resulting in a successive gas accumulation region in this region. The gas carbon isotope values are more negative than those in other regions because of the strong accumulating effect. The Tabei and Mazhatage regions have poor preservation conditions, the gas pools are formed in a short accumulation period, and the gas carbon isotope values are more positive. This is called the transition\|stage gas accumulation region. Because of involvement of low mature gas, even biogenic gas from Upper\|Middle Ordovician, some gas in Ordovician reservoirs on the northern slope of the Tazhong uplift is characterized by much more negative \{δ\{\}\+\{13\}C\-1\}. The \{δ\{\}\+\{13\}C\-2\}\|\{δ\{\}\+\{13\}C\-1\} value is an effective index to measure the gas maturity of the main body in a gas pool. Combining \{δ\{\}\+\{13\}C\-2\}\|\{δ\{\}\+\{13\}C\-1\} with \{δ\{\}\+\{13\}C\-1\} and \{δ\{\}\+\{13\}C\-2\}, the effects described above can be explained very well. Results of our study show that the gas maturity for the Mazhatage region is the highest in the craton basin, followed by Tabei. Gases in the Tazhong area are high\|over mature gases though the gas carbon isotope values are highly negative.
