松辽盆地徐家围子断陷深层天然气成因类型及各种成因气贡献

Deep Gases and Their Genetic Types of the Xujiaweizi Fault Depression Zone,Songliao Basin and Their Contribution

徐家围子断陷深层首次发现以腐泥型气为主的有机成因天然气,不同地区深层天然气成因类型及各种成因气的贡献成为研究重点。采集徐家围子断陷29口井35件深层天然气和26口井33件深层烃源岩样品,采用组分碳同位素和轻烃指纹色谱方法分析天然气和烃源岩样品,实验研究认为深层天然气主要为腐殖型气、腐泥型气、有机深源气3种成因组成的混合气,无机成因烷烃气的贡献较小。首次提出了多种有机成因类型天然气贡献定量测试方法并进行了实验验证,采用天然气甲烷碳同位素、乙烷碳同位素和甲基环己烷指数、环己烷指数、脂烃族参数5个成因类型指标,确定了腐殖型气、腐泥型气、有机深源气的5个成因类型指标端元值,利用天然气的混合配比性建立成因类型指标地球化学模型,采用非线性数学模拟方法建立计算模板,首次定量测试了徐家围子断陷深层天然气样品中3种有机成因气的定量贡献。实验结果表明,徐家围子断陷深层天然气除昌德气藏芳深1井、芳深2井有机深源气贡献为81%外,其他井的腐殖型气、腐泥型气和有机深源气平均贡献分别为62.45%、25.51%、12.02%;不同地区及井段的腐殖型气、腐泥型气和有机深源气贡献有差别,升平—汪家屯地区平均贡献分别为61.63%、20.94%、17.29%,昌德地区平均贡献分别为73.74%、14.48%、11.77%,兴城—徐东地区及断陷中东部平均贡献分别为51.98%、40.99%、7.01%。从断陷北部到中部即从升平—汪家屯、昌德到兴城—徐东地区有机深源气贡献减少、腐泥型气贡献增大,部分井段腐泥型气贡献超过43%且为主要贡献,个别井段腐泥型气贡献最大达74%,与断陷中东部烃源岩Ⅱ型有机质相对发育及断陷地层地质特征相吻合,呈现主要来源于下伏气源岩和天然气藏以垂向运移为主、侧向运移为辅的源岩控型成藏特征。

Sapropel-dominated gas was first found in the Xujiaweizi fault depression zone （XFDZ）, Songliao Basin. This study focuses on various deep gases in different areas and their genesis. 35 deep gas samples from 29 wells and 33 deep source-rock samples from 26 wells in XFDZ north of the Songliao Basin were collected for analysis of stable carbon isotope compositions and hydrocarbon fingerprint chromatogram. The experiment suggests that deep gas contains humic gas, sapropel type gas and organogenic gas, with little or minor abiogenetic alkane gas. Quantitative measurement methods to contribution of various organogenic gas are proposed for the first time in this study and has been verified through experiments. The five genetic type indexes including methane carbon isotope, ethane carbon isotope, methylcyclohexane index, cyclohexane exponent and aliphatic hydrocarbon parameter are used to identify 5 end indexes of humic gas, sapropel type gas and organic anatetic origin gas. The geochemical modeling for genetic type indexes using natural gas blend matching and calculation template using nonlinear mathematical simulation method initially measures the contributions of the three organogenic gases in XFDZ. Experimental results show that besides the contribution of organogenic gas accounting for 81%, which comes from Fangshen 1 well and Fangshen 2 well in the Changde gas reservoir, the average contributions of humic gas, sapropel type gas and organic anatetic origin gas are 62.45%, 25. 51% and 12. 02%, respectively. Humic gas, sapropel type gas and organic anatetic origin gas contribute distinctively in different regions or intervals. The average contribution of the three organogenic gases in the Shengping--Wangjiatun region is 61.63%, 20.94% and 17.29%, respectively; that in the Changde area 73.74%, 14.48% and11.77%; that in the Xingcheng--Xudong and central eastern areas of the fault zone 51. 98G, 40. 99% and 7. 01%. From Shengping to Wangjiatun, and from Dechang to the Xingcheng--Xudong region, the contribution of organic anatetic origin gas decreases while that of sapropel type gas increases, with the contribution of organic anatetic origin gas in some intervals exceeding 40%, and some reaching to 79%. All these features coincide with the type II organic development of hydrocarbon source rock in the middle of the fault depression zone, and geologic features of the fault depression zone. This area is characterized by vertical migration of gas from the underlying source rock, with lateral migration as secondary control on the formation of gas reservoir.